Re: [PATCH v3 0/7] File Sealing & memfd_create()

[David Herrmann <dh.herrmann@gmail.com>]

Hi

On Fri, Jun 13, 2014 at 5:10 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
>> Hi
>>
>> This is v3 of the File-Sealing and memfd_create() patches. You can find v1 with
>> a longer introduction at gmane:
>>   http://thread.gmane.org/gmane.comp.video.dri.devel/102241
>> An LWN article about memfd+sealing is available, too:
>>   https://lwn.net/Articles/593918/
>> v2 with some more discussions can be found here:
>>   http://thread.gmane.org/gmane.linux.kernel.mm/115713
>>
>> This series introduces two new APIs:
>>   memfd_create(): Think of this syscall as malloc() but it returns a
>>                   file-descriptor instead of a pointer. That file-descriptor is
>>                   backed by anon-memory and can be memory-mapped for access.
>>   sealing: The sealing API can be used to prevent a specific set of operations
>>            on a file-descriptor. You 'seal' the file and give thus the
>>            guarantee, that it cannot be modified in the specific ways.
>>
>> A short high-level introduction is also available here:
>>   http://dvdhrm.wordpress.com/2014/06/10/memfd_create2/
>
> Potentially silly question: is it guaranteed that mmapping and reading
> a SEAL_SHRINKed fd within size bounds will not SIGBUS?  If so, should
> this be documented?  (The particular issue here would be reading
> holes.  It should work by using the zero page, but, if so, we should
> probably make it a real documented guarantee.)

No, this is not guaranteed. See the previous discussion in v2 on Patch
2/4 between Hugh and me.

Summary is: If you want mmap-reads to not fail, use mlock(). There are
many situations where a fault might fail (think: OOM) and sealing is
not meant to protect against that. Btw., holes are automatically
filled with fresh pages by shmem. So a read only fails in OOM
situations (or memcg limits, etc.).

Thanks
David