(Only btrfs currently implements dedupe_file_range.)

Instead of checking the mode of the file descriptor, let's check whether
it could have been opened rw.  This allows fixing failures when deduping
a live system: anyone trying to exec a file currently being deduped gets
ETXTBSY.

Issuing this ioctl on a ro file was already allowed for root/cap.

Signed-off-by: Adam Borowski <kilob...@angband.pl>
---
 fs/read_write.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/read_write.c b/fs/read_write.c
index cf377cf..6c414d8 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -1736,7 +1736,7 @@ int vfs_dedupe_file_range(struct file *file, struct 
file_dedupe_range *same)
 
                if (info->reserved) {
                        info->status = -EINVAL;
-               } else if (!(is_admin || (dst_file->f_mode & FMODE_WRITE))) {
+               } else if (!(is_admin || !inode_permission(dst, MAY_WRITE))) {
                        info->status = -EINVAL;
                } else if (file->f_path.mnt != dst_file->f_path.mnt) {
                        info->status = -EXDEV;
-- 
2.8.1

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