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malloc: Fix issue with calloc memory possibly being non-zero

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Since we set #define MORECORE_CLEARS 1, the code assumes 'sbrk' always
returns zero'd out memory.  However since its possible that free()
returns memory back to sbrk() via malloc_trim we could possible get
non-zero'd memory from sbrk().  This is a problem for when code might
call calloc() and expect the memory to have been zero'd out.

There are two possible solutions to this problem.
1. change #define MORECORE_CLEARS 0
2. memset to zero memory returned to sbrk.

We go with the second since the sbrk being called to free up memory
should be pretty rare.

The following code problems an example test to show the issue.  This
test code was inserted right after the call to mem_malloc_init().

...
       u8 *p2;
       int i;

       printf("MALLOC TEST\n");
       p1 = malloc(135176);
       printf("P1 = %p\n", p1);
       memset(p1, 0xab, 135176);

       free(p1);
       p2 = calloc(4097, 1);
       printf("P2 = %p %p\n", p2, p2 + 4097);

       for (i = 0; i < 4097; i++) {
	       if (p2[i] != 0)
		       printf("miscompare at byte %d got %x\n", i, p2[i]);

       free(p2);
       printf("END MALLOC TEST\n\n");
...

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Tested-by: Wolfgang Denk <wd@denx.de>
This commit is contained in:
Kumar Gala 2010-11-15 18:41:43 -06:00 committed by Wolfgang Denk
parent f7ac99fdd9
commit 6163f5b4c8
1 changed files with 7 additions and 0 deletions
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7
common/dlmalloc.c
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@ -1511,6 +1511,13 @@ void *sbrk(ptrdiff_t increment)
ulong old = mem_malloc_brk;
ulong new = old + increment;
/*
* if we are giving memory back make sure we clear it out since
* we set MORECORE_CLEARS to 1
*/
if (increment < 0)
memset((void *)new, 0, -increment);
if ((new < mem_malloc_start) || (new > mem_malloc_end))
return (void *)MORECORE_FAILURE;