Fix dangling smgr_owner pointer when a fake relcache entry is freed.

A fake relcache entry can "own" a SmgrRelation object, like a regular
relcache entry. But when it was free'd, the owner field in SmgrRelation
was not cleared, so it was left pointing to free'd memory.

Amazingly this apparently hasn't caused crashes in practice, or we would've
heard about it earlier. Andres found this with Valgrind.

Report and fix by Andres Freund, with minor modifications by me. Backpatch
to all supported versions.

diff --git a/src/backend/access/transam/xlogutils.c b/src/backend/access/transam/xlogutils.c
index 77a8218a660d4dec3c554a6e47e708db72fa26f1..458e261126ffb802883b16570ba41faa172d3b8f 100644 (file)
--- a/src/backend/access/transam/xlogutils.c
+++ b/src/backend/access/transam/xlogutils.c
@@ -445,6 +445,9 @@ CreateFakeRelcacheEntry(RelFileNode rnode)
 void
 FreeFakeRelcacheEntry(Relation fakerel)
 {
+       /* make sure the fakerel is not referenced by the SmgrRelation anymore */
+       if (fakerel->rd_smgr != NULL)
+               smgrclearowner(&fakerel->rd_smgr, fakerel->rd_smgr);
        pfree(fakerel);
 }
 

diff --git a/src/backend/storage/smgr/smgr.c b/src/backend/storage/smgr/smgr.c
index f7f1437dd8ff7eb5802779d84d4201cc0db0d93e..4cc5d7e1ec468d60f2f807dc71a9f07f20955a75 100644 (file)
--- a/src/backend/storage/smgr/smgr.c
+++ b/src/backend/storage/smgr/smgr.c
@@ -84,6 +84,7 @@ static SMgrRelation first_unowned_reln = NULL;
 
 /* local function prototypes */
 static void smgrshutdown(int code, Datum arg);
+static void add_to_unowned_list(SMgrRelation reln);
 static void remove_from_unowned_list(SMgrRelation reln);
 
 
@@ -174,9 +175,8 @@ smgropen(RelFileNode rnode, BackendId backend)
                for (forknum = 0; forknum <= MAX_FORKNUM; forknum++)
                        reln->md_fd[forknum] = NULL;
 
-               /* place it at head of unowned list (to make smgrsetowner cheap) */
-               reln->next_unowned_reln = first_unowned_reln;
-               first_unowned_reln = reln;
+               /* it has no owner yet */
+               add_to_unowned_list(reln);
        }
 
        return reln;
@@ -191,7 +191,7 @@ smgropen(RelFileNode rnode, BackendId backend)
 void
 smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
 {
-       /* We don't currently support "disowning" an SMgrRelation here */
+       /* We don't support "disowning" an SMgrRelation here, use smgrclearowner */
        Assert(owner != NULL);
 
        /*
@@ -213,6 +213,40 @@ smgrsetowner(SMgrRelation *owner, SMgrRelation reln)
        *owner = reln;
 }
 
+/*
+ * smgrclearowner() -- Remove long-lived reference to an SMgrRelation object
+ *                                        if one exists
+ */
+void
+smgrclearowner(SMgrRelation *owner, SMgrRelation reln)
+{
+       /* Do nothing if the SMgrRelation object is not owned by the owner */
+       if (reln->smgr_owner != owner)
+               return;
+
+       /* unset the owner's reference */
+       *owner = NULL;
+
+       /* unset our reference to the owner */
+       reln->smgr_owner = NULL;
+
+       add_to_unowned_list(reln);
+}
+
+/*
+ * add_to_unowned_list -- link an SMgrRelation onto the unowned list
+ *
+ * Check remove_from_unowned_list()'s comments for performance
+ * considerations.
+ */
+static void
+add_to_unowned_list(SMgrRelation reln)
+{
+       /* place it at head of the list (to make smgrsetowner cheap) */
+       reln->next_unowned_reln = first_unowned_reln;
+       first_unowned_reln = reln;
+}
+
 /*
  * remove_from_unowned_list -- unlink an SMgrRelation from the unowned list
  *

diff --git a/src/include/storage/smgr.h b/src/include/storage/smgr.h
index 98b6f13137422a4106a2e38940fc5cb6712f36b7..6e419f736539cb2a3815208f6ada3f893b050f9a 100644 (file)
--- a/src/include/storage/smgr.h
+++ b/src/include/storage/smgr.h
@@ -80,6 +80,7 @@ extern void smgrinit(void);
 extern SMgrRelation smgropen(RelFileNode rnode, BackendId backend);
 extern bool smgrexists(SMgrRelation reln, ForkNumber forknum);
 extern void smgrsetowner(SMgrRelation *owner, SMgrRelation reln);
+extern void smgrclearowner(SMgrRelation *owner, SMgrRelation reln);
 extern void smgrclose(SMgrRelation reln);
 extern void smgrcloseall(void);
 extern void smgrclosenode(RelFileNodeBackend rnode);