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Adjust Valgrind macro usage to protect chunk headers 

Prior to this commit we only ever protected MemoryChunk's requested_size
field with Valgrind NOACCESS.  This means that if the hdrmask field is
ever accessed accidentally then we're not going to get any warnings from
Valgrind about it.  Valgrind would have warned us about the problem fixed
in 92957ed98 had we already been doing this.

Per suggestion from Tom Lane

Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/1650235.1672694719@sss.pgh.pa.us
Discussion: https://postgr.es/m/CAApHDvr=FZNGbj252Z6M9BSFKoq6BMxgkQ2yEAGUYoo7RquqZg@mail.gmail.com
This commit is contained in:
David Rowley 2023-04-15 11:59:52 +12:00
parent 43a33ef54e
commit 414d66220a
5 changed files with 170 additions and 74 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

36
src/backend/utils/mmgr/alignedalloc.c
View File

@ -31,6 +31,8 @@ AlignedAllocFree(void *pointer)
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
void *unaligned;
VALGRIND_MAKE_MEM_DEFINED(chunk, sizeof(MemoryChunk));
Assert(!MemoryChunkIsExternal(chunk));
/* obtain the original (unaligned) allocated pointer */
@ -58,12 +60,17 @@ void *
AlignedAllocRealloc(void *pointer, Size size)
{
MemoryChunk *redirchunk = PointerGetMemoryChunk(pointer);
Size alignto = MemoryChunkGetValue(redirchunk);
void *unaligned = MemoryChunkGetBlock(redirchunk);
Size alignto;
void *unaligned;
MemoryContext ctx;
Size old_size;
void *newptr;
VALGRIND_MAKE_MEM_DEFINED(redirchunk, sizeof(MemoryChunk));
alignto = MemoryChunkGetValue(redirchunk);
unaligned = MemoryChunkGetBlock(redirchunk);
/* sanity check this is a power of 2 value */
Assert((alignto & (alignto - 1)) == 0);
@ -110,11 +117,18 @@ AlignedAllocRealloc(void *pointer, Size size)
MemoryContext
AlignedAllocGetChunkContext(void *pointer)
{
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
MemoryChunk *redirchunk = PointerGetMemoryChunk(pointer);
MemoryContext cxt;
Assert(!MemoryChunkIsExternal(chunk));
VALGRIND_MAKE_MEM_DEFINED(redirchunk, sizeof(MemoryChunk));
return GetMemoryChunkContext(MemoryChunkGetBlock(chunk));
Assert(!MemoryChunkIsExternal(redirchunk));
cxt = GetMemoryChunkContext(MemoryChunkGetBlock(redirchunk));
VALGRIND_MAKE_MEM_NOACCESS(redirchunk, sizeof(MemoryChunk));
return cxt;
}
/*
@ -126,7 +140,15 @@ Size
AlignedAllocGetChunkSpace(void *pointer)
{
MemoryChunk *redirchunk = PointerGetMemoryChunk(pointer);
void *unaligned = MemoryChunkGetBlock(redirchunk);
void *unaligned;
Size space;
return GetMemoryChunkSpace(unaligned);
VALGRIND_MAKE_MEM_DEFINED(redirchunk, sizeof(MemoryChunk));
unaligned = MemoryChunkGetBlock(redirchunk);
space = GetMemoryChunkSpace(unaligned);
VALGRIND_MAKE_MEM_NOACCESS(redirchunk, sizeof(MemoryChunk));
return space;
}

