Problem with Object Size Checking and reallocarray

[Jon Turney <jon dot turney at dronecode dot org dot uk>]

// gcc test.c -o test.exe -g -O2 -Wp,-D_FORTIFY_SOURCE=2

//
// extracted from InputLineAddChar in xserver/xkb/maprules.c
//

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
  const char *buf[128];
  char *line = reallocarray(NULL, 128, 2);
  // size of line is 128*2 = 256
  printf("%zu\n", __builtin_object_size(line, 0));
  memcpy(line, buf, 128);
  // __mempcy_chk tests against size 2, and terminates
}


reallocarray() is annotated in stdlib.h with '__alloc_size(2) 
__alloc_size(3)'

per [1], this doesn't seem to be the correct syntax when the size is the 
product of the arguments, and the last alloc_size seems to be silently 
winning.

If I change this to '__alloc_size((2,3))' (as in the patch attached), 
__builtin_object_size doesn't seem to be a compile-time constant 
anymore, and so memcpy() evaluates differently, so it's hard to be sure 
that's actually correct...

[1] https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html

Attachment: 0001-Correct-alloc_size-annotation-on-reallocarray.patch
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