[zfs] / module / zfs / zcp.c

/*
 * CDDL HEADER START
 *
 * This file and its contents are supplied under the terms of the
 * Common Development and Distribution License ("CDDL"), version 1.0.
 * You may only use this file in accordance with the terms of version
 * 1.0 of the CDDL.
 *
 * A full copy of the text of the CDDL should have accompanied this
 * source.  A copy of the CDDL is also available via the Internet at
 * http://www.illumos.org/license/CDDL.
 *
 * CDDL HEADER END
 */

/*
 * Copyright (c) 2016, 2017 by Delphix. All rights reserved.
 */

/*
 * ZFS Channel Programs (ZCP)
 *
 * The ZCP interface allows various ZFS commands and operations ZFS
 * administrative operations (e.g. creating and destroying snapshots, typically
 * performed via an ioctl to /dev/zfs by the zfs(8) command and
 * libzfs/libzfs_core) to be run * programmatically as a Lua script.  A ZCP
 * script is run as a dsl_sync_task and fully executed during one transaction
 * group sync.  This ensures that no other changes can be written concurrently
 * with a running Lua script.  Combining multiple calls to the exposed ZFS
 * functions into one script gives a number of benefits:
 *
 * 1. Atomicity.  For some compound or iterative operations, it's useful to be
 * able to guarantee that the state of a pool has not changed between calls to
 * ZFS.
 *
 * 2. Performance.  If a large number of changes need to be made (e.g. deleting
 * many filesystems), there can be a significant performance penalty as a
 * result of the need to wait for a transaction group sync to pass for every
 * single operation.  When expressed as a single ZCP script, all these changes
 * can be performed at once in one txg sync.
 *
 * A modified version of the Lua 5.2 interpreter is used to run channel program
 * scripts. The Lua 5.2 manual can be found at:
 *
 *      http://www.lua.org/manual/5.2/
 *
 * If being run by a user (via an ioctl syscall), executing a ZCP script
 * requires root privileges in the global zone.
 *
 * Scripts are passed to zcp_eval() as a string, then run in a synctask by
 * zcp_eval_sync().  Arguments can be passed into the Lua script as an nvlist,
 * which will be converted to a Lua table.  Similarly, values returned from
 * a ZCP script will be converted to an nvlist.  See zcp_lua_to_nvlist_impl()
 * for details on exact allowed types and conversion.
 *
 * ZFS functionality is exposed to a ZCP script as a library of function calls.
 * These calls are sorted into submodules, such as zfs.list and zfs.sync, for
 * iterators and synctasks, respectively.  Each of these submodules resides in
 * its own source file, with a zcp_*_info structure describing each library
 * call in the submodule.
 *
 * Error handling in ZCP scripts is handled by a number of different methods
 * based on severity:
 *
 * 1. Memory and time limits are in place to prevent a channel program from
 * consuming excessive system or running forever.  If one of these limits is
 * hit, the channel program will be stopped immediately and return from
 * zcp_eval() with an error code. No attempt will be made to roll back or undo
 * any changes made by the channel program before the error occured.
 * Consumers invoking zcp_eval() from elsewhere in the kernel may pass a time
 * limit of 0, disabling the time limit.
 *
 * 2. Internal Lua errors can occur as a result of a syntax error, calling a
 * library function with incorrect arguments, invoking the error() function,
 * failing an assert(), or other runtime errors.  In these cases the channel
 * program will stop executing and return from zcp_eval() with an error code.
 * In place of a return value, an error message will also be returned in the
 * 'result' nvlist containing information about the error. No attempt will be
 * made to roll back or undo any changes made by the channel program before the
 * error occured.
 *
 * 3. If an error occurs inside a ZFS library call which returns an error code,
 * the error is returned to the Lua script to be handled as desired.
 *
 * In the first two cases, Lua's error-throwing mechanism is used, which
 * longjumps out of the script execution with luaL_error() and returns with the
 * error.
 *
 * See zfs-program(8) for more information on high level usage.
 */

#include <sys/lua/lua.h>
#include <sys/lua/lualib.h>
#include <sys/lua/lauxlib.h>

#include <sys/dsl_prop.h>
#include <sys/dsl_synctask.h>
#include <sys/dsl_dataset.h>
#include <sys/zcp.h>
#include <sys/zcp_iter.h>
#include <sys/zcp_prop.h>
#include <sys/zcp_global.h>
#include <util/sscanf.h>

#ifndef KM_NORMALPRI
#define KM_NORMALPRI    0
#endif

#define ZCP_NVLIST_MAX_DEPTH 20

uint64_t zfs_lua_check_instrlimit_interval = 100;
uint64_t zfs_lua_max_instrlimit = ZCP_MAX_INSTRLIMIT;
uint64_t zfs_lua_max_memlimit = ZCP_MAX_MEMLIMIT;

/*
 * Forward declarations for mutually recursive functions
 */
static int zcp_nvpair_value_to_lua(lua_State *, nvpair_t *, char *, int);
static int zcp_lua_to_nvlist_impl(lua_State *, int, nvlist_t *, const char *,
    int);

typedef struct zcp_alloc_arg {
        boolean_t       aa_must_succeed;
        int64_t         aa_alloc_remaining;
        int64_t         aa_alloc_limit;
} zcp_alloc_arg_t;

typedef struct zcp_eval_arg {
        lua_State       *ea_state;
        zcp_alloc_arg_t *ea_allocargs;
        cred_t          *ea_cred;
        nvlist_t        *ea_outnvl;
        int             ea_result;
        uint64_t        ea_instrlimit;
} zcp_eval_arg_t;

/*ARGSUSED*/
static int
zcp_eval_check(void *arg, dmu_tx_t *tx)
{
        return (0);
}

/*
 * The outer-most error callback handler for use with lua_pcall(). On
 * error Lua will call this callback with a single argument that
 * represents the error value. In most cases this will be a string
 * containing an error message, but channel programs can use Lua's
 * error() function to return arbitrary objects as errors. This callback
 * returns (on the Lua stack) the original error object along with a traceback.
 *
 * Fatal Lua errors can occur while resources are held, so we also call any
 * registered cleanup function here.
 */
static int
zcp_error_handler(lua_State *state)
{
        const char *msg;

        zcp_cleanup(state);

