Re: [PATCH 4/4] malloc: Fix sbrk clearing memory after freeing it instead of before

Hi Sean,

On Sun, 2 May 2021 at 20:55, Sean Anderson seanga2@gmail.com wrote:

This fixes memory being cleared after releasing it. Instead, clear memory before releasing it. In addition, suppress valgrind warnings about writing to free'd memory.

Signed-off-by: Sean Anderson seanga2@gmail.com

common/dlmalloc.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/common/dlmalloc.c b/common/dlmalloc.c index 05c8fd87e7..ea51bdf6a6 100644 --- a/common/dlmalloc.c +++ b/common/dlmalloc.c @@ -592,11 +592,13 @@ void *sbrk(ptrdiff_t increment) ulong new = old + increment;

    /*

•    * if we are giving memory back make sure we clear it out since
  

•    * we set MORECORE_CLEARS to 1
  

•    * if we are allocating memory make sure we clear it out since we set
  

•    * MORECORE_CLEARS to 1
     */
  

•   if (increment < 0)
  

•           memset((void *)new, 0, -increment);
  

•   if (increment > 0) {
  

•           VALGRIND_MAKE_MEM_UNDEFINED(old, increment);
  

•           memset((void *)old, 0, increment);
  

•   }
  

Can you explain this a bit more? What is the difference?

Do you need the cast?

    if ((new < mem_malloc_start) || (new > mem_malloc_end))
            return (void *)MORECORE_FAILURE;

-- 2.31.0

Regards, Simon