linux-kernel - Re: RFC: named anonymous vmas

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Message-ID: <CAHGf_=pY=ap-T3R0bK4675THvGikzH1KpMbEz3==_EwPBkebRQ@mail.gmail.com>
Date:	Sat, 3 Aug 2013 19:54:38 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Rich Felker <dalias@...ifal.cx>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Colin Cross <ccross@...gle.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	John Stultz <john.stultz@...aro.org>,
	libc-alpha <libc-alpha@...rceware.org>
Subject: Re: RFC: named anonymous vmas

On Thu, Aug 1, 2013 at 4:36 AM, Rich Felker <dalias@...ifal.cx> wrote:
> On Thu, Aug 01, 2013 at 01:29:51AM -0700, Christoph Hellwig wrote:
>> Btw, FreeBSD has an extension to shm_open to create unnamed but fd
>> passable segments.  From their man page:
>>
>>     As a FreeBSD extension, the constant SHM_ANON may be used for the path
>>     argument to shm_open().  In this case, an anonymous, unnamed shared
>>     memory object is created.  Since the object has no name, it cannot be
>>     removed via a subsequent call to shm_unlink().  Instead, the shared
>>     memory object will be garbage collected when the last reference to the
>>     shared memory object is removed.  The shared memory object may be shared
>>     with other processes by sharing the file descriptor via fork(2) or
>>     sendmsg(2).  Attempting to open an anonymous shared memory object with
>>     O_RDONLY will fail with EINVAL. All other flags are ignored.
>>
>> To me this sounds like the best way to expose this functionality to the
>> user.  Implementing it is another question as shm_open sits in libc,
>> we could either take it and shm_unlink to the kernel, or use O_TMPFILE
>> on tmpfs as the backend.
>
> I'm not sure what the purpose is. shm_open with a long random filename
> and O_EXCL|O_CREAT, followed immediately by shm_unlink, is just as
> good except in the case where you have a malicious user killing the
> process in between these two operations.

Practically, filename length is restricted by NAME_MAX(255bytes). Several
people don't think it is enough long length. The point is, race free API.
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