Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd

[Daniel Colascione]

On Wed, Oct 17, 2018 at 5:08 AM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> On Wed, Oct 17, 2018 at 03:39:58AM -0700, Joel Fernandes wrote:
>> > > This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
>> > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which
>> > > prevents any future mmap and write syscalls from succeeding while
>> > > keeping the existing mmap active. The following program shows the seal
>> > > working in action:
>> >
>> > Where does the FS come from?  I'd rather expect this to be implemented
>> > as a 'force' style flag that applies the seal even if the otherwise
>> > required precondition is not met.
>>
>> The "FS" was meant to convey that the seal is preventing writes at the VFS
>> layer itself, for example vfs_write checks FMODE_WRITE and does not proceed,
>> it instead returns an error if the flag is not set. I could not find a better
>> name for it, I could call it F_SEAL_VFS_WRITE if you prefer?
>
> I don't think there is anything VFS or FS about that - at best that
> is an implementation detail.
>
> Either do something like the force flag I suggested in the last mail,
> or give it a name that matches the intention, e.g F_SEAL_FUTURE_WRITE.

+1

>> > This seems to lack any synchronization for f_mode.
>>
>> The f_mode is set when the struct file is first created and then memfd sets
>> additional flags in memfd_create. Then later we are changing it here at the
>> time of setting the seal. I donot see any possiblity of a race since it is
>> impossible to set the seal before memfd_create returns. Could you provide
>> more details about what kind of synchronization is needed and what is the
>> race condition scenario you were thinking off?
>
> Even if no one changes these specific flags we still need a lock due
> to rmw cycles on the field.  For example fadvise can set or clear
> FMODE_RANDOM.  It seems to use file->f_lock for synchronization.

Compare-and-exchange will suffice, right?