Re: [PATCH] [BZ 20628] make mallinfo saturating

[Paul Eggert <eggert at cs dot ucla dot edu>]

On 10/04/2016 01:04 PM, DJ Delorie wrote:
> +/* Saturated add - add ADD to SUM.  If the result exceeds the range of
> +   "int", set SUM to UINT_MAX instead ((int)-1).  Assumes ADD and SUM
> +   are positive.  The published ABI prevents us from bumping "int" to
> +   a larger type.  */
> +#define SAT_ADD(SUM, ADD) \
> +  ({ INTERNAL_SIZE_T tmp = (INTERNAL_SIZE_T)(SUM) + (INTERNAL_SIZE_T)(ADD); SUM = (tmp > UINT_MAX) ? -1 : tmp; })
> +
> +/* Likewise, but assign ADD to SUM.  */
> +#define SAT_SET(SUM, ADD) \
> +  ({ SUM = ((INTERNAL_SIZE_T)(ADD) > UINT_MAX) ? -1 : (ADD); })
These don't look right, as INTERNAL_SIZE_T might not be wider than int, 
which means the + in SAT_ADD will wrap around before SAT_ADD gets a 
chance to check for overflow. Also, if SUM is INT_MIN then SAT_ADD(SUM, 
1) should set it to INT_MIN + 1, but that doesn't happen when 
INTERNAL_SIZE_T is wider than int.

How about something like the following instead?

  #define MIN(a, b) ((a)<(b)?(a):(b))

  /* Return the sum of the unsigned int A and the nonnegative integer B.
     If the result is not representable as an unsigned int, return 
UINT_MAX.
     B's type may be any integer type.  */
  #define UINT_SATURATED_ADD(a, b) \
     ({ unsigned a1 = a, b1 = MIN (b, UINT_MAX), sum1 = a1 + b1; \
        sum1 < a1 ? UINT_MAX : sum1; })

and then set 'm->smblks = UINT_SATURATED_ADD (m->sm_blks, nfastblocks);' 
and 'm->hblks = UINT_SATURATED_ADD (0, mp_.n_mmaps);', etc.