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

[David Herrmann]

Hi

On Thu, Mar 20, 2014 at 10:01 AM, Pavel Emelyanov <xemul at parallels.com> wrote:
> On 03/20/2014 12:47 PM, Cyrill Gorcunov wrote:
>> If I'm not mistaken in something obvious, this looks similar to /proc/pid/map_files
>> feature, Pavel?
>
> It is, but the map_files will work "in the opposite direction" :) In the memfd
> case one first gets an FD, then mmap()s it; in the /proc/pis/map_files case one
> should first mmap() a region, then open it via /proc/self/map_files.
>
> But I don't know whether this matters.

Yes, you can replace memfd_create() so far with:
  p = mmap(NULL, size, ..., MAP_ANON | MAP_SHARED, -1, 0);
  sprintf(path, "/proc/self/map_files/%lx-%lx", p, p + size);
  fd = open(path, O_RDWR);

However, map_files is only enabled with CONFIG_CHECKPOINT_RESTORE, the
/proc/pid/map_files/ directory is root-only (at least I get EPERM if
non-root), it doesn't provide the "name" argument which is very handy
for debugging, it doesn't explicitly support sealing (it requires
MAP_ANON to be backed by shmem) and it's a very weird API for
something this simple.

Thanks
David