Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Amir Goldstein]

On Mon, Feb 15, 2021 at 2:21 PM Luis Henriques <lhenriques@xxxxxxx> wrote:
>
> Luis Henriques <lhenriques@xxxxxxx> writes:
>
> > Amir Goldstein <amir73il@xxxxxxxxx> writes:
> >
> >> On Fri, Feb 12, 2021 at 2:40 PM Luis Henriques <lhenriques@xxxxxxx> wrote:
> > ...
> >>> Sure, I just wanted to point out that *maybe* there are other options than
> >>> simply reverting that commit :-)
> >>>
> >>> Something like the patch below (completely untested!) should revert to the
> >>> old behaviour in filesystems that don't implement the CFR syscall.
> >>>
> >>> Cheers,
> >>> --
> >>> Luis
> >>>
> >>> diff --git a/fs/read_write.c b/fs/read_write.c
> >>> index 75f764b43418..bf5dccc43cc9 100644
> >>> --- a/fs/read_write.c
> >>> +++ b/fs/read_write.c
> >>> @@ -1406,8 +1406,11 @@ static ssize_t do_copy_file_range(struct file *file_in, loff_t pos_in,
> >>>                                                        file_out, pos_out,
> >>>                                                        len, flags);
> >>>
> >>> -       return generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> >>> -                                      flags);
> >>> +       if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
> >>> +               return -EXDEV;
> >>> +       else
> >>> +               generic_copy_file_range(file_in, pos_in, file_out, pos_out, len,
> >>> +                                       flags);
> >>>  }
> >>>
> >>
> >> Which kernel is this patch based on?
> >
> > It was v5.11-rc7.
> >
> >> At this point, I am with Dave and Darrick on not falling back to
> >> generic_copy_file_range() at all.
> >>
> >> We do not have proof of any workload that benefits from it and the
> >> above patch does not protect from a wierd use case of trying to copy a file
> >> from sysfs to sysfs.
> >>
> >
> > Ok, cool.  I can post a new patch doing just that.  I guess that function
> > do_copy_file_range() can be dropped in that case.
> >
> >> I am indecisive about what should be done with generic_copy_file_range()
> >> called as fallback from within filesystems.
> >>
> >> I think the wise choice is to not do the fallback in any case, but this is up
> >> to the specific filesystem maintainers to decide.
> >
> > I see what you mean.  You're suggesting to have userspace handle all the
> > -EOPNOTSUPP and -EXDEV errors.  Would you rather have a patch that also
> > removes all the calls to generic_copy_file_range() function?  And that
> > function can also be deleted too, of course.
>
> Here's a first stab at this patch.  Hopefully I didn't forgot anything
> here.  Let me know if you prefer the more conservative approach, i.e. not
> touching any of the filesystems and let them use generic_copy_file_range.
>

I'm fine with this one (modulu my comment below).
CC'ing fuse/cifs/nfs maintainers.
Though I don't think the FS maintainers should mind removing the fallback.
It was added by "us" (64bf5ff58dff "vfs: no fallback for ->copy_file_range()")

> Once everyone agrees on the final solution, I can follow-up with the
> manpages update.
>
> Cheers,
> --
> Luis
>
> From e1b37e80b12601d56f792bd19377d3e5208188ef Mon Sep 17 00:00:00 2001
> From: Luis Henriques <lhenriques@xxxxxxx>
> Date: Fri, 12 Feb 2021 18:03:23 +0000
> Subject: [PATCH] vfs: prevent copy_file_range to copy across devices
>
> Nicolas Boichat reported an issue when trying to use the copy_file_range
> syscall on a tracefs file.  It failed silently because the file content is
> generated on-the-fly (reporting a size of zero) and copy_file_range needs
> to know in advance how much data is present.
>
> This commit effectively reverts 5dae222a5ff0 ("vfs: allow copy_file_range to
> copy across devices").  Now the copy is done only if the filesystems for source
> and destination files are the same and they implement this syscall.

This paragraph is not true and confusing.
This is not a revert and it does not revert the important part of cross-fs copy
for which the original commit was for.
Either rephrase this paragraph or remove it.

>
> Fixes: 5dae222a5ff0 ("vfs: allow copy_file_range to copy across devices")

Please add a Link: to this discussion in lore.

