diff mbox series

malloc: ensure set_max_fast never stores zero

Message ID xnk12ver1e.fsf@greed.delorie.com
State Committed
Headers
Series malloc: ensure set_max_fast never stores zero |

Commit Message

DJ Delorie April 4, 2020, 5:52 a.m. UTC 
  The code for set_max_fast() stores an "impossibly small value"
instead of zero, when the parameter is zero.  However, for
small values of the parameter (ex: 1 or 2) the computation
results in a zero being stored anyway.

This patch checks for the parameter being small enough for the
computation to result in zero instead, so that a zero is never
stored.

key values which result in zero being stored:

x86-64:  1..7  (or other 64-bit)
i686:    1..11
armhfp:  1..3  (or other 32-bit)

Comments

Carlos O'Donell April 4, 2020, 4:10 p.m. UTC | #1 
On 4/4/20 1:52 AM, DJ Delorie via Libc-alpha wrote:
> 
> The code for set_max_fast() stores an "impossibly small value"
> instead of zero, when the parameter is zero.  However, for
> small values of the parameter (ex: 1 or 2) the computation
> results in a zero being stored anyway.

Agreed.

> This patch checks for the parameter being small enough for the
> computation to result in zero instead, so that a zero is never
> stored.

Right, because (1 + SIZE_SZ) & ~MALLOC_ALIGN_MASK is zero.

> key values which result in zero being stored:
> 
> x86-64:  1..7  (or other 64-bit)
> i686:    1..11
> armhfp:  1..3  (or other 32-bit)
> 

OK with bug # attached.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> diff --git a/malloc/malloc.c b/malloc/malloc.c
> index 6acb5ad43a..ee87ddbbf9 100644
> --- a/malloc/malloc.c
> +++ b/malloc/malloc.c
> @@ -1632,7 +1632,7 @@ static INTERNAL_SIZE_T global_max_fast;
>   */
>  
>  #define set_max_fast(s) \
> -  global_max_fast = (((s) == 0)						      \
> +  global_max_fast = (((size_t) (s) <= MALLOC_ALIGN_MASK - SIZE_SZ)	\
>                       ? MIN_CHUNK_SIZE / 2 : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
>  
>  static inline INTERNAL_SIZE_T
>
diff mbox series

Patch

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 6acb5ad43a..ee87ddbbf9 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -1632,7 +1632,7 @@  static INTERNAL_SIZE_T global_max_fast;
  */
 
 #define set_max_fast(s) \
-  global_max_fast = (((s) == 0)						      \
+  global_max_fast = (((size_t) (s) <= MALLOC_ALIGN_MASK - SIZE_SZ)	\
                      ? MIN_CHUNK_SIZE / 2 : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
 
 static inline INTERNAL_SIZE_T