Re: [PATCH v8] vfs: fix copy_file_range regression in cross-fs copies

Luis Henriques
2021-02-23T10:32:16Z

On Mon, Feb 22, 2021 at 08:25:27AM -0800, dai.ngo@oracle.com wrote:
>
> On 2/22/21 2:24 AM, Luis Henriques 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://urldefense.com/v3/__https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/__;!!GqivPVa7Brio!P1UWThiSkxbjfjFQWNYJmCxGEkiLFyvHjH6cS-G1ZTt1z-TeqwGQgQmi49dC6w$
> > Link: https://urldefense.com/v3/__https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx*BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/__;Kw!!GqivPVa7Brio!P1UWThiSkxbjfjFQWNYJmCxGEkiLFyvHjH6cS-G1ZTt1z-TeqwGQgQmgCmMHzA$
> > Link: https://urldefense.com/v3/__https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/__;!!GqivPVa7Brio!P1UWThiSkxbjfjFQWNYJmCxGEkiLFyvHjH6cS-G1ZTt1z-TeqwGQgQmzqItkrQ$
> > Reported-by: Nicolas Boichat <drinkcat@chromium.org>
> > Signed-off-by: Luis Henriques <lhenriques@suse.de>
> > ---
> > 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 | 49 ++++++++++++++++++++++++-------------------------
> > 2 files changed, 31 insertions(+), 26 deletions(-)
> >
> > diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> > index 04937e51de56..23dab0fa9087 100644
> > --- a/fs/nfsd/vfs.c
> > +++ b/fs/nfsd/vfs.c
> > @@ -568,6 +568,7 @@ __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
> > @@ -578,7 +579,12 @@ 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 75f764b43418..5a26297fd410 100644
> > --- a/fs/read_write.c
> > +++ b/fs/read_write.c
> > @@ -1388,28 +1388,6 @@ 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
> > *
> > @@ -1427,6 +1405,25 @@ 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;
>
> I think this check is redundant, it's done in vfs_copy_file_range.
> If this check is removed then the else clause below should be removed
> also. Once this check and the else clause are removed then might as
> well move the the check of copy_file_range from here to vfs_copy_file_range.
>

I don't think it's really redundant, although I agree is messy due to the
fact we try to clone first instead of copying them.

So, in the clone path, this is the only place where we return -EXDEV if:

1) we don't have ->copy_file_range *and*
2) we have ->remap_file_range but the i_sb are different.

The check in vfs_copy_file_range() is only executed if:

1) we have *valid* ->copy_file_range ops and/or
2) we have *valid* ->remap_file_range

So... if we remove the check in generic_copy_file_checks() as you suggest
and:
- we don't have ->copy_file_range,
- we have ->remap_file_range but
- the i_sb are different

we'll return the -EOPNOTSUPP (the one set in line "ret = -EOPNOTSUPP;" in
function vfs_copy_file_range() ) instead of -EXDEV.

But I may have got it all wrong. I've looked so many times at this code
that I'm probably useless at finding problems in it :-)

Cheers,
--
Luís

> -Dai
>
> > + } else {
> > + return -EOPNOTSUPP;
> > + }
> > +
> > ret = generic_file_rw_checks(file_in, file_out);
> > if (ret)
> > return ret;
> > @@ -1495,6 +1492,7 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
> > 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).
> > @@ -1513,9 +1511,10 @@ 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);