linux/copy_file_range: properly request a fallback copy on Linux <5.3

Before Linux 5.3, the filesystem's copy_file_range handler had to signal
back to the kernel that we can't fulfill the request and it should
fallback to a content copy. This is done by returning -EOPNOTSUPP.

This commit converts the EXDEV return from zfs_clone_range to
EOPNOTSUPP, to force the kernel to fallback for all the valid reasons it
might be unable to clone. Without it the copy_file_range() syscall will
return EXDEV to userspace, breaking its semantics.

Add test for copy_file_range fallbacks.  copy_file_range should always
fallback to a content copy whenever ZFS can't service the request with
cloning.

Reviewed-by: Brian Atkinson <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Kay Pedersen <[email protected]>
Signed-off-by: Rob Norris <[email protected]>
Closes #15131 

1 files changed, 7 insertions, 0 deletions

diff --git a/module/os/linux/zfs/zpl_file_range.c b/module/os/linux/zfs/zpl_file_range.c
index 18efebfc1..72384b638 100644
--- a/module/os/linux/zfs/zpl_file_range.c
+++ b/module/os/linux/zfs/zpl_file_range.c
@@ -106,6 +106,13 @@ zpl_copy_file_range(struct file *src_file, loff_t src_off,
 	if (ret == -EOPNOTSUPP || ret == -EXDEV)
 		ret = generic_copy_file_range(src_file, src_off, dst_file,
 		    dst_off, len, flags);
+#else
+	/*
+	 * Before Linux 5.3 the filesystem has to return -EOPNOTSUPP to signal
+	 * to the kernel that it should fallback to a content copy.
+	 */
+	if (ret == -EXDEV)
+		ret = -EOPNOTSUPP;
 #endif /* HAVE_VFS_GENERIC_COPY_FILE_RANGE */
 
 	return (ret);