> Cc: Nicolas Boichat <drinkcat@xxxxxxxxxxxx>
> Signed-off-by: Luis Henriques <lhenriques@xxxxxxx>
> ---
>  fs/ceph/file.c     | 21 +++------------
>  fs/cifs/cifsfs.c   |  3 ---
>  fs/fuse/file.c     | 21 +++------------
>  fs/nfs/nfs4file.c  | 20 +++-----------
>  fs/read_write.c    | 65 ++++++++--------------------------------------
>  include/linux/fs.h |  3 ---
>  6 files changed, 20 insertions(+), 113 deletions(-)
>
> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
> index 209535d5b8d3..639bd7bfaea9 100644
> --- a/fs/ceph/file.c
> +++ b/fs/ceph/file.c
> @@ -2261,9 +2261,9 @@ static ssize_t ceph_do_objects_copy(struct ceph_inode_info *src_ci, u64 *src_off
>         return bytes;
>  }
>
> -static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
> -                                     struct file *dst_file, loff_t dst_off,
> -                                     size_t len, unsigned int flags)
> +static ssize_t ceph_copy_file_range(struct file *src_file, loff_t src_off,
> +                                   struct file *dst_file, loff_t dst_off,
> +                                   size_t len, unsigned int flags)
>  {
>         struct inode *src_inode = file_inode(src_file);
>         struct inode *dst_inode = file_inode(dst_file);
> @@ -2456,21 +2456,6 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
>         return ret;
>  }
>
> -static ssize_t ceph_copy_file_range(struct file *src_file, loff_t src_off,
> -                                   struct file *dst_file, loff_t dst_off,
> -                                   size_t len, unsigned int flags)
> -{
> -       ssize_t ret;
> -
> -       ret = __ceph_copy_file_range(src_file, src_off, dst_file, dst_off,
> -                                    len, flags);
> -
> -       if (ret == -EOPNOTSUPP || ret == -EXDEV)
> -               ret = generic_copy_file_range(src_file, src_off, dst_file,
> -                                             dst_off, len, flags);
> -       return ret;
> -}
> -
>  const struct file_operations ceph_file_fops = {
>         .open = ceph_open,
>         .release = ceph_release,
> diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
> index e46da536ed33..8b869cc67443 100644
> --- a/fs/cifs/cifsfs.c
> +++ b/fs/cifs/cifsfs.c
> @@ -1229,9 +1229,6 @@ static ssize_t cifs_copy_file_range(struct file *src_file, loff_t off,
>                                         len, flags);
>         free_xid(xid);
>
> -       if (rc == -EOPNOTSUPP || rc == -EXDEV)
> -               rc = generic_copy_file_range(src_file, off, dst_file,
> -                                            destoff, len, flags);
>         return rc;
>  }
>
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 8cccecb55fb8..0dd703278e49 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -3330,9 +3330,9 @@ static long fuse_file_fallocate(struct file *file, int mode, loff_t offset,
>         return err;
>  }
>
> -static ssize_t __fuse_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)
> +static ssize_t fuse_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)
>  {
>         struct fuse_file *ff_in = file_in->private_data;
>         struct fuse_file *ff_out = file_out->private_data;
> @@ -3439,21 +3439,6 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
>         return err;
>  }
>
> -static ssize_t fuse_copy_file_range(struct file *src_file, loff_t src_off,
> -                                   struct file *dst_file, loff_t dst_off,
> -                                   size_t len, unsigned int flags)
> -{
> -       ssize_t ret;
> -
> -       ret = __fuse_copy_file_range(src_file, src_off, dst_file, dst_off,
> -                                    len, flags);
> -
> -       if (ret == -EOPNOTSUPP || ret == -EXDEV)
> -               ret = generic_copy_file_range(src_file, src_off, dst_file,
> -                                             dst_off, len, flags);
> -       return ret;
> -}
> -
>  static const struct file_operations fuse_file_operations = {
>         .llseek         = fuse_file_llseek,
>         .read_iter      = fuse_file_read_iter,
> diff --git a/fs/nfs/nfs4file.c b/fs/nfs/nfs4file.c
> index 57b3821d975a..60998209e310 100644
> --- a/fs/nfs/nfs4file.c
> +++ b/fs/nfs/nfs4file.c
> @@ -133,9 +133,9 @@ nfs4_file_flush(struct file *file, fl_owner_t id)
>  }
>
>  #ifdef CONFIG_NFS_V4_2
> -static ssize_t __nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
> -                                     struct file *file_out, loff_t pos_out,
> -                                     size_t count, unsigned int flags)
> +static ssize_t nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
> +                                   struct file *file_out, loff_t pos_out,
> +                                   size_t count, unsigned int flags)
>  {
>         struct nfs42_copy_notify_res *cn_resp = NULL;
>         struct nl4_server *nss = NULL;
> @@ -189,20 +189,6 @@ static ssize_t __nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
>         return ret;
>  }
>
> -static ssize_t nfs4_copy_file_range(struct file *file_in, loff_t pos_in,
> -                                   struct file *file_out, loff_t pos_out,
> -                                   size_t count, unsigned int flags)
> -{
> -       ssize_t ret;
> -
> -       ret = __nfs4_copy_file_range(file_in, pos_in, file_out, pos_out, count,
> -                                    flags);
> -       if (ret == -EOPNOTSUPP || ret == -EXDEV)
> -               ret = generic_copy_file_range(file_in, pos_in, file_out,
> -                                             pos_out, count, flags);
> -       return ret;
> -}
> -
>  static loff_t nfs4_file_llseek(struct file *filep, loff_t offset, int whence)
>  {
>         loff_t ret;
> diff --git a/fs/read_write.c b/fs/read_write.c
> index 75f764b43418..87bf9efd7f71 100644
> --- a/fs/read_write.c
> +++ b/fs/read_write.c
> @@ -1358,58 +1358,6 @@ COMPAT_SYSCALL_DEFINE4(sendfile64, int, out_fd, int, in_fd,
>  }
>  #endif
>
> -/**
> - * generic_copy_file_range - copy data between two files
> - * @file_in:   file structure to read from
> - * @pos_in:    file offset to read from
> - * @file_out:  file structure to write data to
> - * @pos_out:   file offset to write data to
> - * @len:       amount of data to copy
> - * @flags:     copy flags
> - *
> - * This is a generic filesystem helper to copy data from one file to another.
> - * It has no constraints on the source or destination file owners - the files
> - * can belong to different superblocks and different filesystem types. Short
> - * copies are allowed.
> - *
> - * This should be called from the @file_out filesystem, as per the
> - * ->copy_file_range() method.
> - *
> - * Returns the number of bytes copied or a negative error indicating the
> - * failure.
> - */
> -
> -ssize_t generic_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)
> -{
> -       return do_splice_direct(file_in, &pos_in, file_out, &pos_out,
> -                               len > MAX_RW_COUNT ? MAX_RW_COUNT : len, 0);
> -}
> -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);