78
src/backend/utils/mmgr/aset.c
View File

@ -188,14 +188,6 @@ typedef struct AllocBlockData
char *endptr; /* end of space in this block */
} AllocBlockData;
/*
* Only the "hdrmask" field should be accessed outside this module.
* We keep the rest of an allocated chunk's header marked NOACCESS when using
* valgrind. But note that chunk headers that are in a freelist are kept
* accessible, for simplicity.
*/
#define ALLOCCHUNK_PRIVATE_LEN offsetof(MemoryChunk, hdrmask)
/*
* AllocPointerIsValid
* True iff pointer is valid allocation pointer.
@ -777,8 +769,8 @@ AllocSetAlloc(MemoryContext context, Size size)
VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
chunk_size - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -801,6 +793,9 @@ AllocSetAlloc(MemoryContext context, Size size)
{
AllocFreeListLink *link = GetFreeListLink(chunk);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
Assert(fidx == MemoryChunkGetValue(chunk));
/* pop this chunk off the freelist */
@ -823,8 +818,8 @@ AllocSetAlloc(MemoryContext context, Size size)
VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
GetChunkSizeFromFreeListIdx(fidx) - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -989,8 +984,8 @@ AllocSetAlloc(MemoryContext context, Size size)
VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
chunk_size - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -1005,8 +1000,8 @@ AllocSetFree(void *pointer)
AllocSet set;
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
@ -1115,8 +1110,8 @@ AllocSetRealloc(void *pointer, Size size)
Size oldchksize;
int fidx;
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
@ -1164,8 +1159,8 @@ AllocSetRealloc(void *pointer, Size size)
block = (AllocBlock) realloc(block, blksize);
if (block == NULL)
{
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return NULL;
}
@ -1232,8 +1227,8 @@ AllocSetRealloc(void *pointer, Size size)
/* Ensure any padding bytes are marked NOACCESS. */
VALGRIND_MAKE_MEM_NOACCESS((char *) pointer + size, chksize - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header . */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return pointer;
}
@ -1305,8 +1300,8 @@ AllocSetRealloc(void *pointer, Size size)
VALGRIND_MAKE_MEM_DEFINED(pointer, size);
#endif
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return pointer;
}
@ -1332,8 +1327,8 @@ AllocSetRealloc(void *pointer, Size size)
/* leave immediately if request was not completed */
if (newPointer == NULL)
{
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return NULL;
}
@ -1374,11 +1369,17 @@ AllocSetGetChunkContext(void *pointer)
AllocBlock block;
AllocSet set;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
block = ExternalChunkGetBlock(chunk);
else
block = (AllocBlock) MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
Assert(AllocBlockIsValid(block));
set = block->aset;
@ -1396,16 +1397,27 @@ AllocSetGetChunkSpace(void *pointer)
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
int fidx;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
AllocBlock block = ExternalChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
Assert(AllocBlockIsValid(block));
return block->endptr - (char *) chunk;
}
fidx = MemoryChunkGetValue(chunk);
Assert(FreeListIdxIsValid(fidx));
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
return GetChunkSizeFromFreeListIdx(fidx) + ALLOC_CHUNKHDRSZ;
}
@ -1471,7 +1483,10 @@ AllocSetStats(MemoryContext context,
{
AllocFreeListLink *link = GetFreeListLink(chunk);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
Assert(MemoryChunkGetValue(chunk) == fidx);
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
freechunks++;
freespace += chksz + ALLOC_CHUNKHDRSZ;
@ -1566,8 +1581,8 @@ AllocSetCheck(MemoryContext context)
Size chsize,
dsize;
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOCCHUNK_PRIVATE_LEN);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, ALLOC_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
@ -1617,12 +1632,9 @@ AllocSetCheck(MemoryContext context)
elog(WARNING, "problem in alloc set %s: detected write past chunk end in block %p, chunk %p",
name, block, chunk);
/*
* If chunk is allocated, disallow external access to private part
* of chunk header.
*/
/* if chunk is allocated, disallow access to the chunk header */
if (dsize != InvalidAllocSize)
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOCCHUNK_PRIVATE_LEN);
VALGRIND_MAKE_MEM_NOACCESS(chunk, ALLOC_CHUNKHDRSZ);
blk_data += chsize;
nchunks++;

