Re: [PATCH v3] vfs: fix copy_file_range regression in cross-fs copies

[Nicolas Boichat <drinkcat@chromium.org>]

On Thu, Feb 18, 2021 at 1:25 AM Luis Henriques <lhenriques@suse.de> 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-filesystems copy restrictions that existed
> prior to commit 5dae222a5ff0 ("vfs: allow copy_file_range to copy across
> devices").

Note that you also fix intra-filesystem copy_file_range on these
generated filesystems. This is IMHO great, but needs to be mentioned
in the commit message.

>  It also introduces a flag (COPY_FILE_SPLICE) that can be used
> by filesystems calling directly into the vfs copy_file_range to override
> these restrictions.  Right now, only NFS needs to set this flag.
>
> Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")

So technically this fixes something much older, presumably ever since
copy_file_range was introduced.

> Link: https://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/
> Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/
> Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/
> Reported-by: Nicolas Boichat <drinkcat@chromium.org>

Tested-by: Nicolas Boichat <drinkcat@chromium.org>
but I guess you should not add to the next revision, I'll keep testing
further revisions ,-)

> Signed-off-by: Luis Henriques <lhenriques@suse.de>
> ---
> Ok, I've tried to address all the issues and comments.  Hopefully this v3
> is a bit closer to the final fix.
>
> 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      |  3 ++-
>  fs/read_write.c    | 44 +++++++++++++++++++++++++++++++++++++++++---
>  include/linux/fs.h |  7 +++++++
>  3 files changed, 50 insertions(+), 4 deletions(-)
>
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 04937e51de56..14e55822c223 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -578,7 +578,8 @@ 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);
> +       return vfs_copy_file_range(src, src_pos, dst, dst_pos, count,
> +                                  COPY_FILE_SPLICE);
>  }
>
>  __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..40a16003fb05 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1410,6 +1410,33 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
>                                        flags);
>  }
>
> +/*
> + * This helper function checks whether copy_file_range can actually be used,
> + * depending on the source and destination filesystems being the same.
> + *
> + * In-kernel callers may set COPY_FILE_SPLICE to override these checks.
> + */
> +static int fops_copy_file_checks(struct file *file_in, struct file *file_out,

fops_copy_file_range_checks ?

> +                                unsigned int flags)
> +{
> +       if (WARN_ON_ONCE(flags & ~COPY_FILE_SPLICE))
> +               return -EINVAL;
> +
> +       if (flags & COPY_FILE_SPLICE)
> +               return 0;
> +       /*
> +        * We got here from userspace, so forbid copies if copy_file_range isn't
> +        * implemented or if we're doing a cross-fs copy.
> +        */
> +       if (!file_out->f_op->copy_file_range)
> +               return -EOPNOTSUPP;

After this is merged, should this be added as an error code to the man page?

> +       else if (file_out->f_op->copy_file_range !=
> +                file_in->f_op->copy_file_range)

Just note, this could be a cross-fs copy (just not a cross-fs_type copy).

> +               return -EXDEV;
> +
> +       return 0;
> +}
> +
>  /*
>   * Performs necessary checks before doing a file copy
>   *
> @@ -1427,6 +1454,14 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
>         loff_t size_in;
>         int ret;
>
> +       /* Only check f_ops if we're not trying to clone */
> +       if (!file_in->f_op->remap_file_range ||
> +           (file_inode(file_in)->i_sb == file_inode(file_out)->i_sb)) {
> +               ret = fops_copy_file_checks(file_in, file_out, flags);
> +               if (ret)
> +                       return ret;
> +       }
> +
>         ret = generic_file_rw_checks(file_in, file_out);
>         if (ret)
>                 return ret;
> @@ -1474,9 +1509,6 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>  {
>         ssize_t ret;
>
> -       if (flags != 0)
> -               return -EINVAL;
> -
>         ret = generic_copy_file_checks(file_in, pos_in, file_out, pos_out, &len,
>                                        flags);
>         if (unlikely(ret))
> @@ -1511,6 +1543,9 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>                         ret = cloned;
>                         goto done;
>                 }
> +               ret = fops_copy_file_checks(file_in, file_out, flags);
> +               if (ret)
> +                       return ret;
>         }
>
>         ret = do_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> @@ -1543,6 +1578,9 @@ SYSCALL_DEFINE6(copy_file_range, int, fd_in, loff_t __user *, off_in,
>         struct fd f_out;
>         ssize_t ret = -EBADF;
>
> +       if (flags != 0)
> +               return -EINVAL;
> +
>         f_in = fdget(fd_in);
>         if (!f_in.file)
>                 goto out2;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index fd47deea7c17..6f604926d955 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1815,6 +1815,13 @@ struct dir_context {
>   */
>  #define REMAP_FILE_ADVISORY            (REMAP_FILE_CAN_SHORTEN)
>
> +/*
> + * This flag control the behavior of copy_file_range from internal (kernel)
> + * users.  It can be used to override the policy of forbidding copies when
> + * source and destination filesystems are different.
> + */
> +#define COPY_FILE_SPLICE               (1 << 0)

nit: BIT(0) ?


> +
>  struct iov_iter;
>
>  struct file_operations {