Please do not remove the big comment above - it is there for a reason.

The case of !file_out->f_op->copy_file_range should return -EOPNOTSUPP
because the outcome of -EXDEV for copy to/from the same fs is confusing.

Either leave this helper and only remove generic_copy_file_range() or
open code the same logic (including comment) in the caller.

> -}
> -
>  /*
>   * Performs necessary checks before doing a file copy
>   *
> @@ -1474,6 +1422,14 @@ ssize_t vfs_copy_file_range(struct file *file_in, loff_t pos_in,
>  {
>         ssize_t ret;
>
> +       /*
> +        * Allow the copy only if the filesystems for file_in and file_out are
> +        * the same, and copy_file_range is implemented.
> +        */

This comment is wrong. See the big fat comment above to understand why.
Also this check is in the wrong place because it misses the case of
file_in->f_op->remap_file_range && !file_out->f_op->copy_file_range

As I wrote above either leave the helper do_copy_file_range() or open
code it in its current location.

> +       if (!file_out->f_op->copy_file_range ||
> +           (file_out->f_op->copy_file_range != file_in->f_op->copy_file_range))
> +               return -EXDEV;
> +
>         if (flags != 0)
>                 return -EINVAL;
>
> @@ -1513,8 +1469,9 @@ 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);
> +       ret = file_out->f_op->copy_file_range(file_in, pos_in,
> +                                             file_out, pos_out,
> +                                             len, flags);
>         WARN_ON_ONCE(ret == -EOPNOTSUPP);

Please remove this line.
filesystem ops can now return -EOPNOTSUPP.

Thanks,
Amir.