From dbc5d26b29a15721884c53467fed16f30716e2f9 Mon Sep 17 00:00:00 2001
From: Eugene Syromyatnikov <evgsyr@gmail.com>
Date: Mon, 7 May 2018 08:28:38 +0200
Subject: [PATCH] nlattr: add ability to pass nla_type to decoder in
 decode_nlattr

As of now, it's impossible to introduce a dispatching nlattr decoder
that performs actions based on the type of the message, so let's use
a combination of zero decoder list size and non-zero decoder pointer
for this.

This is going to be used later in IFLA_AF_SPEC decoding.

* nlattr.c (decode_nlattr_with_data): Handle zero size and non-NULL
decoders in a special way.
(decode_nlattr): Add an error message about ignoring of opaque_data
argument when zero size and non-NULL decoders are provided.
* nlattr.h (decode_nlattr): Add a comment about the new special case.
---
 nlattr.c | 22 +++++++++++++++++-----
 nlattr.h |  8 +++++++-
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/nlattr.c b/nlattr.c
index 8dea4847..f65d9cf6 100644
--- a/nlattr.c
+++ b/nlattr.c
@@ -101,13 +101,18 @@ decode_nlattr_with_data(struct tcb *const tcp,
 	print_nlattr(nla, table, dflt);
 
 	if (nla_len > NLA_HDRLEN) {
+		const unsigned int idx = size ? nla->nla_type : 0;
+
 		tprints(", ");
 		if (!decoders
-		    || nla->nla_type >= size
-		    || !decoders[nla->nla_type]
-		    || !decoders[nla->nla_type](tcp, addr + NLA_HDRLEN,
-						nla_len - NLA_HDRLEN,
-						opaque_data))
+		    || (size && idx >= size)
+		    || !decoders[idx]
+		    || !decoders[idx](
+				tcp, addr + NLA_HDRLEN,
+				nla_len - NLA_HDRLEN,
+				size ? opaque_data
+				     : (const void *) (uintptr_t) nla->nla_type)
+		    )
 			printstr_ex(tcp, addr + NLA_HDRLEN,
 				    nla_len - NLA_HDRLEN, QUOTE_FORCE_HEX);
 		tprints("}");
@@ -128,6 +133,13 @@ decode_nlattr(struct tcb *const tcp,
 	bool is_array = false;
 	unsigned int elt;
 
+	if (decoders && !size && opaque_data)
+		error_func_msg("[xlat %p, dflt \"%s\", decoders %p] "
+			       "size is zero (going to pass nla_type as "
+			       "decoder argument), but opaque data (%p) is not "
+			       "- will be ignored",
+			       table, dflt, decoders, opaque_data);
+
 	for (elt = 0; fetch_nlattr(tcp, &nla, addr, len, is_array); elt++) {
 		if (abbrev(tcp) && elt == max_strlen) {
 			tprints("...");
diff --git a/nlattr.h b/nlattr.h
index 7f0ee0cd..765aba82 100644
--- a/nlattr.h
+++ b/nlattr.h
@@ -42,13 +42,19 @@ struct decode_nla_xlat_opts {
 
 typedef bool (*nla_decoder_t)(struct tcb *, kernel_ulong_t addr,
 			      unsigned int len, const void *opaque_data);
+
+/**
+ * The case of non-NULL decoders and zero size is handled in a special way:
+ * the zeroth decoder is always called with nla_type being passed as opaque
+ * data.
+ */
 extern void
 decode_nlattr(struct tcb *,
 	      kernel_ulong_t addr,
 	      unsigned int len,
 	      const struct xlat *,
 	      const char *dflt,
-	      const nla_decoder_t *,
+	      const nla_decoder_t *decoders,
 	      unsigned int size,
 	      const void *opaque_data);
 
-- 
2.50.0