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From: Michal Hocko <mhocko@kernel.org>
To: Jann Horn <jannh@google.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-man <linux-man@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved
Date: Mon, 16 Apr 2018 21:18:05 +0200	[thread overview]
Message-ID: <20180416191805.GS17484@dhcp22.suse.cz> (raw)
In-Reply-To: <CAG48ez3DwRXMtiinUWKnan6hAppLYLdx-w+VzXG6ubioZUacQg@mail.gmail.com>

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

  reply	other threads:[~2018-04-16 19:18 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-12 15:39 Jann Horn
2018-04-12 18:32 ` Michael Kerrisk (man-pages)
2018-04-12 18:33 ` John Hubbard
2018-04-12 18:37   ` Michael Kerrisk (man-pages)
2018-04-12 18:49     ` Jann Horn
2018-04-12 18:59       ` John Hubbard
2018-04-12 19:18         ` Jann Horn
2018-04-12 19:24           ` John Hubbard
2018-04-13  6:43             ` Michael Kerrisk (man-pages)
2018-04-13  6:49               ` Michal Hocko
2018-04-13 15:04                 ` Jann Horn
2018-04-13 16:04                   ` Michal Hocko
2018-04-13 16:05                     ` Jann Horn
2018-04-13 16:17                       ` Jann Horn
2018-04-16 10:07                         ` Michal Hocko
2018-04-16 13:55                           ` Jann Horn
2018-04-16 19:18                             ` Michal Hocko [this message]
2018-04-16 19:30                               ` Jann Horn
2018-04-16 19:57                                 ` Michal Hocko
2018-04-16 20:17                                   ` Jann Horn
2018-04-16 21:11                                     ` Michal Hocko
2018-04-16 21:12                                       ` Jann Horn
2018-04-17  6:23                                         ` Michal Hocko
2018-05-02 13:06                                       ` Michael Kerrisk (man-pages)

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