Re: [RFC v3 7/7] shm: isolate pinned pages when sealing files

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

Hi

On Fri, Jun 13, 2014 at 5:06 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> On Fri, Jun 13, 2014 at 3:36 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
>> When setting SEAL_WRITE, we must make sure nobody has a writable reference
>> to the pages (via GUP or similar). We currently check references and wait
>> some time for them to be dropped. This, however, might fail for several
>> reasons, including:
>>  - the page is pinned for longer than we wait
>>  - while we wait, someone takes an already pinned page for read-access
>>
>> Therefore, this patch introduces page-isolation. When sealing a file with
>> SEAL_WRITE, we copy all pages that have an elevated ref-count. The newpage
>> is put in place atomically, the old page is detached and left alone. It
>> will get reclaimed once the last external user dropped it.
>>
>> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
>
> Won't this have unexpected effects?
>
> Thread 1:  start read into mapping backed by fd
>
> Thread 2:  SEAL_WRITE
>
> Thread 1: read finishes.  now the page doesn't match the sealed page

Just to be clear: you're talking about read() calls that write into
the memfd? (like my FUSE example does) Your language might be
ambiguous to others as "read into" actually implies a write.

No, this does not have unexpected effects. But yes, your conclusion is
right. To be clear, this behavior would be part of the API. Any
asynchronous write might be cut off by SEAL_WRITE _iff_ you unmap your
buffer before the write finishes. But you actually have to extend your
example:

Thread 1: p = mmap(memfd, SIZE);
Thread 1: h = async_read(some_fd, p, SIZE);
Thread 1: munmap(p, SIZE);
Thread 2: SEAL_WRITE
Thread 1: async_wait(h);

If you don't do the unmap(), then SEAL_WRITE will fail due to an
elevated i_mmap_writable. I think this is fine. In fact, I remember
reading that async-IO is not required to resolve user-space addresses
at the time of the syscall, but might delay it to the time of the
actual write. But you're right, it would be misleading that the AIO
operation returns success. This would be part of the memfd-API,
though. And if you mess with your address space while running an
async-IO operation on it, you're screwed anyway.

Btw., your sealing use-case is really odd. No-one guarantees that the
SEAL_WRITE happens _after_ you schedule your async-read. In case you
have some synchronization there, you just have to move it after
waiting for your async-io to finish.

Does that clear things up?
Thanks
David

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