From ffae5cc5a6024b4e25ec920ed5c4dfac649605f8 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Mon, 25 Sep 2006 22:01:10 +0000 Subject: [PATCH] Add a check to prevent overwriting valid data if smgrnblocks() gives a wrong answer, as has been seen to occur with a buggy Linux kernel. Not really our bug, but it's a simple test in a seldom-used control path, so might as well have a defense. --- src/backend/storage/buffer/bufmgr.c | 25 +++++++++++++++++++++---- 1 file changed, 21 insertions(+), 4 deletions(-) diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index 147b2ef8d8..aaa0dff684 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -8,7 +8,7 @@ * * * IDENTIFICATION - * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.210 2006/09/17 22:16:22 tgl Exp $ + * $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.211 2006/09/25 22:01:10 tgl Exp $ * *------------------------------------------------------------------------- */ @@ -175,9 +175,26 @@ ReadBuffer(Relation reln, BlockNumber blockNum) /* * We get here only in the corner case where we are trying to extend * the relation but we found a pre-existing buffer marked BM_VALID. - * (This can happen because mdread doesn't complain about reads - * beyond EOF --- which is arguably bogus, but changing it seems - * tricky.) We *must* do smgrextend before succeeding, else the + * This can happen because mdread doesn't complain about reads beyond + * EOF --- which is arguably bogus, but changing it seems tricky --- + * and so a previous attempt to read a block just beyond EOF could + * have left a "valid" zero-filled buffer. Unfortunately, we have + * also seen this case occurring because of buggy Linux kernels that + * sometimes return an lseek(SEEK_END) result that doesn't account for + * a recent write. In that situation, the pre-existing buffer would + * contain valid data that we don't want to overwrite. Since the + * legitimate cases should always have left a zero-filled buffer, + * complain if not PageIsNew. + */ + bufBlock = isLocalBuf ? LocalBufHdrGetBlock(bufHdr) : BufHdrGetBlock(bufHdr); + if (!PageIsNew((PageHeader) bufBlock)) + ereport(ERROR, + (errmsg("unexpected data beyond EOF in block %u of relation \"%s\"", + blockNum, RelationGetRelationName(reln)), + errhint("This has been seen to occur with buggy kernels; consider updating your system."))); + + /* + * We *must* do smgrextend before succeeding, else the * page will not be reserved by the kernel, and the next P_NEW call * will decide to return the same page. Clear the BM_VALID bit, * do the StartBufferIO call that BufferAlloc didn't, and proceed. -- 2.40.0