        VERIFY3U(1, ==, lua_gettop(state));
        msg = lua_tostring(state, 1);
        luaL_traceback(state, state, msg, 1);
        return (1);
}

int
zcp_argerror(lua_State *state, int narg, const char *msg, ...)
{
        va_list alist;

        va_start(alist, msg);
        const char *buf = lua_pushvfstring(state, msg, alist);
        va_end(alist);

        return (luaL_argerror(state, narg, buf));
}

/*
 * Install a new cleanup function, which will be invoked with the given
 * opaque argument if a fatal error causes the Lua interpreter to longjump out
 * of a function call.
 *
 * If an error occurs, the cleanup function will be invoked exactly once and
 * then unreigstered.
 */
void
zcp_register_cleanup(lua_State *state, zcp_cleanup_t cleanfunc, void *cleanarg)
{
        zcp_run_info_t *ri = zcp_run_info(state);
        /*
         * A cleanup function should always be explicitly removed before
         * installing a new one to avoid accidental clobbering.
         */
        ASSERT3P(ri->zri_cleanup, ==, NULL);

        ri->zri_cleanup = cleanfunc;
        ri->zri_cleanup_arg = cleanarg;
}

void
zcp_clear_cleanup(lua_State *state)
{
        zcp_run_info_t *ri = zcp_run_info(state);

        ri->zri_cleanup = NULL;
        ri->zri_cleanup_arg = NULL;
}

/*
 * If it exists, execute the currently set cleanup function then unregister it.
 */
void
zcp_cleanup(lua_State *state)
{
        zcp_run_info_t *ri = zcp_run_info(state);

        if (ri->zri_cleanup != NULL) {
                ri->zri_cleanup(ri->zri_cleanup_arg);
                zcp_clear_cleanup(state);
        }
}

/*
 * Convert the lua table at the given index on the Lua stack to an nvlist
 * and return it.
 *
 * If the table can not be converted for any reason, NULL is returned and
 * an error message is pushed onto the Lua stack.
 */
static nvlist_t *
zcp_table_to_nvlist(lua_State *state, int index, int depth)
{
        nvlist_t *nvl;
        /*
         * Converting a Lua table to an nvlist with key uniqueness checking is
         * O(n^2) in the number of keys in the nvlist, which can take a long
         * time when we return a large table from a channel program.
         * Furthermore, Lua's table interface *almost* guarantees unique keys
         * on its own (details below). Therefore, we don't use fnvlist_alloc()
         * here to avoid the built-in uniqueness checking.
         *
         * The *almost* is because it's possible to have key collisions between
         * e.g. the string "1" and the number 1, or the string "true" and the
         * boolean true, so we explicitly check that when we're looking at a
         * key which is an integer / boolean or a string that can be parsed as
         * one of those types. In the worst case this could still devolve into
         * O(n^2), so we only start doing these checks on boolean/integer keys
         * once we've seen a string key which fits this weird usage pattern.
         *
         * Ultimately, we still want callers to know that the keys in this
         * nvlist are unique, so before we return this we set the nvlist's
         * flags to reflect that.
         */
        VERIFY0(nvlist_alloc(&nvl, 0, KM_SLEEP));

        /*
         * Push an empty stack slot where lua_next() will store each
         * table key.
         */
        lua_pushnil(state);
        boolean_t saw_str_could_collide = B_FALSE;
        while (lua_next(state, index) != 0) {
                /*
                 * The next key-value pair from the table at index is
                 * now on the stack, with the key at stack slot -2 and
                 * the value at slot -1.
                 */
                int err = 0;
                char buf[32];
                const char *key = NULL;
                boolean_t key_could_collide = B_FALSE;

                switch (lua_type(state, -2)) {
                case LUA_TSTRING:
                        key = lua_tostring(state, -2);

                        /* check if this could collide with a number or bool */
                        long long tmp;
                        int parselen;
                        if ((sscanf(key, "%lld%n", &tmp, &parselen) > 0 &&
                            parselen == strlen(key)) ||
                            strcmp(key, "true") == 0 ||
                            strcmp(key, "false") == 0) {
                                key_could_collide = B_TRUE;
                                saw_str_could_collide = B_TRUE;
                        }
                        break;
                case LUA_TBOOLEAN:
                        key = (lua_toboolean(state, -2) == B_TRUE ?
                            "true" : "false");
                        if (saw_str_could_collide) {
                                key_could_collide = B_TRUE;
                        }
                        break;
                case LUA_TNUMBER:
                        VERIFY3U(sizeof (buf), >,
                            snprintf(buf, sizeof (buf), "%lld",
                            (longlong_t)lua_tonumber(state, -2)));
                        key = buf;
                        if (saw_str_could_collide) {
                                key_could_collide = B_TRUE;
                        }
                        break;
                default:
                        fnvlist_free(nvl);
                        (void) lua_pushfstring(state, "Invalid key "
                            "type '%s' in table",
                            lua_typename(state, lua_type(state, -2)));
                        return (NULL);
                }
                /*
                 * Check for type-mismatched key collisions, and throw an error.
                 */
                if (key_could_collide && nvlist_exists(nvl, key)) {
                        fnvlist_free(nvl);
                        (void) lua_pushfstring(state, "Collision of "
                            "key '%s' in table", key);
                        return (NULL);
                }
                /*
                 * Recursively convert the table value and insert into
                 * the new nvlist with the parsed key.  To prevent
                 * stack overflow on circular or heavily nested tables,
                 * we track the current nvlist depth.
                 */
                if (depth >= ZCP_NVLIST_MAX_DEPTH) {
                        fnvlist_free(nvl);
                        (void) lua_pushfstring(state, "Maximum table "
                            "depth (%d) exceeded for table",
                            ZCP_NVLIST_MAX_DEPTH);
                        return (NULL);
                }
                err = zcp_lua_to_nvlist_impl(state, -1, nvl, key,
                    depth + 1);
                if (err != 0) {
                        fnvlist_free(nvl);
                        /*
                         * Error message has been pushed to the lua
                         * stack by the recursive call.
                         */
                        return (NULL);
                }
                /*
                 * Pop the value pushed by lua_next().
                 */
                lua_pop(state, 1);
        }

