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

[dai.ngo@oracle.com]

On 2/23/21 9:33 AM, Amir Goldstein wrote:
> On Tue, Feb 23, 2021 at 7:31 PM <dai.ngo@oracle.com> wrote:
>> On 2/23/21 8:57 AM, dai.ngo@oracle.com wrote:
>>
>>
>> On 2/23/21 8:47 AM, Amir Goldstein wrote:
>>
>> On Tue, Feb 23, 2021 at 6:02 PM <dai.ngo@oracle.com> wrote:
>>
>>
>> On 2/23/21 7:29 AM, dai.ngo@oracle.com wrote:
>>
>> On 2/23/21 2:32 AM, Luis Henriques wrote:
>>
>> 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.
>>
>> 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.

I'm fine with the patch as it is, as long as it does not break NFS.

I just think it's easier to read if the checks are done in
vfs_copy_file_range such as:

@@ -1495,6 +1473,11 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
  
         file_start_write(file_out);
  
+       if (file_out->f_op->copy_file_range == NULL &&
+           file_in->f_op->remap_file_range == NULL)
+               return -EOPNOTSUPP;     /* not sure this error is needed */
+
+       ret = -EXDEV;
         /*
          * 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 +1496,10 @@ 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 &&
+           file_out->f_op->copy_file_range == file_in->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);

Thanks,
-Dai

>
> Thanks,
> Amir.