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Make more effort to put a sentinel at the end of allocated memory

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Traditionally, in MEMORY_CONTEXT_CHECKING builds, we only ever marked a
sentinel byte just beyond the requested size if there happened to be
enough space on the chunk to do so.  For Slab and Generation context
types, we only rounded the size of the chunk up to the next maxalign
boundary, so it was often not that likely that those would ever have space
for the sentinel given that the majority of allocation requests are going
to be for sizes which are maxaligned.  For AllocSet, it was a little
different as smaller allocations are rounded up to the next power-of-2
value rather than the next maxalign boundary, so we're a bit more likely
to have space for the sentinel byte, especially when we get away from tiny
sized allocations such as 8 or 16 bytes.

Here we make more of an effort to allow space so that there is enough room
for the sentinel byte in more cases.  This makes it more likely that we'll
detect when buggy code accidentally writes beyond the end of any of its
memory allocations.

Each of the 3 MemoryContext types has been changed as follows:

The Slab allocator will now always set a sentinel byte.  Both the current
usages of this MemoryContext type happen to use chunk sizes which were on
the maxalign boundary, so these never used sentinel bytes previously.

For the Generation allocator, we now always ensure there's enough space in
the allocation for a sentinel byte.

For AllocSet, this commit makes an adjustment for allocation sizes which
are greater than allocChunkLimit.  We now ensure there is always space for
a sentinel byte.  We don't alter the sentinel behavior for request sizes
<= allocChunkLimit.  Making way for the sentinel byte for power-of-2
request sizes would require doubling up to the next power of 2.  Some
analysis done on the request sizes made during installcheck shows that a
fairly large portion of allocation requests are for power-of-2 sizes.  The
amount of additional memory for the sentinel there seems prohibitive, so
we do nothing for those here.

Author: David Rowley
Discussion: https://postgr.es/m/3478405.1661824539@sss.pgh.pa.us
This commit is contained in:
David Rowley 2022-09-07 15:46:57 +12:00
parent f98d074245
commit 0e480385ec
3 changed files with 74 additions and 47 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

41
src/backend/utils/mmgr/aset.c
View File

@ -705,7 +705,13 @@ AllocSetAlloc(MemoryContext context, Size size)
*/
if (size > set->allocChunkLimit)
{
#ifdef MEMORY_CONTEXT_CHECKING
/* ensure there's always space for the sentinel byte */
chunk_size = MAXALIGN(size + 1);
#else
chunk_size = MAXALIGN(size);
#endif
blksize = chunk_size + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
block = (AllocBlock) malloc(blksize);
if (block == NULL)
@ -724,8 +730,8 @@ AllocSetAlloc(MemoryContext context, Size size)
#ifdef MEMORY_CONTEXT_CHECKING
chunk->requested_size = size;
/* set mark to catch clobber of "unused" space */
if (size < chunk_size)
set_sentinel(MemoryChunkGetPointer(chunk), size);
Assert(size < chunk_size);
set_sentinel(MemoryChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
/* fill the allocated space with junk */
@ -766,6 +772,12 @@ AllocSetAlloc(MemoryContext context, Size size)
* corresponding free list to see if there is a free chunk we could reuse.
* If one is found, remove it from the free list, make it again a member
* of the alloc set and return its data address.
*
* Note that we don't attempt to ensure there's space for the sentinel
* byte here. We expect a large proportion of allocations to be for sizes
* which are already a power of 2. If we were to always make space for a
* sentinel byte in MEMORY_CONTEXT_CHECKING builds, then we'd end up
* doubling the memory requirements for such allocations.
*/
fidx = AllocSetFreeIndex(size);
chunk = set->freelist[fidx];
@ -992,10 +1004,10 @@ AllocSetFree(void *pointer)
Size chunk_size = block->endptr - (char *) pointer;
/* Test for someone scribbling on unused space in chunk */
if (chunk->requested_size < chunk_size)
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
set->header.name, chunk);
Assert(chunk->requested_size < chunk_size);
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
set->header.name, chunk);
}
#endif
@ -1098,10 +1110,10 @@ AllocSetRealloc(void *pointer, Size size)
#ifdef MEMORY_CONTEXT_CHECKING
/* Test for someone scribbling on unused space in chunk */
if (chunk->requested_size < oldsize)
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
set->header.name, chunk);
Assert(chunk->requested_size < oldsize);
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
set->header.name, chunk);
#endif
/*
@ -1111,7 +1123,12 @@ AllocSetRealloc(void *pointer, Size size)
if (block->freeptr != block->endptr)
elog(ERROR, "could not find block containing chunk %p", chunk);
#ifdef MEMORY_CONTEXT_CHECKING
/* ensure there's always space for the sentinel byte */
chksize = MAXALIGN(size + 1);
#else
chksize = MAXALIGN(size);
#endif
/* Do the realloc */
blksize = chksize + ALLOC_BLOCKHDRSZ + ALLOC_CHUNKHDRSZ;
@ -1162,8 +1179,8 @@ AllocSetRealloc(void *pointer, Size size)
chunk->requested_size = size;
/* set mark to catch clobber of "unused" space */
if (size < chksize)
set_sentinel(pointer, size);
Assert(size < chksize);
set_sentinel(pointer, size);
#else /* !MEMORY_CONTEXT_CHECKING */
/*

