With the register_printf_type/register_printf_modifier/register_printf_specifier
APIs the C library is just told the size of the argument and is provided with
a callback to fetch the argument from va_list using va_arg into C library provided
memory.  The C library isn't told what alignment requirement it has, but we were
using direct load of a __float128 value from that memory which assumes
__alignof (__float128) alignment.

The following patch fixes that by using memcpy instead.

I haven't been able to reproduce an actual crash, tried
 #include <quadmath.h>
 #include <stdlib.h>
 #include <stdio.h>

int main ()
{
  __float128 r;
  int prec = 20;
  int width = 46;
  char buf[128];

  r = 2.0q;
  r = sqrtq (r);
  int n = quadmath_snprintf (buf, sizeof buf, "%+-#*.20Qe", width, r);
  if ((size_t) n < sizeof buf)
    printf ("%s\n", buf);
    /* Prints: +1.41421356237309504880e+00 */
  quadmath_snprintf (buf, sizeof buf, "%Qa", r);
  if ((size_t) n < sizeof buf)
    printf ("%s\n", buf);
    /* Prints: 0x1.6a09e667f3bcc908b2fb1366ea96p+0 */
  n = quadmath_snprintf (NULL, 0, "%+-#46.*Qe", prec, r);
  if (n > -1)
    {
      char *str = malloc (n + 1);
      if (str)
        {
          quadmath_snprintf (str, n + 1, "%+-#46.*Qe", prec, r);
          printf ("%s\n", str);
          /* Prints: +1.41421356237309504880e+00 */
        }
      free (str);
    }
  printf ("%+-#*.20Qe\n", width, r);
  printf ("%Qa\n", r);
  printf ("%+-#46.*Qe\n", prec, r);
  printf ("%d %Qe %d %Qe %d %Qe\n", 1, r, 2, r, 3, r);
  return 0;
}
In any case, I think memcpy for loading from it is right.

2024-04-03  Simon Chopin  <simon.chopin@canonical.com>
	    Jakub Jelinek  <jakub@redhat.com>

	PR libquadmath/114533
	* printf/printf_fp.c (__quadmath_printf_fp): Use memcpy to copy
	__float128 out of args.
	* printf/printf_fphex.c (__quadmath_printf_fphex): Likewise.

Signed-off-by: Simon Chopin <simon.chopin@canonical.com>