Re: [PATCHv4 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP_32BIT)

[Dmitry Safonov]

On 02/11/2017 11:13 PM, Thomas Gleixner wrote:
> On Mon, 30 Jan 2017, Dmitry Safonov wrote:
>
> > At this momet, logic in arch_get_unmapped_area{,_topdown} for mmaps with
> > MAP_32BIT flag checks TIF_ADDR32 which means:
> > o if 32-bit ELF changes mode to 64-bit on x86_64 and then tries to
> >   mmap() with MAP_32BIT it'll result in addr over 4Gb (as default is
> >   top-down allocation)
> > o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
> >   it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).
> >
> > Fix it by handeling MAP_32BIT in 64-bit syscalls only.
>
> I really have a hard time to understand what is fixed and how that is
> related to the $subject.
>
> Again. Please explain the problem first properly so one can understand the
> issue immediately.

Ok, rewrote the changes log.

> > As a little bonus it'll make thread flag a little less used.
>
> I really do not understand the bonus part here. You replace the thread flag
> check with a different one and AFAICT this looks like oart of the 'fix'.

It's a part of the fix, right.
What I meant here is that after those patches TIF_ADDR32 is no more
used after exec() time. That's bonus as if we manage to change exec()
code in some way (i.e., pass address restriction as a parameter), we'll
have additional free thread info flag.

> Thanks,
>
> 	tglx
>
> > @@ -101,7 +101,7 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
> >  static void find_start_end(unsigned long flags, unsigned long *begin,
> >  			   unsigned long *end)
> >  {
> > -	if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT)) {
> > +	if (!in_compat_syscall() && (flags & MAP_32BIT)) {
> >  		/* This is usually used needed to map code in small
> >  		   model, so it needs to be in the first 31bit. Limit
> >  		   it to that.  This means we need to move the
> > @@ -195,7 +195,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
> >  		return addr;
> >
> >  	/* for MAP_32BIT mappings we force the legacy mmap base */
> > -	if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT))
> > +	if (!in_compat_syscall() && (flags & MAP_32BIT))
> >  		goto bottomup;
> >
> >  	/* requesting a specific address */
> > --
> > 2.11.0


--
             Dmitry