43
src/backend/utils/mmgr/generation.c
View File

@ -342,8 +342,16 @@ GenerationAlloc(MemoryContext context, Size size)
GenerationContext *set = (GenerationContext *) context;
GenerationBlock *block;
MemoryChunk *chunk;
Size chunk_size = MAXALIGN(size);
Size required_size = chunk_size + Generation_CHUNKHDRSZ;
Size chunk_size;
Size required_size;
#ifdef MEMORY_CONTEXT_CHECKING
/* ensure there's always space for the sentinel byte */
chunk_size = MAXALIGN(size + 1);
#else
chunk_size = MAXALIGN(size);
#endif
required_size = chunk_size + Generation_CHUNKHDRSZ;
/* is it an over-sized chunk? if yes, allocate special block */
if (chunk_size > set->allocChunkLimit)
@ -373,8 +381,8 @@ GenerationAlloc(MemoryContext context, Size size)
#ifdef MEMORY_CONTEXT_CHECKING
chunk->requested_size = size;
/* set mark to catch clobber of "unused" space */
if (size < chunk_size)
set_sentinel(MemoryChunkGetPointer(chunk), size);
Assert(size < chunk_size);
set_sentinel(MemoryChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
/* fill the allocated space with junk */
@ -491,8 +499,8 @@ GenerationAlloc(MemoryContext context, Size size)
#ifdef MEMORY_CONTEXT_CHECKING
chunk->requested_size = size;
/* set mark to catch clobber of "unused" space */
if (size < chunk_size)
set_sentinel(MemoryChunkGetPointer(chunk), size);
Assert(size < chunk_size);
set_sentinel(MemoryChunkGetPointer(chunk), size);
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
/* fill the allocated space with junk */
@ -634,10 +642,10 @@ GenerationFree(void *pointer)
#ifdef MEMORY_CONTEXT_CHECKING
/* Test for someone scribbling on unused space in chunk */
if (chunk->requested_size < chunksize)
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
((MemoryContext) block->context)->name, chunk);
Assert(chunk->requested_size < chunksize);
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
((MemoryContext) block->context)->name, chunk);
#endif
#ifdef CLOBBER_FREED_MEMORY
@ -727,10 +735,10 @@ GenerationRealloc(void *pointer, Size size)
#ifdef MEMORY_CONTEXT_CHECKING
/* Test for someone scribbling on unused space in chunk */
if (chunk->requested_size < oldsize)
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
((MemoryContext) set)->name, chunk);
Assert(chunk->requested_size < oldsize);
if (!sentinel_ok(pointer, chunk->requested_size))
elog(WARNING, "detected write past chunk end in %s %p",
((MemoryContext) set)->name, chunk);
#endif
/*
@ -769,8 +777,7 @@ GenerationRealloc(void *pointer, Size size)
oldsize - size);
/* set mark to catch clobber of "unused" space */
if (size < oldsize)
set_sentinel(pointer, size);
set_sentinel(pointer, size);
#else /* !MEMORY_CONTEXT_CHECKING */
/*
@ -1034,8 +1041,8 @@ GenerationCheck(MemoryContext context)
name, block, chunk);
/* check sentinel */
if (chunk->requested_size < chunksize &&
!sentinel_ok(chunk, Generation_CHUNKHDRSZ + chunk->requested_size))
Assert(chunk->requested_size < chunksize);
if (!sentinel_ok(chunk, Generation_CHUNKHDRSZ + chunk->requested_size))
elog(WARNING, "problem in Generation %s: detected write past chunk end in block %p, chunk %p",
name, block, chunk);
}

