Re: [PATCH 3/6] shm: add memfd_create() syscall

[David Herrmann]

Hi

On Wed, Apr 2, 2014 at 3:38 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
> On Wed, Mar 19, 2014 at 11:06 PM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
>> memfd_create() is similar to mmap(MAP_ANON), but returns a file-descriptor
>> that you can pass to mmap(). It explicitly allows sealing and
>> avoids any connection to user-visible mount-points. Thus, it's not
>> subject to quotas on mounted file-systems, but can be used like
>> malloc()'ed memory, but with a file-descriptor to it.
>>
>> memfd_create() does not create a front-FD, but instead returns the raw
>> shmem file, so calls like ftruncate() can be used. Also calls like fstat()
>> will return proper information and mark the file as regular file. Sealing
>> is explicitly supported on memfds.
>>
>> Compared to O_TMPFILE, it does not require a tmpfs mount-point and is not
>> subject to quotas and alike.
>
> Instead of adding new syscall we can extend existing openat() a little
> bit more:
>
> openat(AT_FDSHM, "name", O_TMPFILE | O_RDWR, 0666)

O_TMPFILE requires an existing directory as "name". So you have to use:
  open("/run/", O_TMPFILE | O_RDWR, 0666)
instead of
  open("/run/new_file", O_TMPFILE | O_RDWR, 0666)

We _really_ want to set a name for the inode, though. Otherwise,
debug-info via /proc/pid/fd/ is useless.

Furthermore, Linus requested to allow sealing only on files that
_explicitly_ allow sealing. So v2 of this series will have
MFD_ALLOW_SEALING as memfd_create() flag. I don't think we can do this
with linkat() (or is that meant to be implicit for the new AT_FDSHM?).
Last but not least, you now need a separate syscall to set the
file-size.

I could live with most of these issues, except for the name-thing. Ideas?

Thanks
David
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