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

[Amir Goldstein]

On Tue, Feb 23, 2021 at 6:02 PM <dai.ngo@xxxxxxxxxx> wrote:
>
>
> On 2/23/21 7:29 AM, dai.ngo@xxxxxxxxxx wrote:
> >
> > On 2/23/21 2:32 AM, Luis Henriques wrote:
> >> On Mon, Feb 22, 2021 at 08:25:27AM -0800, dai.ngo@xxxxxxxxxx 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@xxxxxxxxxxxx/__;!!GqivPVa7Brio!P1UWThiSkxbjfjFQWNYJmCxGEkiLFyvHjH6cS-G1ZTt1z-TeqwGQgQmi49dC6w$
> >>>> Link:
> >>>> https://urldefense.com/v3/__https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx*BnvW=ZmpsG47CyHFJwnw7zSX6Q@xxxxxxxxxxxxxx/__;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@xxxxxxxxxxxx>
> >>>> Signed-off-by: Luis Henriques <lhenriques@xxxxxxx>
> >>>> ---
> >>>> 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.
> >
> > Yes, this is the different.The NFS code handles both -EOPNOTSUPP and
> > -EXDEVV by doing generic_copy_file_range.  Do any other consumers of
> > vfs_copy_file_range rely on -EXDEV and not -EOPNOTSUPP and which is
> > the correct error code for this case? It seems to me that -EOPNOTSUPP
> > is more appropriate than EXDEV when (sb1 != sb2).
>

EXDEV is the right code for:
filesystem supports the operation but not for sb1 != sb1.

> So with the current patch, for a clone operation across 2 filesystems:
>
>    . if src and dst filesystem support both copy_file_range and
>      map_file_range then the code returns -ENOTSUPPORT.
>

Why do you say that?
Which code are you referring to exactly?
Did you see this behavior in a test?

>    . if the filesystems only support map_file_range then the
>      code returns -EXDEV
>
> This seems confusing, shouldn't only 1 error code returned for this case?
>

>From my read of the code, user will get -EXDEV in both the cases you
listed.

Thanks,
Amir.