        /*
         * Mark the nvlist as having unique keys. This is a little ugly, but we
         * ensured above that there are no duplicate keys in the nvlist.
         */
        nvl->nvl_nvflag |= NV_UNIQUE_NAME;

        return (nvl);
}

/*
 * Convert a value from the given index into the lua stack to an nvpair, adding
 * it to an nvlist with the given key.
 *
 * Values are converted as follows:
 *
 *   string -> string
 *   number -> int64
 *   boolean -> boolean
 *   nil -> boolean (no value)
 *
 * Lua tables are converted to nvlists and then inserted. The table's keys
 * are converted to strings then used as keys in the nvlist to store each table
 * element.  Keys are converted as follows:
 *
 *   string -> no change
 *   number -> "%lld"
 *   boolean -> "true" | "false"
 *   nil -> error
 *
 * In the case of a key collision, an error is thrown.
 *
 * If an error is encountered, a nonzero error code is returned, and an error
 * string will be pushed onto the Lua stack.
 */
static int
zcp_lua_to_nvlist_impl(lua_State *state, int index, nvlist_t *nvl,
    const char *key, int depth)
{
        /*
         * Verify that we have enough remaining space in the lua stack to parse
         * a key-value pair and push an error.
         */
        if (!lua_checkstack(state, 3)) {
                (void) lua_pushstring(state, "Lua stack overflow");
                return (1);
        }

        index = lua_absindex(state, index);

        switch (lua_type(state, index)) {
        case LUA_TNIL:
                fnvlist_add_boolean(nvl, key);
                break;
        case LUA_TBOOLEAN:
                fnvlist_add_boolean_value(nvl, key,
                    lua_toboolean(state, index));
                break;
        case LUA_TNUMBER:
                fnvlist_add_int64(nvl, key, lua_tonumber(state, index));
                break;
        case LUA_TSTRING:
                fnvlist_add_string(nvl, key, lua_tostring(state, index));
                break;
        case LUA_TTABLE: {
                nvlist_t *value_nvl = zcp_table_to_nvlist(state, index, depth);
                if (value_nvl == NULL)
                        return (EINVAL);

                fnvlist_add_nvlist(nvl, key, value_nvl);
                fnvlist_free(value_nvl);
                break;
        }
        default:
                (void) lua_pushfstring(state,
                    "Invalid value type '%s' for key '%s'",
                    lua_typename(state, lua_type(state, index)), key);
                return (EINVAL);
        }

        return (0);
}

/*
 * Convert a lua value to an nvpair, adding it to an nvlist with the given key.
 */
void
zcp_lua_to_nvlist(lua_State *state, int index, nvlist_t *nvl, const char *key)
{
        /*
         * On error, zcp_lua_to_nvlist_impl pushes an error string onto the Lua
         * stack before returning with a nonzero error code. If an error is
         * returned, throw a fatal lua error with the given string.
         */
        if (zcp_lua_to_nvlist_impl(state, index, nvl, key, 0) != 0)
                (void) lua_error(state);
}

int
zcp_lua_to_nvlist_helper(lua_State *state)
{
        nvlist_t *nv = (nvlist_t *)lua_touserdata(state, 2);
        const char *key = (const char *)lua_touserdata(state, 1);
        zcp_lua_to_nvlist(state, 3, nv, key);
        return (0);
}

void
zcp_convert_return_values(lua_State *state, nvlist_t *nvl,
    const char *key, zcp_eval_arg_t *evalargs)
{
        int err;
        lua_pushcfunction(state, zcp_lua_to_nvlist_helper);
        lua_pushlightuserdata(state, (char *)key);
        lua_pushlightuserdata(state, nvl);
        lua_pushvalue(state, 1);
        lua_remove(state, 1);
        err = lua_pcall(state, 3, 0, 0); /* zcp_lua_to_nvlist_helper */
        if (err != 0) {
                zcp_lua_to_nvlist(state, 1, nvl, ZCP_RET_ERROR);
                evalargs->ea_result = SET_ERROR(ECHRNG);
        }
}

/*
 * Push a Lua table representing nvl onto the stack.  If it can't be
 * converted, return EINVAL, fill in errbuf, and push nothing. errbuf may
 * be specified as NULL, in which case no error string will be output.
 *
 * Most nvlists are converted as simple key->value Lua tables, but we make
 * an exception for the case where all nvlist entries are BOOLEANs (a string
 * key without a value). In Lua, a table key pointing to a value of Nil
 * (no value) is equivalent to the key not existing, so a BOOLEAN nvlist
 * entry can't be directly converted to a Lua table entry. Nvlists of entirely
 * BOOLEAN entries are frequently used to pass around lists of datasets, so for
 * convenience we check for this case, and convert it to a simple Lua array of
 * strings.
 */
int
zcp_nvlist_to_lua(lua_State *state, nvlist_t *nvl,
    char *errbuf, int errbuf_len)
{
        nvpair_t *pair;
        lua_newtable(state);
        boolean_t has_values = B_FALSE;
        /*
         * If the list doesn't have any values, just convert it to a string
         * array.
         */
        for (pair = nvlist_next_nvpair(nvl, NULL);
            pair != NULL; pair = nvlist_next_nvpair(nvl, pair)) {
                if (nvpair_type(pair) != DATA_TYPE_BOOLEAN) {
                        has_values = B_TRUE;
                        break;
                }
        }
        if (!has_values) {
                int i = 1;
                for (pair = nvlist_next_nvpair(nvl, NULL);
                    pair != NULL; pair = nvlist_next_nvpair(nvl, pair)) {
                        (void) lua_pushinteger(state, i);
                        (void) lua_pushstring(state, nvpair_name(pair));
                        (void) lua_settable(state, -3);
                        i++;
                }
        } else {
                for (pair = nvlist_next_nvpair(nvl, NULL);
                    pair != NULL; pair = nvlist_next_nvpair(nvl, pair)) {
                        int err = zcp_nvpair_value_to_lua(state, pair,
                            errbuf, errbuf_len);
                        if (err != 0) {
                                lua_pop(state, 1);
                                return (err);
                        }
                        (void) lua_setfield(state, -2, nvpair_name(pair));
                }
        }
        return (0);
}

/*
 * Push a Lua object representing the value of "pair" onto the stack.
 *
 * Only understands boolean_value, string, int64, nvlist,
 * string_array, and int64_array type values.  For other
 * types, returns EINVAL, fills in errbuf, and pushes nothing.
 */
static int
zcp_nvpair_value_to_lua(lua_State *state, nvpair_t *pair,
    char *errbuf, int errbuf_len)
{
        int err = 0;

        if (pair == NULL) {
                lua_pushnil(state);
                return (0);
        }

        switch (nvpair_type(pair)) {
        case DATA_TYPE_BOOLEAN_VALUE:
                (void) lua_pushboolean(state,
                    fnvpair_value_boolean_value(pair));
                break;
        case DATA_TYPE_STRING:
                (void) lua_pushstring(state, fnvpair_value_string(pair));
                break;
        case DATA_TYPE_INT64:
                (void) lua_pushinteger(state, fnvpair_value_int64(pair));
                break;
        case DATA_TYPE_NVLIST:
                err = zcp_nvlist_to_lua(state,
                    fnvpair_value_nvlist(pair), errbuf, errbuf_len);
                break;
        case DATA_TYPE_STRING_ARRAY: {
                char **strarr;
                uint_t nelem;
                (void) nvpair_value_string_array(pair, &strarr, &nelem);
                lua_newtable(state);
                for (int i = 0; i < nelem; i++) {
                        (void) lua_pushinteger(state, i + 1);
                        (void) lua_pushstring(state, strarr[i]);
                        (void) lua_settable(state, -3);
                }
                break;
        }
        case DATA_TYPE_UINT64_ARRAY: {
                uint64_t *intarr;
                uint_t nelem;
                (void) nvpair_value_uint64_array(pair, &intarr, &nelem);
                lua_newtable(state);
                for (int i = 0; i < nelem; i++) {
                        (void) lua_pushinteger(state, i + 1);
                        (void) lua_pushinteger(state, intarr[i]);
                        (void) lua_settable(state, -3);
                }
                break;
        }
        case DATA_TYPE_INT64_ARRAY: {
                int64_t *intarr;
                uint_t nelem;
                (void) nvpair_value_int64_array(pair, &intarr, &nelem);
                lua_newtable(state);
                for (int i = 0; i < nelem; i++) {
                        (void) lua_pushinteger(state, i + 1);
                        (void) lua_pushinteger(state, intarr[i]);
                        (void) lua_settable(state, -3);
                }
                break;
        }
        default: {
                if (errbuf != NULL) {
                        (void) snprintf(errbuf, errbuf_len,
                            "Unhandled nvpair type %d for key '%s'",
                            nvpair_type(pair), nvpair_name(pair));
                }
                return (EINVAL);
        }
        }
        return (err);
}

int
zcp_dataset_hold_error(lua_State *state, dsl_pool_t *dp, const char *dsname,
    int error)
{
        if (error == ENOENT) {
                (void) zcp_argerror(state, 1, "no such dataset '%s'", dsname);
                return (0); /* not reached; zcp_argerror will longjmp */
        } else if (error == EXDEV) {
                (void) zcp_argerror(state, 1,
                    "dataset '%s' is not in the target pool '%s'",
                    dsname, spa_name(dp->dp_spa));
                return (0); /* not reached; zcp_argerror will longjmp */
        } else if (error == EIO) {
                (void) luaL_error(state,
                    "I/O error while accessing dataset '%s'", dsname);
                return (0); /* not reached; luaL_error will longjmp */
        } else if (error != 0) {
                (void) luaL_error(state,
                    "unexpected error %d while accessing dataset '%s'",
                    error, dsname);
                return (0); /* not reached; luaL_error will longjmp */
        }
        return (0);
}

/*
 * Note: will longjmp (via lua_error()) on error.
 * Assumes that the dsname is argument #1 (for error reporting purposes).
 */
dsl_dataset_t *
zcp_dataset_hold(lua_State *state, dsl_pool_t *dp, const char *dsname,
    void *tag)
{
        dsl_dataset_t *ds;
        int error = dsl_dataset_hold(dp, dsname, tag, &ds);
        (void) zcp_dataset_hold_error(state, dp, dsname, error);
        return (ds);
}

static int zcp_debug(lua_State *);
static zcp_lib_info_t zcp_debug_info = {
        .name = "debug",
        .func = zcp_debug,
        .pargs = {
            { .za_name = "debug string", .za_lua_type = LUA_TSTRING},
            {NULL, 0}
        },
        .kwargs = {
            {NULL, 0}
        }
};

static int
zcp_debug(lua_State *state)
{
        const char *dbgstring;
        zcp_run_info_t *ri = zcp_run_info(state);
        zcp_lib_info_t *libinfo = &zcp_debug_info;

        zcp_parse_args(state, libinfo->name, libinfo->pargs, libinfo->kwargs);

        dbgstring = lua_tostring(state, 1);

        zfs_dbgmsg("txg %lld ZCP: %s", ri->zri_tx->tx_txg, dbgstring);

        return (0);
}

static int zcp_exists(lua_State *);
static zcp_lib_info_t zcp_exists_info = {
        .name = "exists",
        .func = zcp_exists,
        .pargs = {
            { .za_name = "dataset", .za_lua_type = LUA_TSTRING},
            {NULL, 0}
        },
        .kwargs = {
            {NULL, 0}
        }
};

static int
zcp_exists(lua_State *state)
{
        zcp_run_info_t *ri = zcp_run_info(state);
        dsl_pool_t *dp = ri->zri_pool;
        zcp_lib_info_t *libinfo = &zcp_exists_info;

        zcp_parse_args(state, libinfo->name, libinfo->pargs, libinfo->kwargs);

        const char *dsname = lua_tostring(state, 1);

        dsl_dataset_t *ds;
        int error = dsl_dataset_hold(dp, dsname, FTAG, &ds);
        if (error == 0) {
                dsl_dataset_rele(ds, FTAG);
                lua_pushboolean(state, B_TRUE);
        } else if (error == ENOENT) {
                lua_pushboolean(state, B_FALSE);
        } else if (error == EXDEV) {
                return (luaL_error(state, "dataset '%s' is not in the "
                    "target pool", dsname));
        } else if (error == EIO) {
                return (luaL_error(state, "I/O error opening dataset '%s'",
                    dsname));
        } else if (error != 0) {
                return (luaL_error(state, "unexpected error %d", error));
        }

        return (1);
}

/*
 * Allocate/realloc/free a buffer for the lua interpreter.
 *
 * When nsize is 0, behaves as free() and returns NULL.
 *
 * If ptr is NULL, behaves as malloc() and returns an allocated buffer of size
 * at least nsize.
 *
 * Otherwise, behaves as realloc(), changing the allocation from osize to nsize.
 * Shrinking the buffer size never fails.
 *
 * The original allocated buffer size is stored as a uint64 at the beginning of
 * the buffer to avoid actually reallocating when shrinking a buffer, since lua
 * requires that this operation never fail.
 */
static void *
zcp_lua_alloc(void *ud, void *ptr, size_t osize, size_t nsize)
{
        zcp_alloc_arg_t *allocargs = ud;
        int flags = (allocargs->aa_must_succeed) ?
            KM_SLEEP : (KM_NOSLEEP | KM_NORMALPRI);

        if (nsize == 0) {
                if (ptr != NULL) {
                        int64_t *allocbuf = (int64_t *)ptr - 1;
                        int64_t allocsize = *allocbuf;
                        ASSERT3S(allocsize, >, 0);
                        ASSERT3S(allocargs->aa_alloc_remaining + allocsize, <=,
                            allocargs->aa_alloc_limit);
                        allocargs->aa_alloc_remaining += allocsize;
                        vmem_free(allocbuf, allocsize);
                }
                return (NULL);
        } else if (ptr == NULL) {
                int64_t *allocbuf;
                int64_t allocsize = nsize + sizeof (int64_t);

                if (!allocargs->aa_must_succeed &&
                    (allocsize <= 0 ||
                    allocsize > allocargs->aa_alloc_remaining)) {
                        return (NULL);
                }

                allocbuf = vmem_alloc(allocsize, flags);
                if (allocbuf == NULL) {
                        return (NULL);
                }
                allocargs->aa_alloc_remaining -= allocsize;

                *allocbuf = allocsize;
                return (allocbuf + 1);
        } else if (nsize <= osize) {
                /*
                 * If shrinking the buffer, lua requires that the reallocation
                 * never fail.
                 */
                return (ptr);
        } else {
                ASSERT3U(nsize, >, osize);

                uint64_t *luabuf = zcp_lua_alloc(ud, NULL, 0, nsize);
                if (luabuf == NULL) {
                        return (NULL);
                }
                (void) memcpy(luabuf, ptr, osize);
                VERIFY3P(zcp_lua_alloc(ud, ptr, osize, 0), ==, NULL);
                return (luabuf);
        }
}

/* ARGSUSED */
static void
zcp_lua_counthook(lua_State *state, lua_Debug *ar)
{
        /*
         * If we're called, check how many instructions the channel program has
         * executed so far, and compare against the limit.
         */
        lua_getfield(state, LUA_REGISTRYINDEX, ZCP_RUN_INFO_KEY);
        zcp_run_info_t *ri = lua_touserdata(state, -1);

        ri->zri_curinstrs += zfs_lua_check_instrlimit_interval;
        if (ri->zri_maxinstrs != 0 && ri->zri_curinstrs > ri->zri_maxinstrs) {
                ri->zri_timed_out = B_TRUE;
                (void) lua_pushstring(state,
                    "Channel program timed out.");
                (void) lua_error(state);
        }
}

static int
zcp_panic_cb(lua_State *state)
{
        panic("unprotected error in call to Lua API (%s)\n",
            lua_tostring(state, -1));
        return (0);
}

static void
zcp_eval_sync(void *arg, dmu_tx_t *tx)
{
        int err;
        zcp_run_info_t ri;
        zcp_eval_arg_t *evalargs = arg;
        lua_State *state = evalargs->ea_state;

        /*
         * Open context should have setup the stack to contain:
         * 1: Error handler callback
         * 2: Script to run (converted to a Lua function)
         * 3: nvlist input to function (converted to Lua table or nil)
         */
        VERIFY3U(3, ==, lua_gettop(state));

        /*
         * Store the zcp_run_info_t struct for this run in the Lua registry.
         * Registry entries are not directly accessible by the Lua scripts but
         * can be accessed by our callbacks.
         */
        ri.zri_space_used = 0;
        ri.zri_pool = dmu_tx_pool(tx);
        ri.zri_cred = evalargs->ea_cred;
        ri.zri_tx = tx;
        ri.zri_timed_out = B_FALSE;
        ri.zri_cleanup = NULL;
        ri.zri_cleanup_arg = NULL;
        ri.zri_curinstrs = 0;
        ri.zri_maxinstrs = evalargs->ea_instrlimit;

        lua_pushlightuserdata(state, &ri);
        lua_setfield(state, LUA_REGISTRYINDEX, ZCP_RUN_INFO_KEY);
        VERIFY3U(3, ==, lua_gettop(state));

        /*
         * Tell the Lua interpreter to call our handler every count
         * instructions. Channel programs that execute too many instructions
         * should die with ETIME.
         */
        (void) lua_sethook(state, zcp_lua_counthook, LUA_MASKCOUNT,
            zfs_lua_check_instrlimit_interval);

        /*
         * Tell the Lua memory allocator to stop using KM_SLEEP before handing
         * off control to the channel program. Channel programs that use too
         * much memory should die with ENOSPC.
         */
        evalargs->ea_allocargs->aa_must_succeed = B_FALSE;

        /*
         * Call the Lua function that open-context passed us. This pops the
         * function and its input from the stack and pushes any return
         * or error values.
         */
        err = lua_pcall(state, 1, LUA_MULTRET, 1);

        /*
         * Let Lua use KM_SLEEP while we interpret the return values.
         */
        evalargs->ea_allocargs->aa_must_succeed = B_TRUE;

        /*
         * Remove the error handler callback from the stack. At this point,
         * if there is a cleanup function registered, then it was registered
         * but never run or removed, which should never occur.
         */
        ASSERT3P(ri.zri_cleanup, ==, NULL);
        lua_remove(state, 1);

        switch (err) {
        case LUA_OK: {
                /*
                 * Lua supports returning multiple values in a single return
                 * statement.  Return values will have been pushed onto the
                 * stack:
                 * 1: Return value 1
                 * 2: Return value 2
                 * 3: etc...
                 * To simplify the process of retrieving a return value from a
                 * channel program, we disallow returning more than one value
                 * to ZFS from the Lua script, yielding a singleton return
                 * nvlist of the form { "return": Return value 1 }.
                 */
                int return_count = lua_gettop(state);

                if (return_count == 1) {
                        evalargs->ea_result = 0;
                        zcp_convert_return_values(state, evalargs->ea_outnvl,
                            ZCP_RET_RETURN, evalargs);
                } else if (return_count > 1) {
                        evalargs->ea_result = SET_ERROR(ECHRNG);
                        (void) lua_pushfstring(state, "Multiple return "
                            "values not supported");
                        zcp_convert_return_values(state, evalargs->ea_outnvl,
                            ZCP_RET_ERROR, evalargs);
                }
                break;
        }
        case LUA_ERRRUN:
        case LUA_ERRGCMM: {
                /*
                 * The channel program encountered a fatal error within the
                 * script, such as failing an assertion, or calling a function
                 * with incompatible arguments. The error value and the
                 * traceback generated by zcp_error_handler() should be on the
                 * stack.
                 */
                VERIFY3U(1, ==, lua_gettop(state));
                if (ri.zri_timed_out) {
                        evalargs->ea_result = SET_ERROR(ETIME);
                } else {
                        evalargs->ea_result = SET_ERROR(ECHRNG);
                }

                zcp_convert_return_values(state, evalargs->ea_outnvl,
                    ZCP_RET_ERROR, evalargs);

                if (evalargs->ea_result == ETIME &&
                    evalargs->ea_outnvl != NULL) {
                        (void) nvlist_add_uint64(evalargs->ea_outnvl,
                            ZCP_ARG_INSTRLIMIT, ri.zri_curinstrs);
                }
                break;
        }
        case LUA_ERRERR: {
                /*
                 * The channel program encountered a fatal error within the
                 * script, and we encountered another error while trying to
                 * compute the traceback in zcp_error_handler(). We can only
                 * return the error message.
                 */
                VERIFY3U(1, ==, lua_gettop(state));
                if (ri.zri_timed_out) {
                        evalargs->ea_result = SET_ERROR(ETIME);
                } else {
                        evalargs->ea_result = SET_ERROR(ECHRNG);
                }

                zcp_convert_return_values(state, evalargs->ea_outnvl,
                    ZCP_RET_ERROR, evalargs);
                break;
        }
        case LUA_ERRMEM:
                /*
                 * Lua ran out of memory while running the channel program.
                 * There's not much we can do.
                 */
                evalargs->ea_result = SET_ERROR(ENOSPC);
                break;
        default:
                VERIFY0(err);
        }
}

int
zcp_eval(const char *poolname, const char *program, uint64_t instrlimit,
    uint64_t memlimit, nvpair_t *nvarg, nvlist_t *outnvl)
{
        int err;
        lua_State *state;
        zcp_eval_arg_t evalargs;

        if (instrlimit > zfs_lua_max_instrlimit)
                return (SET_ERROR(EINVAL));
        if (memlimit == 0 || memlimit > zfs_lua_max_memlimit)
                return (SET_ERROR(EINVAL));

        zcp_alloc_arg_t allocargs = {
                .aa_must_succeed = B_TRUE,
                .aa_alloc_remaining = (int64_t)memlimit,
                .aa_alloc_limit = (int64_t)memlimit,
        };

        /*
         * Creates a Lua state with a memory allocator that uses KM_SLEEP.
         * This should never fail.
         */
        state = lua_newstate(zcp_lua_alloc, &allocargs);
        VERIFY(state != NULL);
        (void) lua_atpanic(state, zcp_panic_cb);

        /*
         * Load core Lua libraries we want access to.
         */
        VERIFY3U(1, ==, luaopen_base(state));
        lua_pop(state, 1);
        VERIFY3U(1, ==, luaopen_coroutine(state));
        lua_setglobal(state, LUA_COLIBNAME);
        VERIFY0(lua_gettop(state));
        VERIFY3U(1, ==, luaopen_string(state));
        lua_setglobal(state, LUA_STRLIBNAME);
        VERIFY0(lua_gettop(state));
        VERIFY3U(1, ==, luaopen_table(state));
        lua_setglobal(state, LUA_TABLIBNAME);
        VERIFY0(lua_gettop(state));

        /*
         * Load globally visible variables such as errno aliases.
         */
        zcp_load_globals(state);
        VERIFY0(lua_gettop(state));

