On Thu, Feb 18, 2021 at 5:16 PM Luis Henriques [off-list ref] 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://lore.kernel.org/linux-fsdevel/20210212044405.4120619-1-drinkcat@chromium.org/ (local)
Link: https://lore.kernel.org/linux-fsdevel/CANMq1KDZuxir2LM5jOTm0xx+BnvW=ZmpsG47CyHFJwnw7zSX6Q@mail.gmail.com/ (local)
Link: https://lore.kernel.org/linux-fsdevel/20210126135012.1.If45b7cdc3ff707bc1efa17f5366057d60603c45f@changeid/ (local)
Reported-by: Nicolas Boichat <redacted>
Signed-off-by: Luis Henriques <redacted>
---
And v5!  Sorry.  Sure, it makes sense to go through the all the vfs_cfr()
checks first.

You missed my other comment on v4...

not checking NULL copy_file_range case.

Thanks,
Amir.