RFC: named anonymous vmas

[Rich Felker]

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.

Rich