        /*
         * Load ZFS-specific modules.
         */
        lua_newtable(state);
        VERIFY3U(1, ==, zcp_load_list_lib(state));
        lua_setfield(state, -2, "list");
        VERIFY3U(1, ==, zcp_load_synctask_lib(state, B_FALSE));
        lua_setfield(state, -2, "check");
        VERIFY3U(1, ==, zcp_load_synctask_lib(state, B_TRUE));
        lua_setfield(state, -2, "sync");
        VERIFY3U(1, ==, zcp_load_get_lib(state));
        lua_pushcclosure(state, zcp_debug_info.func, 0);
        lua_setfield(state, -2, zcp_debug_info.name);
        lua_pushcclosure(state, zcp_exists_info.func, 0);
        lua_setfield(state, -2, zcp_exists_info.name);
        lua_setglobal(state, "zfs");
        VERIFY0(lua_gettop(state));

        /*
         * Push the error-callback that calculates Lua stack traces on
         * unexpected failures.
         */
        lua_pushcfunction(state, zcp_error_handler);
        VERIFY3U(1, ==, lua_gettop(state));

        /*
         * Load the actual script as a function onto the stack as text ("t").
         * The only valid error condition is a syntax error in the script.
         * ERRMEM should not be possible because our allocator is using
         * KM_SLEEP.  ERRGCMM should not be possible because we have not added
         * any objects with __gc metamethods to the interpreter that could
         * fail.
         */
        err = luaL_loadbufferx(state, program, strlen(program),
            "channel program", "t");
        if (err == LUA_ERRSYNTAX) {
                fnvlist_add_string(outnvl, ZCP_RET_ERROR,
                    lua_tostring(state, -1));
                lua_close(state);
                return (SET_ERROR(EINVAL));
        }
        VERIFY0(err);
        VERIFY3U(2, ==, lua_gettop(state));

        /*
         * Convert the input nvlist to a Lua object and put it on top of the
         * stack.
         */
        char errmsg[128];
        err = zcp_nvpair_value_to_lua(state, nvarg,
            errmsg, sizeof (errmsg));
        if (err != 0) {
                fnvlist_add_string(outnvl, ZCP_RET_ERROR, errmsg);
                lua_close(state);
                return (SET_ERROR(EINVAL));
        }
        VERIFY3U(3, ==, lua_gettop(state));

        evalargs.ea_state = state;
        evalargs.ea_allocargs = &allocargs;
        evalargs.ea_instrlimit = instrlimit;
        evalargs.ea_cred = CRED();
        evalargs.ea_outnvl = outnvl;
        evalargs.ea_result = 0;

        VERIFY0(dsl_sync_task(poolname, zcp_eval_check,
            zcp_eval_sync, &evalargs, 0, ZFS_SPACE_CHECK_NONE));

        lua_close(state);

        return (evalargs.ea_result);
}

/*
 * Retrieve metadata about the currently running channel program.
 */
zcp_run_info_t *
zcp_run_info(lua_State *state)
{
        zcp_run_info_t *ri;

        lua_getfield(state, LUA_REGISTRYINDEX, ZCP_RUN_INFO_KEY);
        ri = lua_touserdata(state, -1);
        lua_pop(state, 1);
        return (ri);
}

/*
 * Argument Parsing
 * ================
 *
 * The Lua language allows methods to be called with any number
 * of arguments of any type. When calling back into ZFS we need to sanitize
 * arguments from channel programs to make sure unexpected arguments or
 * arguments of the wrong type result in clear error messages. To do this
 * in a uniform way all callbacks from channel programs should use the
 * zcp_parse_args() function to interpret inputs.
 *
 * Positional vs Keyword Arguments
 * ===============================
 *
 * Every callback function takes a fixed set of required positional arguments
 * and optional keyword arguments. For example, the destroy function takes
 * a single positional string argument (the name of the dataset to destroy)
 * and an optional "defer" keyword boolean argument. When calling lua functions
 * with parentheses, only positional arguments can be used:
 *
 *     zfs.sync.snapshot("rpool@snap")
 *
 * To use keyword arguments functions should be called with a single argument
 * that is a lua table containing mappings of integer -> positional arguments
 * and string -> keyword arguments:
 *
 *     zfs.sync.snapshot({1="rpool@snap", defer=true})
 *
 * The lua language allows curly braces to be used in place of parenthesis as
 * syntactic sugar for this calling convention:
 *
 *     zfs.sync.snapshot{"rpool@snap", defer=true}
 */

/*
 * Throw an error and print the given arguments.  If there are too many
 * arguments to fit in the output buffer, only the error format string is
 * output.
 */
static void
zcp_args_error(lua_State *state, const char *fname, const zcp_arg_t *pargs,
    const zcp_arg_t *kwargs, const char *fmt, ...)
{
        int i;
        char errmsg[512];
        size_t len = sizeof (errmsg);
        size_t msglen = 0;
        va_list argp;

        va_start(argp, fmt);
        VERIFY3U(len, >, vsnprintf(errmsg, len, fmt, argp));
        va_end(argp);

        /*
         * Calculate the total length of the final string, including extra
         * formatting characters. If the argument dump would be too large,
         * only print the error string.
         */
        msglen = strlen(errmsg);
        msglen += strlen(fname) + 4; /* : + {} + null terminator */
        for (i = 0; pargs[i].za_name != NULL; i++) {
                msglen += strlen(pargs[i].za_name);
                msglen += strlen(lua_typename(state, pargs[i].za_lua_type));
                if (pargs[i + 1].za_name != NULL || kwargs[0].za_name != NULL)
                        msglen += 5; /* < + ( + )> + , */
                else
                        msglen += 4; /* < + ( + )> */
        }
        for (i = 0; kwargs[i].za_name != NULL; i++) {
                msglen += strlen(kwargs[i].za_name);
                msglen += strlen(lua_typename(state, kwargs[i].za_lua_type));
                if (kwargs[i + 1].za_name != NULL)
                        msglen += 4; /* =( + ) + , */
                else
                        msglen += 3; /* =( + ) */
        }

        if (msglen >= len)
                (void) luaL_error(state, errmsg);

