Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved

[Michal Hocko <mhocko@kernel.org>]

On Mon 16-04-18 22:17:40, Jann Horn wrote:
> On Mon, Apr 16, 2018 at 9:57 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > On Mon 16-04-18 21:30:09, Jann Horn wrote:
> >> On Mon, Apr 16, 2018 at 9:18 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > [...]
> >> > Yes, reasonably well written application will not have this problem.
> >> > That, however, requires an external synchronization and that's why
> >> > called it error prone and racy. I guess that was the main motivation for
> >> > that part of the man page.
> >>
> >> What requires external synchronization? I still don't understand at
> >> all what you're talking about.
> >>
> >> The following code:
> >>
> >> void *try_to_alloc_addr(void *hint, size_t len) {
> >>   char *x = mmap(hint, len, ...);
> >>   if (x == MAP_FAILED) return NULL;
> >>   if (x == hint) return x;
> >
> > Any other thread can modify the address space at this moment.
> 
> But not parts of the address space that were returned by this mmap() call.
?
> > Just
> > consider that another thread would does mmap(x, MAP_FIXED) (or any other
> > address overlapping [x, x+len] range)
> 
> If the other thread does that without previously having created a
> mapping covering the area in question, that would be a bug in the
> other thread.

MAP_FIXED is sometimes used without preallocated address ranges.

> MAP_FIXED on an unmapped address is almost always a bug
> (excluding single-threaded cases with no library code, and even then
> it's quite weird) - for example, any malloc() call could also cause
> libc to start using the memory range you're trying to map with
> MAP_FIXED.

Yeah and that's why we there is such a large paragraph in the man page
;)

> > becaus it is seemingly safe as x
> > != hint.
> 
> I don't understand this part. Are you talking about a hypothetical
> scenario in which a programmer attempts to segment the virtual memory
> space into areas that are exclusively used by threads without creating
> memory mappings for those areas?

Yeah, that doesn't sound all that over-exaggerated, right? And yes,
such a code would be subtle and most probably buggy. I am not trying to
argue for those hypothetical cases. All I am saying is that MAP_FIXED is
subtle.

I _do_ agree that using it solely on the preallocated and _properly_
managed address ranges is safe. I still maintain my position on error
prone though. And besides that there are usecases which do not operate
on preallocated address ranges so people really have to be careful.

I do not really care what is the form. I find the current wording quite
informative and showing examples of how things might be broken. I do
agree with your remark that "MAP_FIXED on preallocated ranges is safe"
should be added. But MAP_FIXED is dangerous API and should have few big
fat warnings.
-- 
Michal Hocko
SUSE Labs