Message-ID: <CAN-5tyGO1-21HTU4TjXiE9dF3rD_vVt9owd0-8y=S8HeMuf9mA@mail.gmail.com>
Date:   Thu, 1 Jul 2021 14:06:53 -0400
From:   Olga Kornievskaia <aglo@...ch.edu>
To:     Luis Henriques <lhenriques@...e.de>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Nicolas Boichat <drinkcat@...omium.org>,
        Amir Goldstein <amir73il@...il.com>,
        Petr Vorel <pvorel@...e.cz>, Steve French <sfrench@...ba.org>,
        kernel test robot <oliver.sang@...el.com>,
        linux-nfs <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH v11] vfs: fix copy_file_range regression in cross-fs copies

On Thu, Jul 1, 2021 at 5:06 AM Luis Henriques <lhenriques@...e.de> wrote:
>
> On Wed, Jun 30, 2021 at 05:06:49PM -0400, Olga Kornievskaia wrote:
> > adding linux-nfs to the recipients as well (seems to have been dropped)
> >
> > On Wed, Jun 30, 2021 at 12:22 PM Luis Henriques <lhenriques@...e.de> wrote:
> > >
> > > A regression has been reported by Nicolas Boichat, found while using the
> > > copy_file_range syscall to copy a tracefs file.  Before commit
> > > 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices") the
> > > kernel would return -EXDEV to userspace when trying to copy a file across
> > > different filesystems.  After this commit, the syscall doesn't fail anymore
> > > and instead returns zero (zero bytes copied), as this file's content is
> > > generated on-the-fly and thus reports a size of zero.
> > >
> > > This patch restores some cross-filesystem copy restrictions that existed
> > > prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
> > > devices").  Filesystems are still allowed to fall-back to the VFS
> > > generic_copy_file_range() implementation, but that has now to be done
> > > explicitly.
> > >
> > > nfsd is also modified to fall-back into generic_copy_file_range() in case
> > > vfs_copy_file_range() fails with -EOPNOTSUPP or -EXDEV.
> > >
> > > Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")
> > > Link: https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/
> > > Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/
> > > Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/
> > > Reported-by: Nicolas Boichat <drinkcat@...omium.org>
> > > Reported-by: kernel test robot <oliver.sang@...el.com>
> > > Signed-off-by: Luis Henriques <lhenriques@...e.de>
> > > ---
> > > Changes since v10
> > > - simply remove the "if (len == 0)" short-circuit instead of checking if
> > >   the filesystem implements the syscall.  This is because a filesystem may
> > >   implement it but a particular instance (hint: overlayfs!) may not.
> > > Changes since v9
> > > - the early return from the syscall when len is zero now checks if the
> > >   filesystem is implemented, returning -EOPNOTSUPP if it is not and 0
> > >   otherwise.  Issue reported by test robot.
> > >   (obviously, dropped Amir's Reviewed-by and Olga's Tested-by tags)
> > > Changes since v8
> > > - Simply added Amir's Reviewed-by and Olga's Tested-by
> > > Changes since v7
> > > - set 'ret' to '-EOPNOTSUPP' before the clone 'if' statement so that the
> > >   error returned is always related to the 'copy' operation
> > > Changes since v6
> > > - restored i_sb checks for the clone operation
> > > Changes since v5
> > > - check if ->copy_file_range is NULL before calling it
> > > Changes since v4
> > > - nfsd falls-back to generic_copy_file_range() only *if* it gets -EOPNOTSUPP
> > >   or -EXDEV.
> > > Changes since v3
> > > - dropped the COPY_FILE_SPLICE flag
> > > - kept the f_op's checks early in generic_copy_file_checks, implementing
> > >   Amir's suggestions
> > > - modified nfsd to use generic_copy_file_range()
> > > Changes since v2
> > > - do all the required checks earlier, in generic_copy_file_checks(),
> > >   adding new checks for ->remap_file_range
> > > - new COPY_FILE_SPLICE flag
> > > - don't remove filesystem's fallback to generic_copy_file_range()
> > > - updated commit changelog (and subject)
> > > Changes since v1 (after Amir review)
> > > - restored do_copy_file_range() helper
> > > - return -EOPNOTSUPP if fs doesn't implement CFR
> > > - updated commit description
> > >
> > >  fs/nfsd/vfs.c   |  8 +++++++-
> > >  fs/read_write.c | 52 +++++++++++++++++++++++--------------------------
> > >  2 files changed, 31 insertions(+), 29 deletions(-)
> > >
> > > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > > index 15adf1f6ab21..f54a88b3b4a2 100644
> > > --- a/fs/nfsd/vfs.c
> > > +++ b/fs/nfsd/vfs.c
> > > @@ -569,6 +569,7 @@ __be32 nfsd4_clone_file_range(struct nfsd_file *nf_src, u64 src_pos,
> > >  ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
> > >                              u64 dst_pos, u64 count)
> > >  {
> > > +       ssize_t ret;
> > >
> > >         /*
> > >          * Limit copy to 4MB to prevent indefinitely blocking an nfsd
> > > @@ -579,7 +580,12 @@ ssize_t nfsd_copy_file_range(struct file *src, u64 src_pos, struct file *dst,
> > >          * limit like this and pipeline multiple COPY requests.
> > >          */
> > >         count = min_t(u64, count, 1 << 22);
> > > -       return vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
> > > +       ret = vfs_copy_file_range(src, src_pos, dst, dst_pos, count, 0);
> > > +
> > > +       if (ret == -EOPNOTSUPP || ret == -EXDEV)
> > > +               ret = generic_copy_file_range(src, src_pos, dst, dst_pos,
> > > +                                             count, 0);
> > > +       return ret;
> > >  }
> > >
> > >  __be32 nfsd4_vfs_fallocate(struct svc_rqst *rqstp, struct svc_fh *fhp,
> > > diff --git a/fs/read_write.c b/fs/read_write.c
> > > index 9db7adf160d2..049a2dda29f7 100644
> > > --- a/fs/read_write.c
> > > +++ b/fs/read_write.c
> > > @@ -1395,28 +1395,6 @@ ssize_t generic_copy_file_range(struct file *file_in, loff_t pos_in,
> > >  }
> > >  EXPORT_SYMBOL(generic_copy_file_range);
> > >
> > > -static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
> > > -                                 struct file *file_out, loff_t pos_out,
> > > -                                 size_t len, unsigned int flags)
> > > -{
> > > -       /*
> > > -        * Although we now allow filesystems to handle cross sb copy, passing
> > > -        * a file of the wrong filesystem type to filesystem driver can result
> > > -        * in an attempt to dereference the wrong type of ->private_data, so
> > > -        * avoid doing that until we really have a good reason.  NFS defines
> > > -        * several different file_system_type structures, but they all end up
> > > -        * using the same ->copy_file_range() function pointer.
> > > -        */
> > > -       if (file_out->f_op->copy_file_range &&
> > > -           file_out->f_op->copy_file_range == file_in->f_op->copy_file_range)
> > > -               return file_out->f_op->copy_file_range(file_in, pos_in,
> > > -                                                      file_out, pos_out,
> > > -                                                      len, flags);
> > > -
> > > -       return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> > > -                                      flags);
> > > -}
> > > -
> > >  /*
> > >   * Performs necessary checks before doing a file copy
> > >   *
> > > @@ -1434,6 +1412,25 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
> > >         loff_t size_in;
> > >         int ret;
> > >
> > > +       /*
> > > +        * Although we now allow filesystems to handle cross sb copy, passing
> > > +        * a file of the wrong filesystem type to filesystem driver can result
> > > +        * in an attempt to dereference the wrong type of ->private_data, so
> > > +        * avoid doing that until we really have a good reason.  NFS defines
> > > +        * several different file_system_type structures, but they all end up
> > > +        * using the same ->copy_file_range() function pointer.
> > > +        */
> > > +       if (file_out->f_op->copy_file_range) {
> > > +               if (file_in->f_op->copy_file_range !=
> > > +                   file_out->f_op->copy_file_range)
> > > +                       return -EXDEV;
> > > +       } else if (file_in->f_op->remap_file_range) {
> > > +               if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
> > > +                       return -EXDEV;
> > > +       } else {
> > > +                return -EOPNOTSUPP;
> > > +       }
> > > +
> > >         ret = generic_file_rw_checks(file_in, file_out);
> > >         if (ret)
> > >                 return ret;
> > > @@ -1497,11 +1494,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> > >         if (unlikely(ret))
> > >                 return ret;
> > >
> > > -       if (len == 0)
> > > -               return 0;
> >
> > Can somebody please explain this change to me? Is this an attempt to
> > support "whole" file copy?
>
> No, this was a bug reported in this thread:
>
> https://lore.kernel.org/linux-fsdevel/877dk1zibo.fsf@suse.de/
>
> (I'm also adding back Steve to the Cc: list.)

