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

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

Hi

On Tue, Jun 17, 2014 at 2:13 PM, Florian Weimer <fweimer@redhat.com> wrote:
> On 06/17/2014 12:10 PM, David Herrmann wrote:
>
>>>> The file might have holes, therefore, you'd have to allocate backing
>>>> pages. This might hit a soft-limit and fail. To avoid this, use
>>>> fallocate() to allocate pages prior to mmap()
>>>
>>>
>>> This does not work because the consuming side does not know how the
>>> descriptor was set up if sealing does not imply that.
>>
>>
>> The consuming side has to very seals via F_GET_SEALS. After that, it
>> shall do a simple fallocate() on the whole file if it wants to go sure
>> that all pages are allocated. Why shouldn't that be possible? Please
>> elaborate.
>
>
> Hmm.  You permit general fallocate even for WRITE seals.  That's really
> unexpected.

SEAL_WRITE prevents modifications of file-content. fallocate() does
not modify file-contents, so I think it's not unexpected that
fallocate() is still allowed.

> The inode_newsize_ok check in shmem_fallocate can result in SIGXFSZ, which
> doesn't seem to be what's intended here.

It can only result in SIGXFSZ if you _increase_ the file-size with
fallocate(). You shouldn't do that if you only verify that holes are
allocated. Hence, a simple fallocate(st.st_size) cannot result in
SIGXFSZ. Obviously, this requires SEAL_SHRINK to prevent the remote
site to shrink the file while you call fallocate(). But SEAL_WRITE
usually goes together with SEAL_SHRINK for obvious reasons.

> Will the new pages attributed to the process calling fallocate, or to the
> process calling memfd_create?

Pages are always allocated by the caller and charged on current->mm
(current process).

Thanks
David