fs: skip copy_file_range write limits when there is no data

generic_copy_file_checks() reduces count to zero for a source offset at
or beyond EOF, but still checks the destination write limits. If pos_out
is at RLIMIT_FSIZE, this sends SIGXFSZ and returns -EFBIG even though
there is no data to write.

For example, GNU cat copying a 1024-byte regular file to another regular
file with RLIMIT_FSIZE=1024 copies all bytes and is then killed by its
next copy_file_range() call at EOF. A zero-length request can fail in
the same way.

Check write limits only when count is nonzero, as the ordinary write
checks already do. Keep the other copy validation in place.

Fixes: 96e6e8f4a68d ("vfs: add missing checks to copy_file_range")
Cc: stable@vger.kernel.org
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Link: https://patch.msgid.link/20261026015738.46230-1-dennylin0707@gmail.com
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions