Re: Building Emacs on WSL

[Paul Eggert <eggert@cs.ucla.edu>]

[-- Attachment #1: Type: text/plain, Size: 1665 bytes --]

On 12/20/2016 04:40 AM, Fabrice Popineau wrote:
>
> (gdb) run  --batch  --load loadup bootstrap
> Starting program: /mnt/d/Source/emacs/build-emacs-ubuntu/src/temacs 
> --batch  --load loadup bootstrap
> warning: Error disabling address space randomization: Succès

This is a warning from GDB, not from Emacs. GDB normally attempts to 
disable ASLR, to make debugging easier. Looking at the sequence of 
system calls in GDB 7.12, GDB calls 'personality' in some funky and 
undocumented ways, e.g., GDB assumes that errno is not changed when 
'personality' succeeds.

Please compile and run the attached program under WSL, using the same 
flags you use to compile Emacs, and let me know what it does. On Fedora 
24 x86-64, this program outputs:

personality (0xffffffff) returns 0x0, errno=0 (Success)
ADDR_NO_RANDOMIZE was clear
personality (pers | ADDR_NO_RANDOMIZE) returns 0x0, errno=0 (Success)
ADDR_NO_RANDOMIZE set successfully

When run under GDB, the same program outputs:

personality (0xffffffff) returns 0x40000, errno=0 (Success)
ADDR_NO_RANDOMIZE was set

because GDB already disabled ASLR. You can look at GDB's file 
gdb/nat/linux-personality.c to see how GDB differs from Emacs in this 
area; perhaps there's something there that Emacs can use to figure out 
that it is running with a buggy kernel.

>
> there is a simple fix:

That fix cannot be installed as-is since ASLR needs to be disabled on 
ordinary GNU/Linux platforms. It appears that WSL pretends to support 
the Linux 'personality' system call and its ADDR_NO_RANDOMIZE flag, and 
that this support does not actually work and makes Emacs crash.


[-- Attachment #2: personality.c --]
[-- Type: text/plain, Size: 834 bytes --]

#include <sys/personality.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>

static int
print_personality (unsigned long persona, char const *msg)
{
  int pers, err;
  errno = 0;
  pers = personality (persona);
  err = errno;
  printf ("personality (%s) returns 0x%x, errno=%d (%s)\n",
	  msg, pers, err, strerror (err));
  errno = err;
  return pers;
}

int
main (void)
{
  int pers;
  pers = print_personality (0xffffffff, "0xffffffff");
  if (pers & ADDR_NO_RANDOMIZE)
    printf ("ADDR_NO_RANDOMIZE was set\n");
  else
    {
      printf ("ADDR_NO_RANDOMIZE was clear\n");
      pers = print_personality (pers | ADDR_NO_RANDOMIZE,
				"pers | ADDR_NO_RANDOMIZE");
      if (0 <= personality)
	printf ("ADDR_NO_RANDOMIZE set successfully");
      else
	printf ("ADDR_NO_RANDOMIZE could not be set");
    }
  return 0;
}