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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 22:17:40 +0200	[thread overview]
Message-ID: <CAG48ez1bV_zZP3Y2ioDndP+H8mLCcxOtU1vCbWe7Q8myEGfXQQ@mail.gmail.com> (raw)
In-Reply-To: <20180416195726.GT17484@dhcp22.suse.cz>

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 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.

> 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?

> This will succeed and ...
>>   munmap(x, len);
> ... now you are munmaping somebody's else memory range
>
>>   return NULL;
>
> Do code _is_ buggy but it is not obvious at all.
>
>> }
>>
>> has no need for any form of external synchronization.
>
> If the above mmap/munmap section was protected by a lock and _all_ other
> mmaps (direct or indirect) would use the same lock then you are safe
> against that.

  reply	other threads:[~2018-04-16 20:18 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
2018-04-16 19:30                               ` Jann Horn
2018-04-16 19:57                                 ` Michal Hocko
2018-04-16 20:17                                   ` Jann Horn [this message]
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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