Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 16 Apr 2018 21:57:26 +0200
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
Message-ID: <20180416195726.GT17484@dhcp22.suse.cz>
References: <9c714917-fc29-4d12-b5e8-cff28761a2c1@gmail.com>
 <20180413064917.GC17484@dhcp22.suse.cz>
 <CAG48ez2w+3FDh9LM3+P2EHowicjM2Xw6giR6uq=26JfWHYsTAQ@mail.gmail.com>
 <20180413160435.GA17484@dhcp22.suse.cz>
 <CAG48ez3-xtmAt2EpRFR8GNKKPcsDsyg7XdwQ=D5w3Ym6w4Krjw@mail.gmail.com>
 <CAG48ez1PdzMs8hkatbzSLBWYucjTc75o8ovSmeC66+e9mLvSfA@mail.gmail.com>
 <20180416100736.GG17484@dhcp22.suse.cz>
 <CAG48ez3DwRXMtiinUWKnan6hAppLYLdx-w+VzXG6ubioZUacQg@mail.gmail.com>
 <20180416191805.GS17484@dhcp22.suse.cz>
 <CAG48ez1nf96nHj8a+aZy22RwqYTUZBGrsGFcz=ZhZBUWzaEZ9w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAG48ez1nf96nHj8a+aZy22RwqYTUZBGrsGFcz=ZhZBUWzaEZ9w@mail.gmail.com>
User-Agent: Mutt/1.9.4 (2018-02-28)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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. Just
consider that another thread would does mmap(x, MAP_FIXED) (or any other
address overlapping [x, x+len] range) becaus it is seemingly safe as x
!= hint. 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.
-- 
Michal Hocko
SUSE Labs