From 97651b0139cfa511eaf063b60c900c2ba81b8ba2 Mon Sep 17 00:00:00 2001 From: David Rowley Date: Sat, 6 Jul 2024 13:59:34 +1200 Subject: [PATCH] Fix incorrect sentinel byte logic in GenerationRealloc() This only affects MEMORY_CONTEXT_CHECKING builds. This fixes an off-by-one issue in GenerationRealloc() where the fast-path code which tries to reuse the existing allocation if the existing chunk is >= the new requested size. The code there thought it was always ok to use the existing chunk, but when oldsize == size there isn't enough space to store the sentinel byte. If both sizes matched exactly set_sentinel() would overwrite the first byte beyond the chunk and then subsequent GenerationRealloc() calls could then fail the Assert(chunk->requested_size < oldsize) check which is trying to ensure the chunk is large enough to store the sentinel. The same issue does not exist in aset.c as the sentinel checking code only adds a sentinel byte if there's enough space in the chunk. Reported-by: Alexander Lakhin Discussion: https://postgr.es/m/49275921-7b39-41af-5eb8-97b50ce3312e@gmail.com Backpatch-through: 16, where the problem was introduced by 0e480385e --- src/backend/utils/mmgr/generation.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/mmgr/generation.c b/src/backend/utils/mmgr/generation.c index b858b8d0f7b..0238c111d2f 100644 --- a/src/backend/utils/mmgr/generation.c +++ b/src/backend/utils/mmgr/generation.c @@ -846,8 +846,8 @@ GenerationRealloc(void *pointer, Size size, int flags) #endif /* - * Maybe the allocated area already is >= the new size. (In particular, - * we always fall out here if the requested size is a decrease.) + * Maybe the allocated area already big enough. (In particular, we always + * fall out here if the requested size is a decrease.) * * This memory context does not use power-of-2 chunk sizing and instead * carves the chunks to be as small as possible, so most repalloc() calls @@ -855,7 +855,12 @@ GenerationRealloc(void *pointer, Size size, int flags) * * XXX Perhaps we should annotate this condition with unlikely()? */ +#ifdef MEMORY_CONTEXT_CHECKING + /* With MEMORY_CONTEXT_CHECKING, we need an extra byte for the sentinel */ + if (oldsize > size) +#else if (oldsize >= size) +#endif { #ifdef MEMORY_CONTEXT_CHECKING Size oldrequest = chunk->requested_size;