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

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Jun 13, 2014 at 8:15 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
> 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.).
>

Isn't the point of SEAL_SHRINK to allow servers to mmap and read
safely without worrying about SIGBUS?

--Andy

> Thanks
> David



-- 
Andy Lutomirski
AMA Capital Management, LLC

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