Re: [PATCH 08/11] vfs: push EXDEV check down into ->remap_file_range

Darrick J. Wong
2018-12-03T23:58:47Z

On Tue, Dec 04, 2018 at 10:37:14AM +1100, Dave Chinner wrote:
> On Mon, Dec 03, 2018 at 11:11:30AM -0800, Darrick J. Wong wrote:
> > On Mon, Dec 03, 2018 at 01:04:07PM +0200, Amir Goldstein wrote:
> > > On Mon, Dec 3, 2018 at 10:34 AM Dave Chinner <david@fromorbit.com> wrote:
> > > >
> > > > From: Dave Chinner <dchinner@redhat.com>
> > > >
> > > > before we can enable cross-device copies into copy_file_range(),
> > > > we have to ensure that ->remap_file_range() implemenations will
> > > > correctly reject attempts to do cross filesystem clones. Currently
> > >
> > > But you only fixed remap_file_range() implemenations of xfs and ocfs2...
> > >
> > > > these checks are done above calls to ->remap_file_range(), but
> > > > we need to drive them inwards so that we get EXDEV protection for all
> > > > callers of ->remap_file_range().
> > > >
> > > > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> > > > ---
> > > > fs/read_write.c | 21 +++++++++++++--------
> > > > 1 file changed, 13 insertions(+), 8 deletions(-)
> > > >
> > > > diff --git a/fs/read_write.c b/fs/read_write.c
> > > > index 3288db1d5f21..174cf92eea1d 100644
> > > > --- a/fs/read_write.c
> > > > +++ b/fs/read_write.c
> > > > @@ -1909,6 +1909,19 @@ int generic_remap_file_range_prep(struct file *file_in, loff_t pos_in,
> > > > bool same_inode = (inode_in == inode_out);
> > > > int ret;
> > > >
> > > > + /*
> > > > + * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
> > > > + * the same mount. Practically, they only need to be on the same file
> > > > + * system. We check this here rather than at the ioctl layers because
> > > > + * this is effectively a limitation of the fielsystem implementations,
> > > > + * not so much the API itself. Further, ->remap_file_range() can be
> > > > + * called from syscalls that don't have cross device copy restrictions
> > > > + * (such as copy_file_range()) and so we need to catch them before we
> > > > + * do any damage.
> > > > + */
> > > > + if (inode_in->i_sb != inode_out->i_sb)
> > > > + return -EXDEV;
> > > > +
> > > > /* Don't touch certain kinds of inodes */
> > > > if (IS_IMMUTABLE(inode_out))
> > > > return -EPERM;
> > > > @@ -2013,14 +2026,6 @@ loff_t do_clone_file_range(struct file *file_in, loff_t pos_in,
> > > > if (!S_ISREG(inode_in->i_mode) || !S_ISREG(inode_out->i_mode))
> > > > return -EINVAL;
> > > >
> > > > - /*
> > > > - * FICLONE/FICLONERANGE ioctls enforce that src and dest files are on
> > > > - * the same mount. Practically, they only need to be on the same file
> > > > - * system.
> > > > - */
> > > > - if (inode_in->i_sb != inode_out->i_sb)
> > > > - return -EXDEV;
> > > > -
> > >
> >
> > I think this is sort of backwards -- the checks should stay in
> > do_clone_file_range, and vfs_copy_file_range should be calling that
> > instead of directly calling ->remap_range():
> >
> > vfs_copy_file_range()
> > {
> > file_start_write(...);
> > ret = do_clone_file_range(...);
> > if (ret > 0)
> > return ret;
> > ret = do_copy_file_range(...);
> > file_end_write(...);
> > return ret;
> > }
>
> I'm already confused by the way we weave in and out of "vfs_/do_*"
> functions, and this just makes it worse.
>
> Just what the hell is supposed to be in a "vfs_" prefixed function,
> and why the hell is it considered a "vfs" level function if we then
> export it's internal functions for individual filesystems to use?

I /think/ vfs_ functions are file_start_write()/file_end_write()
wrappers around a similarly named function that lacks the freeze
protection??

(AFAICT Amir made that split so that overlayfs could use these
functions, though I do not know if everything vfs_ was made that way
/specifically/ for overlayfs or if that's the way things have been and
ovlfs simply takes advantage of it...)

Guhhh, none of this is documented......

--D

> Cheers,
>
> Dave.
> --
> Dave Chinner
> david@fromorbit.com