[PATCH] Add Prefer_MAP_32BIT_EXEC for Silvermont

[Adhemerval Zanella]

On 11-12-2015 13:59, H.J. Lu wrote:
> On Fri, Dec 11, 2015 at 7:39 AM, Joseph Myers <joseph@codesourcery.com> wrote:
>> > On Fri, 11 Dec 2015, H.J. Lu wrote:
>> >
>>> >> +++ b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
>>> >> @@ -0,0 +1,49 @@
>>> >> +/* Copyright (C) 2015 Free Software Foundation, Inc.
>> >
>> > All new files should have a descriptive first line before the copyright
>> > notice.
>> >
> Here is the updated patch.

> 
> diff --git a/sysdeps/unix/sysv/linux/x86_64/64/mmap.c b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c
> new file mode 100644
> index 0000000..c34f633
> --- /dev/null
> +++ b/sysdeps/unix/sysv/linux/x86_64/64/mmap.c

> +
> +__ptr_t
> +__mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
> +{
> +  /* If the Prefer_MAP_32BIT_EXEC bit is set, try to map executable pages
> +     with MAP_32BIT first.  */
> +  if (addr == NULL
> +      && (prot & PROT_EXEC) != 0
> +      && HAS_ARCH_FEATURE (Prefer_MAP_32BIT_EXEC))
> +    {
> +      addr = (__ptr_t) INLINE_SYSCALL (mmap, 6, addr, len, prot,
> +				       flags | MAP_32BIT,
> +				       fd, offset);
> +      if (addr != MAP_FAILED)
> +	return addr;
> +    }
> +  return (__ptr_t) INLINE_SYSCALL (mmap, 6, addr, len, prot, flags,
> +				   fd, offset);
> +}
> +

I would advise not add another syscall variant implementation, but rather 
work on make a generic implementation with adjustments made by each platform.
Something like

__ptr_t
__mmap (__ptr_t addr, size_t len, int prot, int flags, int fd, off_t offset)
{
  flags = MMAP_ARCH_FLAGS (flags);
  return (__ptr_t) INLINE_SYSCALL( mmap, 6, addr, len, prot, flags, fd,  offset);
}

And then define MMAP_ARCH_FLAG for x86 to add the MAP_32BIT when required.

I am proposing it because I am reworking the cancellable syscalls implementations
to get rid of the sysdep-cancel assembly macros (which currently are not really
required) and this kind of code fragmentation only add complexity in the project.