Comment 13 for bug 1020210

Carlos, this is faster to debug on paper than trying debug optimized program.

For minimal example what is wrong I could trigger assert for unoptimized version of malloc. In optimized version you need go to assembly to see where gcc scheduled loads.

Idea is simple, while we free one chunk then a chunk on top of fastbin could be in other thread allocated, resized and then returned back into top of fastbin to trigger assertion or seqfault when trim unmaps corresponding page.

A program is following,

#include <stdlib.h>
#include <pthread.h>

void * freea (void *p)
{
  free (p); // 1
}

int main ()
{
  pthread_t x;
  char *u, *v;
  u = malloc (16);
  pthread_create (&x, NULL, freea, u);

  v = malloc (16);
  free (v); // 2

  malloc_trim (0);
  v = malloc (512); // 3
  free (v);

  malloc_trim (0);
  v = malloc (16);
  free (v); // 4
}

First step into free 1 until you get to this fragment.

Here run free 2 so v gets into top of fastbin.

    unsigned int idx = fastbin_index(size); // 32 >> 4 = 2
    fb = &fastbin (av, idx);

    mchunkptr fd;
    mchunkptr old = *fb; // v
    unsigned int old_idx = ~0u;
    do
      {
        /* Another simple check: make sure the top of the bin is not the
           record we are going to add (i.e., double free). */
        if (__builtin_expect (old == p, 0))
          {
            errstr = "double free or corruption (fasttop)";
            goto errout;
          }

Now here run step 3 where v is chunk of size 528

        if (old != NULL)
          old_idx = fastbin_index(chunksize(old)); // 528 >> 4 = 33
        p->fd = fd = old;

And continue by step 4 which returns v into top of fastbin. which is same state as at 2.

      }
    while ((old = catomic_compare_and_exchange_val_rel (fb, p, fd)) != fd);

And as 33 != 2 we cause an error.

    if (fd != NULL && __builtin_expect (old_idx != idx, 0))
      {
        errstr = "invalid fastbin entry (free)";
        goto errout;
      }