Re: [RFC PATCH 2/3] fs: add RWF_ENCODED for writing compressed data

[Omar Sandoval <osandov@osandov.com>]

On Tue, Sep 24, 2019 at 10:15:13AM -0700, Omar Sandoval wrote:
> On Thu, Sep 19, 2019 at 05:44:12PM +0200, Jann Horn wrote:
> > On Thu, Sep 19, 2019 at 8:54 AM Omar Sandoval <osandov@osandov.com> wrote:
> > > Btrfs can transparently compress data written by the user. However, we'd
> > > like to add an interface to write pre-compressed data directly to the
> > > filesystem. This adds support for so-called "encoded writes" via
> > > pwritev2().
> > >
> > > A new RWF_ENCODED flags indicates that a write is "encoded". If this
> > > flag is set, iov[0].iov_base points to a struct encoded_iov which
> > > contains metadata about the write: namely, the compression algorithm and
> > > the unencoded (i.e., decompressed) length of the extent. iov[0].iov_len
> > > must be set to sizeof(struct encoded_iov), which can be used to extend
> > > the interface in the future. The remaining iovecs contain the encoded
> > > extent.
> > >
> > > A similar interface for reading encoded data can be added to preadv2()
> > > in the future.
> > >
> > > Filesystems must indicate that they support encoded writes by setting
> > > FMODE_ENCODED_IO in ->file_open().
> > [...]
> > > +int import_encoded_write(struct kiocb *iocb, struct encoded_iov *encoded,
> > > +                        struct iov_iter *from)
> > > +{
> > > +       if (iov_iter_single_seg_count(from) != sizeof(*encoded))
> > > +               return -EINVAL;
> > > +       if (copy_from_iter(encoded, sizeof(*encoded), from) != sizeof(*encoded))
> > > +               return -EFAULT;
> > > +       if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
> > > +           encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE) {
> > > +               iocb->ki_flags &= ~IOCB_ENCODED;
> > > +               return 0;
> > > +       }
> > > +       if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
> > > +           encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
> > > +               return -EINVAL;
> > > +       if (!capable(CAP_SYS_ADMIN))
> > > +               return -EPERM;
> > 
> > How does this capable() check interact with io_uring? Without having
> > looked at this in detail, I suspect that when an encoded write is
> > requested through io_uring, the capable() check might be executed on
> > something like a workqueue worker thread, which is probably running
> > with a full capability set.
> 
> I discussed this more with Jens. You're right, per-IO permission checks
> aren't going to work. In fully-polled mode, we never get an opportunity
> to check capabilities in right context. So, this will probably require a
> new open flag.

Actually, file_ns_capable() accomplishes the same thing without a new
open flag. Changing the capable() check to file_ns_capable() in
init_user_ns should be enough.