Ok so this is a problem. As I mentioned a zero size copy means in NFS
copy the whole file.  Current copy_file_range system doesn't have the
same semantics. I don't expect the same semantics exist in other file
systems but, if they do, then perhaps semantics of copy_file_range can
be changed to reflect that. If not, then I would like to put back the
return 0 if len=0 somehow or you need to put it explicitly in all file
systems (or at least in NFS).

>
> Cheers,
> --
> Luís
>
> > I believe previously file systems relied
> > on the fact that they don't need to handle 0 size copy_file_range size
> > call. If this is being changed why not individual implementors (nfs,
> > etc) were modified to keep the same behavior? I mean is CIFS ok with
> > getting count=0 copy_file_range request?
> >
> > In the NFS spec of COPY (copy_file_range), length of 0 means (or could
> > mean) "whole file" copy. While the linux NFS server did put in support
> > for doing "whole file" copy, it's not present before 5.13 in the linux
> > server. It makes it now confusing that a copy of length 0 previously
> > would return 0 and now it could copy whole file.
> > > -
> > >         file_start_write(file_out);
> > >
> > > +       ret = -EOPNOTSUPP;
> > >         /*
> > >          * Try cloning first, this is supported by more file systems, and
> > >          * more efficient if both clone and copy are supported (e.g. NFS).
> > > @@ -1520,9 +1515,10 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> > >                 }
> > >         }
> > >
> > > -       ret = do_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> > > -                               flags);
> > > -       WARN_ON_ONCE(ret == -EOPNOTSUPP);
> > > +       if (file_out->f_op->copy_file_range)
> > > +               ret = file_out->f_op->copy_file_range(file_in, pos_in,
> > > +                                                     file_out, pos_out,
> > > +                                                     len, flags);
> > >  done:
> > >         if (ret > 0) {
> > >                 fsnotify_access(file_in);

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