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

[Amir Goldstein]

On Tue, Feb 23, 2021 at 7:31 PM <dai.ngo@xxxxxxxxxx> wrote:
>
> On 2/23/21 8:57 AM, dai.ngo@xxxxxxxxxx wrote:
>
>
> On 2/23/21 8:47 AM, Amir Goldstein wrote:
>
> 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?
>
>
> If the filesystems support both copy_file_range and map_file_range,
> it passes the check in generic_file_check but it fails with the
> check in vfs_copy_file_range and returns -ENOTSUPPORT (added by
> the v8 patch)
>
> Ok, I misread the code here. If it passes the check in generic_copy_file_checks
> and it fails the sb check in vfs_copy_file_range then it tries copy_file_range
> so it's ok.
>
> I think having the check in both generic_copy_file_checks and vfs_copy_file_range
> making the code hard to read. What's the reason not to do the check only in
> vfs_copy_file_range?
>

You are going in circles.
I already answered that.
Please re-read the entire thread on all patch versions before commenting.

Thanks,
Amir.