37
src/backend/utils/mmgr/slab.c
View File

@ -145,7 +145,12 @@ SlabContextCreate(MemoryContext parent,
chunkSize = sizeof(int);
/* chunk, including SLAB header (both addresses nicely aligned) */
#ifdef MEMORY_CONTEXT_CHECKING
/* ensure there's always space for the sentinel byte */
fullChunkSize = Slab_CHUNKHDRSZ + MAXALIGN(chunkSize + 1);
#else
fullChunkSize = Slab_CHUNKHDRSZ + MAXALIGN(chunkSize);
#endif
/* Make sure the block can store at least one chunk. */
if (blockSize < fullChunkSize + Slab_BLOCKHDRSZ)
@ -420,14 +425,12 @@ SlabAlloc(MemoryContext context, Size size)
MCTX_SLAB_ID);
#ifdef MEMORY_CONTEXT_CHECKING
/* slab mark to catch clobber of "unused" space */
if (slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ))
{
set_sentinel(MemoryChunkGetPointer(chunk), size);
VALGRIND_MAKE_MEM_NOACCESS(((char *) chunk) +
Slab_CHUNKHDRSZ + slab->chunkSize,
slab->fullChunkSize -
(slab->chunkSize + Slab_CHUNKHDRSZ));
}
Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
set_sentinel(MemoryChunkGetPointer(chunk), size);
VALGRIND_MAKE_MEM_NOACCESS(((char *) chunk) +
Slab_CHUNKHDRSZ + slab->chunkSize,
slab->fullChunkSize -
(slab->chunkSize + Slab_CHUNKHDRSZ));
#endif
#ifdef RANDOMIZE_ALLOCATED_MEMORY
@ -454,10 +457,10 @@ SlabFree(void *pointer)
#ifdef MEMORY_CONTEXT_CHECKING
/* Test for someone scribbling on unused space in chunk */
if (slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ))
if (!sentinel_ok(pointer, slab->chunkSize))
elog(WARNING, "detected write past chunk end in %s %p",
slab->header.name, chunk);
Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
if (!sentinel_ok(pointer, slab->chunkSize))
elog(WARNING, "detected write past chunk end in %s %p",
slab->header.name, chunk);
#endif
/* compute index of the chunk with respect to block start */
@ -744,11 +747,11 @@ SlabCheck(MemoryContext context)
elog(WARNING, "problem in slab %s: bogus block link in block %p, chunk %p",
name, block, chunk);
/* there might be sentinel (thanks to alignment) */
if (slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ))
if (!sentinel_ok(chunk, Slab_CHUNKHDRSZ + slab->chunkSize))
elog(WARNING, "problem in slab %s: detected write past chunk end in block %p, chunk %p",
name, block, chunk);
/* check the sentinel byte is intact */
Assert(slab->chunkSize < (slab->fullChunkSize - Slab_CHUNKHDRSZ));
if (!sentinel_ok(chunk, Slab_CHUNKHDRSZ + slab->chunkSize))
elog(WARNING, "problem in slab %s: detected write past chunk end in block %p, chunk %p",
name, block, chunk);
}
}