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From: Jann Horn <jannh@google.com>
To: Michal Hocko <mhocko@kernel.org>
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 23:12:48 +0200	[thread overview]
Message-ID: <CAG48ez2fXcQS7sw_aCC6_wBKjYphOxFRN4rrRzSk2+-T_mFaxw@mail.gmail.com> (raw)
In-Reply-To: <20180416211115.GU17484@dhcp22.suse.cz>

On Mon, Apr 16, 2018 at 11:11 PM, Michal Hocko <mhocko@kernel.org> wrote:
> 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.

Wow, really? Can you point to an example?

>> 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

  reply	other threads:[~2018-04-16 21:13 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
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 [this message]
2018-04-17  6:23                                         ` Michal Hocko
2018-05-02 13:06                                       ` Michael Kerrisk (man-pages)

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