Re: RFC: named anonymous vmas

From: Rich Felker
Date: Thu Aug 01 2013 - 04:36:47 EST

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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at