57
src/backend/utils/mmgr/generation.c
View File

@ -98,14 +98,6 @@ struct GenerationBlock
char *endptr; /* end of space in this block */
};
/*
* Only the "hdrmask" field should be accessed outside this module.
* We keep the rest of an allocated chunk's header marked NOACCESS when using
* valgrind. But note that freed chunk headers are kept accessible, for
* simplicity.
*/
#define GENERATIONCHUNK_PRIVATE_LEN offsetof(MemoryChunk, hdrmask)
/*
* GenerationIsValid
* True iff set is valid generation set.
@ -407,8 +399,8 @@ GenerationAlloc(MemoryContext context, Size size)
VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
chunk_size - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -522,8 +514,8 @@ GenerationAlloc(MemoryContext context, Size size)
VALGRIND_MAKE_MEM_NOACCESS((char *) MemoryChunkGetPointer(chunk) + size,
chunk_size - size);
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -634,6 +626,9 @@ GenerationFree(void *pointer)
Size chunksize;
#endif
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
block = ExternalChunkGetBlock(chunk);
@ -667,9 +662,6 @@ GenerationFree(void *pointer)
#endif
}
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
#ifdef MEMORY_CONTEXT_CHECKING
/* Test for someone scribbling on unused space in chunk */
Assert(chunk->requested_size < chunksize);
@ -747,8 +739,8 @@ GenerationRealloc(void *pointer, Size size)
GenerationPointer newPointer;
Size oldsize;
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
@ -835,8 +827,8 @@ GenerationRealloc(void *pointer, Size size)
VALGRIND_MAKE_MEM_DEFINED(pointer, size);
#endif
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
return pointer;
}
@ -847,8 +839,8 @@ GenerationRealloc(void *pointer, Size size)
/* leave immediately if request was not completed */
if (newPointer == NULL)
{
/* Disallow external access to private part of chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
return NULL;
}
@ -886,11 +878,17 @@ GenerationGetChunkContext(void *pointer)
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
GenerationBlock *block;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
block = ExternalChunkGetBlock(chunk);
else
block = (GenerationBlock *) MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
Assert(GenerationBlockIsValid(block));
return &block->context->header;
}
@ -906,6 +904,9 @@ GenerationGetChunkSpace(void *pointer)
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
Size chunksize;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
GenerationBlock *block = ExternalChunkGetBlock(chunk);
@ -916,6 +917,9 @@ GenerationGetChunkSpace(void *pointer)
else
chunksize = MemoryChunkGetValue(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
return Generation_CHUNKHDRSZ + chunksize;
}
@ -1057,8 +1061,8 @@ GenerationCheck(MemoryContext context)
GenerationBlock *chunkblock;
Size chunksize;
/* Allow access to private part of chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, GENERATIONCHUNK_PRIVATE_LEN);
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Generation_CHUNKHDRSZ);
if (MemoryChunkIsExternal(chunk))
{
@ -1101,12 +1105,9 @@ GenerationCheck(MemoryContext context)
else
nfree += 1;
/*
* If chunk is allocated, disallow external access to private part
* of chunk header.
*/
/* if chunk is allocated, disallow access to the chunk header */
if (chunk->requested_size != InvalidAllocSize)
VALGRIND_MAKE_MEM_NOACCESS(chunk, GENERATIONCHUNK_PRIVATE_LEN);
VALGRIND_MAKE_MEM_NOACCESS(chunk, Generation_CHUNKHDRSZ);
}
/*

