Re: POWER: Unexpected fault when writing to brk-allocated memory

[Florian Weimer]

On 11/05/2017 01:18 PM, Nicholas Piggin wrote:
Something like the following patch may help if you could test.

The patch appears to fix it:

# /lib64/ld64.so.1 ./a.out
initial brk value: 0x7fffe4590000
probing at 0x80000001fffc

I used the follow simplified reproducer:

#include <err.h>
#include <unistd.h>
#include <inttypes.h>
#include <errno.h>
#include <stdio.h>

int
main (void)
{
  errno = 0;
  void *p = sbrk (0);
  if (errno != 0)
    err (1, "sbrk (0)");
  printf ("initial brk value: %p\n", p);
  unsigned long long target = 0x800000020000ULL;
  if ((uintptr_t) p >= target)
    errx (1, "initial brk value is already above target");
  unsigned long long increment = target - (uintptr_t) p;
  errno = 0;
  sbrk (increment);
  if (errno != 0)
    err (1, "sbrk (0x%llx)", increment);
  volatile int *pi = (volatile int *) (target - 4);
  printf ("probing at %p\n", pi);
  *pi = 1;
}


It is still probabilistic because if the increment is too large, the 
second sbrk call will fail with an out of memory error (which is 
expected), so you'll have to run it a couple of times.

If the test fails, the write at the will segfault.

Thanks,
Florian