[PATCH] fs: skip copy_file_range write limits when there is no data

[Hungyu Lin]

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@xxxxxxxxxxxxxxx
Signed-off-by: Hungyu Lin <dennylin0707@xxxxxxxxx>
---
 fs/read_write.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index 4da846a2bf17..c05ad2c42e75 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1531,9 +1531,11 @@ static int generic_copy_file_checks(struct file *file_in, loff_t pos_in,
 	else
 		count = min(count, size_in - (uint64_t)pos_in);
 
-	ret = generic_write_check_limits(file_out, pos_out, &count);
-	if (ret)
-		return ret;
+	if (count) {
+		ret = generic_write_check_limits(file_out, pos_out, &count);
+		if (ret)
+			return ret;
+	}
 
 	/* Don't allow overlapped copying within the same file. */
 	if (inode_in == inode_out &&
-- 
2.43.0