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

[dai.ngo@oracle.com]

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).

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.

   . 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?

-Dai

>
>>
>> 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 :-)
>
> You're not alone, we all try to do the right thing :-)
>
> -Dai
>
>>
>> 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);