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SubjectRe: RFC: named anonymous vmas
On Thu, Aug 1, 2013 at 4:36 AM, Rich Felker <dalias@aerifal.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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