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Re: RFC: named anonymous vmas
- From: KOSAKI Motohiro <kosaki dot motohiro at gmail dot com>
- To: Rich Felker <dalias at aerifal dot cx>
- Cc: Christoph Hellwig <hch at infradead dot org>, Colin Cross <ccross at google dot com>, lkml <linux-kernel at vger dot kernel dot org>, Linux-MM <linux-mm at kvack dot org>, Android Kernel Team <kernel-team at android dot com>, John Stultz <john dot stultz at linaro dot org>, libc-alpha <libc-alpha at sourceware dot org>
- Date: Sat, 3 Aug 2013 19:54:38 -0400
- Subject: Re: RFC: named anonymous vmas
- References: <CAMbhsRQU=xrcum+ZUbG3S+JfFUJK_qm_VB96Vz=PpL=vQYhUvg at mail dot gmail dot com> <20130622103158 dot GA16304 at infradead dot org> <CAMbhsRTz246dWPQOburNor2HvrgbN-AWb2jT_AEywtJHFbKWsA at mail dot gmail dot com> <20130801082951 dot GA23563 at infradead dot org> <20130801083608 dot GJ221 at brightrain dot aerifal dot cx>
On Thu, Aug 1, 2013 at 4:36 AM, Rich Felker <firstname.lastname@example.org> 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.