[U-Boot] [PATCH] malloc: Fix issue with calloc memory possibly being non-zero

[Kumar Gala]

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 ack to sbrk() via malloc_trim we could possible get
non-zero'd memory from sbrk() if it allocates back memory that was
previously freed to it.

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(). we
could possible get non-zero'd memory from sbrk() if it allocates back
memory that was previously freed to it.

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 at kernel.crashing.org>
---
 common/dlmalloc.c |    7 +++++++
 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/common/dlmalloc.c b/common/dlmalloc.c
index 4871f4b..e9bab09 100644
--- a/common/dlmalloc.c
+++ b/common/dlmalloc.c
@@ -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;
 
-- 
1.7.2.3