[PATCH v3 31/33] ext4: allow remounting read-only in the emergency state

[Christian Brauner <brauner@kernel.org>]

__ext4_remount() refuses any change of the read-only state with EROFS
while ext4_emergency_state() is set, i.e. after a shutdown or an error.
Neither sets SB_RDONLY anymore, so a remount to read-only fails and the
filesystem stays read-write. Emergency remount (sysrq u) asks with
SB_FORCE and reconfigure_super() WARNs, a panic with panic_on_warn:

  forced remount of a ext4 fs returned -30

Becoming read-only doesn't need to write anything. So allow it and skip
everything in that branch that would touch the disk: sync_filesystem(),
dquot_suspend(), the s_state update, ext4_mark_recovery_complete() and
the superblock commit without a journal. Going read-write is still
refused.

Note that ext4_emergency_state() only exists since v6.15. Older stable
trees need ext4_forced_shutdown() instead.

Fixes: 95257987a638 ("ext4: drop EXT4_MF_FS_ABORTED flag")
Fixes: d3476f3dad4a ("ext4: don't set SB_RDONLY after filesystem errors")
Cc: stable@vger.kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/ext4/super.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 9cae52471dee9..ae5ba6823c01c 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -6761,11 +6761,16 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 
 	if ((bool)(fc->sb_flags & SB_RDONLY) != sb_rdonly(sb)) {
 		if (ext4_emergency_state(sb)) {
-			err = -EROFS;
-			goto restore_opts;
-		}
-
-		if (fc->sb_flags & SB_RDONLY) {
+			/*
+			 * Going read-only must work for emergency remount
+			 * (sysrq u) but nothing may be written, just flip it.
+			 */
+			if (!(fc->sb_flags & SB_RDONLY)) {
+				err = -EROFS;
+				goto restore_opts;
+			}
+			sb->s_flags |= SB_RDONLY;
+		} else if (fc->sb_flags & SB_RDONLY) {
 			err = sync_filesystem(sb);
 			if (err < 0)
 				goto restore_opts;
@@ -6876,7 +6881,8 @@ static int __ext4_remount(struct fs_context *fc, struct super_block *sb)
 			goto restore_opts;
 	}
 
-	if (sbi->s_journal == NULL && !(old_sb_flags & SB_RDONLY)) {
+	if (sbi->s_journal == NULL && !(old_sb_flags & SB_RDONLY) &&
+	    !ext4_emergency_state(sb)) {
 		err = ext4_commit_super(sb);
 		if (err)
 			goto restore_opts;

-- 
2.53.0