[PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont

[H.J. Lu]

On Fri, Dec 11, 2015 at 10:11 AM, Zack Weinberg <zackw@panix.com> wrote:
> On Fri, Dec 11, 2015 at 12:02 PM, Adhemerval Zanella
> <adhemerval.zanella@linaro.org> wrote:
>> Another issue is this is basically limiting ALSR really hard on x86_64.
>> I also would prefer to make the default to *not* include this flag and
>> set the env. variable to actually enable it. If the cpu is slow doing
>> what's intended because it is buggy, let it be slow at default. Do not
>> break what was intended (full ALSR).
>
> FWIW, I was about to post the exact same objection.  Relatedly, the
> environment variable should be handled through the normal ld.so-tuning
> environment variable mechanism (and, in particular, ineffective for
> set*id binaries).
>

We have discussed it internally.  Since this is very critical for performance
on Silvermont based platforms, we want to keep it op-out for normal
programs and disable it for SUID programs.  Reduced address range is no
worse than 32-bit programs.

Like this.

-- 
H.J.
---
/* Prefer_MAP_32BIT_EXEC reduces bits available for address space layout
   randomization (ASLR).  Prefer_MAP_32BIT_EXEC is disabled for SUID
   programs and can also be disabled by setting environment variable,
   LD_DISABLE_PREFER_MAP_32BIT_EXEC.  */

static inline unsigned int
get_prefer_map_32bit_exec (void)
{
#if defined __LP64__ && IS_IN (rtld)
  extern char **__environ attribute_hidden;
  extern int __libc_enable_secure;
  if (__builtin_expect (__libc_enable_secure, 0))
    return 0;
  for (char **current = __environ; *current != NULL; ++current)
    {
      /* Check LD_DISABLE_PREFER_MAP_32BIT_EXEC=.  */
      static const char *disable = "LD_DISABLE_PREFER_MAP_32BIT_EXEC=";
      for (size_t i = 0; ; i++)
{
 if (disable[i] != (*current)[i])
   break;
 if ((*current)[i] == '=')
   return 0;
}
    }
  return bit_Prefer_MAP_32BIT_EXEC;
#else
  return 0;
#endif
}