btrfs: allow defrag on a file opened read-only that has rw permissions

Requiring a read-write descriptor conflicts both ways with exec,
returning ETXTBSY whenever you try to defrag a program that's currently
being run, or causing intermittent exec failures on a live system being
defragged.

As defrag doesn't change the file's contents in any way, there's no
reason to consider it a rw operation.  Thus, let's check only whether
the file could have been opened rw.  Such access control is still needed
as currently defrag can use extra disk space, and might trigger bugs.

We return EINVAL when the request is invalid; here it's ok but merely
the user has insufficient privileges.  Thus, the EPERM return value
reflects the error better -- as discussed in the identical case for
dedupe.

According to codesearch.debian.net, no userspace program distinguishes
these values beyond strerror().

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Reviewed-by: David Sterba <dsterba@suse.com>
[ fold the EPERM patch from Adam ]
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 4482cf35522e9304b78fcb668b73b9b4fa534bd0..80fe3c654612be17d3e85b9cafc4bc8ad52a66b4 100644 (file)
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2928,8 +2928,14 @@ static int btrfs_ioctl_defrag(struct file *file, void __user *argp)
                ret = btrfs_defrag_root(root);
                break;
        case S_IFREG:
-               if (!(file->f_mode & FMODE_WRITE)) {
-                       ret = -EINVAL;
+               /*
+                * Note that this does not check the file descriptor for write
+                * access. This prevents defragmenting executables that are
+                * running and allows defrag on files open in read-only mode.
+                */
+               if (!capable(CAP_SYS_ADMIN) &&
+                   inode_permission(inode, MAY_WRITE)) {
+                       ret = -EPERM;
                        goto out;
                }