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 15:55:36, Jann Horn wrote:
> On Mon, Apr 16, 2018 at 12:07 PM, Michal Hocko <mhocko@kernel.org> wrote:
> > On Fri 13-04-18 18:17:36, Jann Horn wrote:
> >> On Fri, Apr 13, 2018 at 6:05 PM, Jann Horn <jannh@google.com> wrote:
> >> > On Fri, Apr 13, 2018 at 6:04 PM, Michal Hocko <mhocko@kernel.org> wrote:
> >> >> On Fri 13-04-18 17:04:09, Jann Horn wrote:
> >> >>> On Fri, Apr 13, 2018 at 8:49 AM, Michal Hocko <mhocko@kernel.org> wrote:
> >> >>> > On Fri 13-04-18 08:43:27, Michael Kerrisk wrote:
> >> >>> > [...]
> >> >>> >> So, you mean remove this entire paragraph:
> >> >>> >>
> >> >>> >>               For cases in which the specified memory region has not been
> >> >>> >>               reserved using an existing mapping,  newer  kernels  (Linux
> >> >>> >>               4.17  and later) provide an option MAP_FIXED_NOREPLACE that
> >> >>> >>               should be used instead; older kernels require the caller to
> >> >>> >>               use addr as a hint (without MAP_FIXED) and take appropriate
> >> >>> >>               action if the kernel places the new mapping at a  different
> >> >>> >>               address.
> >> >>> >>
> >> >>> >> It seems like some version of the first half of the paragraph is worth
> >> >>> >> keeping, though, so as to point the reader in the direction of a remedy.
> >> >>> >> How about replacing that text with the following:
> >> >>> >>
> >> >>> >>               Since  Linux 4.17, the MAP_FIXED_NOREPLACE flag can be used
> >> >>> >>               in a multithreaded program to avoid  the  hazard  described
> >> >>> >>               above.
> >> >>> >
> >> >>> > Yes, that sounds reasonable to me.
> >> >>>
> >> >>> But that kind of sounds as if you can't avoid it before Linux 4.17,
> >> >>> when actually, you just have to call mmap() with the address as hint,
> >> >>> and if mmap() returns a different address, munmap() it and go on your
> >> >>> normal error path.
> >> >>
> >> >> This is still racy in multithreaded application which is the main point
> >> >> of the whole section, no?
> >> >
> >> > No, it isn't.
> >
> > I could have been more specific, sorry.
> >
> >> mmap() with a hint (without MAP_FIXED) will always non-racily allocate
> >> a memory region for you or return an error code. If it does allocate a
> >> memory region, it belongs to you until you deallocate it. It might be
> >> at a different address than you requested -
> >
> > Yes, this all is true. Except the atomicity is guaranteed only for the
> > syscall. Once you return to the userspace any error handling is error
> > prone and racy because your mapping might change under you feet. So...
> 
> Can you please elaborate on why you think anything could change the
> mapping returned by mmap() under the caller's feet?

Because as soon as the mmap_sem is dropped then any other thread can
modify the shared address space.

> When mmap() returns a memory area to the caller, that memory area
> belongs to the caller. No unrelated code will touch it, unless that
> code is buggy.

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.
-- 
Michal Hocko
SUSE Labs