22
src/backend/utils/mmgr/mcxt.c
View File

@ -190,8 +190,14 @@ GetMemoryChunkMethodID(const void *pointer)
*/
Assert(pointer == (const void *) MAXALIGN(pointer));
/* Allow access to the uint64 header */
VALGRIND_MAKE_MEM_DEFINED((char *) pointer - sizeof(uint64), sizeof(uint64));
header = *((const uint64 *) ((const char *) pointer - sizeof(uint64)));
/* Disallow access to the uint64 header */
VALGRIND_MAKE_MEM_NOACCESS((char *) pointer - sizeof(uint64), sizeof(uint64));
return (MemoryContextMethodID) (header & MEMORY_CONTEXT_METHODID_MASK);
}
@ -204,7 +210,17 @@ GetMemoryChunkMethodID(const void *pointer)
static inline uint64
GetMemoryChunkHeader(const void *pointer)
{
return *((const uint64 *) ((const char *) pointer - sizeof(uint64)));
uint64 header;
/* Allow access to the uint64 header */
VALGRIND_MAKE_MEM_DEFINED((char *) pointer - sizeof(uint64), sizeof(uint64));
header = *((const uint64 *) ((const char *) pointer - sizeof(uint64)));
/* Disallow access to the uint64 header */
VALGRIND_MAKE_MEM_NOACCESS((char *) pointer - sizeof(uint64), sizeof(uint64));
return header;
}
/*
@ -1404,6 +1420,10 @@ MemoryContextAllocAligned(MemoryContext context,
/* Mark the bytes before the redirection header as noaccess */
VALGRIND_MAKE_MEM_NOACCESS(unaligned,
(char *) alignedchunk - (char *) unaligned);
/* Disallow access to the redirection chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(alignedchunk, sizeof(MemoryChunk));
return aligned;
}

51
src/backend/utils/mmgr/slab.c
View File

@ -630,6 +630,9 @@ SlabAlloc(MemoryContext context, Size size)
randomize_mem((char *) MemoryChunkGetPointer(chunk), size);
#endif
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Slab_CHUNKHDRSZ);
return MemoryChunkGetPointer(chunk);
}
@ -641,11 +644,16 @@ void
SlabFree(void *pointer)
{
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
SlabBlock *block = MemoryChunkGetBlock(chunk);
SlabBlock *block;
SlabContext *slab;
int curBlocklistIdx;
int newBlocklistIdx;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Slab_CHUNKHDRSZ);
block = MemoryChunkGetBlock(chunk);
/*
* For speed reasons we just Assert that the referenced block is good.
* Future field experience may show that this Assert had better become a
@ -761,9 +769,17 @@ void *
SlabRealloc(void *pointer, Size size)
{
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
SlabBlock *block = MemoryChunkGetBlock(chunk);
SlabBlock *block;
SlabContext *slab;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Slab_CHUNKHDRSZ);
block = MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Slab_CHUNKHDRSZ);
/*
* Try to verify that we have a sane block pointer: the block header
* should reference a slab context. (We use a test-and-elog, not just
@ -790,9 +806,18 @@ MemoryContext
SlabGetChunkContext(void *pointer)
{
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
SlabBlock *block = MemoryChunkGetBlock(chunk);
SlabBlock *block;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Slab_CHUNKHDRSZ);
block = MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Slab_CHUNKHDRSZ);
Assert(SlabBlockIsValid(block));
return &block->slab->header;
}
@ -805,9 +830,17 @@ Size
SlabGetChunkSpace(void *pointer)
{
MemoryChunk *chunk = PointerGetMemoryChunk(pointer);
SlabBlock *block = MemoryChunkGetBlock(chunk);
SlabBlock *block;
SlabContext *slab;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Slab_CHUNKHDRSZ);
block = MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Slab_CHUNKHDRSZ);
Assert(SlabBlockIsValid(block));
slab = block->slab;
@ -1017,7 +1050,15 @@ SlabCheck(MemoryContext context)
if (!slab->isChunkFree[j])
{
MemoryChunk *chunk = SlabBlockGetChunk(slab, block, j);
SlabBlock *chunkblock = (SlabBlock *) MemoryChunkGetBlock(chunk);
SlabBlock *chunkblock;
/* Allow access to the chunk header. */
VALGRIND_MAKE_MEM_DEFINED(chunk, Slab_CHUNKHDRSZ);
chunkblock = (SlabBlock *) MemoryChunkGetBlock(chunk);
/* Disallow access to the chunk header. */
VALGRIND_MAKE_MEM_NOACCESS(chunk, Slab_CHUNKHDRSZ);
/*
* check the chunk's blockoffset correctly points back to