        VERIFY3U(len, >, strlcat(errmsg, ": ", len));
        VERIFY3U(len, >, strlcat(errmsg, fname, len));
        VERIFY3U(len, >, strlcat(errmsg, "{", len));
        for (i = 0; pargs[i].za_name != NULL; i++) {
                VERIFY3U(len, >, strlcat(errmsg, "<", len));
                VERIFY3U(len, >, strlcat(errmsg, pargs[i].za_name, len));
                VERIFY3U(len, >, strlcat(errmsg, "(", len));
                VERIFY3U(len, >, strlcat(errmsg,
                    lua_typename(state, pargs[i].za_lua_type), len));
                VERIFY3U(len, >, strlcat(errmsg, ")>", len));
                if (pargs[i + 1].za_name != NULL || kwargs[0].za_name != NULL) {
                        VERIFY3U(len, >, strlcat(errmsg, ", ", len));
                }
        }
        for (i = 0; kwargs[i].za_name != NULL; i++) {
                VERIFY3U(len, >, strlcat(errmsg, kwargs[i].za_name, len));
                VERIFY3U(len, >, strlcat(errmsg, "=(", len));
                VERIFY3U(len, >, strlcat(errmsg,
                    lua_typename(state, kwargs[i].za_lua_type), len));
                VERIFY3U(len, >, strlcat(errmsg, ")", len));
                if (kwargs[i + 1].za_name != NULL) {
                        VERIFY3U(len, >, strlcat(errmsg, ", ", len));
                }
        }
        VERIFY3U(len, >, strlcat(errmsg, "}", len));

        (void) luaL_error(state, errmsg);
        panic("unreachable code");
}

static void
zcp_parse_table_args(lua_State *state, const char *fname,
    const zcp_arg_t *pargs, const zcp_arg_t *kwargs)
{
        int i;
        int type;

        for (i = 0; pargs[i].za_name != NULL; i++) {
                /*
                 * Check the table for this positional argument, leaving it
                 * on the top of the stack once we finish validating it.
                 */
                lua_pushinteger(state, i + 1);
                lua_gettable(state, 1);

                type = lua_type(state, -1);
                if (type == LUA_TNIL) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "too few arguments");
                        panic("unreachable code");
                } else if (type != pargs[i].za_lua_type) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "arg %d wrong type (is '%s', expected '%s')",
                            i + 1, lua_typename(state, type),
                            lua_typename(state, pargs[i].za_lua_type));
                        panic("unreachable code");
                }

                /*
                 * Remove the positional argument from the table.
                 */
                lua_pushinteger(state, i + 1);
                lua_pushnil(state);
                lua_settable(state, 1);
        }

        for (i = 0; kwargs[i].za_name != NULL; i++) {
                /*
                 * Check the table for this keyword argument, which may be
                 * nil if it was omitted. Leave the value on the top of
                 * the stack after validating it.
                 */
                lua_getfield(state, 1, kwargs[i].za_name);

                type = lua_type(state, -1);
                if (type != LUA_TNIL && type != kwargs[i].za_lua_type) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "kwarg '%s' wrong type (is '%s', expected '%s')",
                            kwargs[i].za_name, lua_typename(state, type),
                            lua_typename(state, kwargs[i].za_lua_type));
                        panic("unreachable code");
                }

                /*
                 * Remove the keyword argument from the table.
                 */
                lua_pushnil(state);
                lua_setfield(state, 1, kwargs[i].za_name);
        }

        /*
         * Any entries remaining in the table are invalid inputs, print
         * an error message based on what the entry is.
         */
        lua_pushnil(state);
        if (lua_next(state, 1)) {
                if (lua_isnumber(state, -2) && lua_tointeger(state, -2) > 0) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "too many positional arguments");
                } else if (lua_isstring(state, -2)) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "invalid kwarg '%s'", lua_tostring(state, -2));
                } else {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "kwarg keys must be strings");
                }
                panic("unreachable code");
        }

        lua_remove(state, 1);
}

static void
zcp_parse_pos_args(lua_State *state, const char *fname, const zcp_arg_t *pargs,
    const zcp_arg_t *kwargs)
{
        int i;
        int type;

        for (i = 0; pargs[i].za_name != NULL; i++) {
                type = lua_type(state, i + 1);
                if (type == LUA_TNONE) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "too few arguments");
                        panic("unreachable code");
                } else if (type != pargs[i].za_lua_type) {
                        zcp_args_error(state, fname, pargs, kwargs,
                            "arg %d wrong type (is '%s', expected '%s')",
                            i + 1, lua_typename(state, type),
                            lua_typename(state, pargs[i].za_lua_type));
                        panic("unreachable code");
                }
        }
        if (lua_gettop(state) != i) {
                zcp_args_error(state, fname, pargs, kwargs,
                    "too many positional arguments");
                panic("unreachable code");
        }

        for (i = 0; kwargs[i].za_name != NULL; i++) {
                lua_pushnil(state);
        }
}

/*
 * Checks the current Lua stack against an expected set of positional and
 * keyword arguments. If the stack does not match the expected arguments
 * aborts the current channel program with a useful error message, otherwise
 * it re-arranges the stack so that it contains the positional arguments
 * followed by the keyword argument values in declaration order. Any missing
 * keyword argument will be represented by a nil value on the stack.
 *
 * If the stack contains exactly one argument of type LUA_TTABLE the curly
 * braces calling convention is assumed, otherwise the stack is parsed for
 * positional arguments only.
 *
 * This function should be used by every function callback. It should be called
 * before the callback manipulates the Lua stack as it assumes the stack
 * represents the function arguments.
 */
void
zcp_parse_args(lua_State *state, const char *fname, const zcp_arg_t *pargs,
    const zcp_arg_t *kwargs)
{
        if (lua_gettop(state) == 1 && lua_istable(state, 1)) {
                zcp_parse_table_args(state, fname, pargs, kwargs);
        } else {
                zcp_parse_pos_args(state, fname, pargs, kwargs);
        }
}