I've been debugging a "memory corruption" issue and it seems to come down to
how we set MORECORE_CLEARS
from malloc.h:
MORECORE (default: sbrk)
The name of the routine to call to obtain more memory from the system.
MORECORE_FAILURE (default: -1)
The value returned upon failure of MORECORE.
MORECORE_CLEARS (default 1)
True (1) if the routine mapped to MORECORE zeroes out memory (which
holds for sbrk).
So the issue I'm seeing is that we zero out memory in mem_malloc_init(). Thus
our sbrk implementation just does simple record keeping. However we can get
into the following situation:
a1 = malloc(LARGE_SZ); /* causes internals to call sbrk */
...
/* use memory at 'a1' */
...
free(a1); /* causes internals to call malloc_trim, malloc_trim decides to
return memory to sbrk, memory is now nonzero */
a2 = calloc(SIZE); /* internals assume sbrk memory is zero because of
MORECORE_CLEARS setting, however its not */
... BAD THINGS HAPPEN ...
I'm guessing we haven't seen much of this because there aren't that many users
of calloc today. I happen to see it related to NAND code which has kzalloc
defined as calloc.
- k
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