ext4: only dirty folios when data journaling regular files

fstest generic/388 occasionally reproduces a crash that looks as
follows:

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
Call Trace:
 <TASK>
 ext4_block_zero_page_range+0x30c/0x380 [ext4]
 ext4_truncate+0x436/0x440 [ext4]
 ext4_process_orphan+0x5d/0x110 [ext4]
 ext4_orphan_cleanup+0x124/0x4f0 [ext4]
 ext4_fill_super+0x262d/0x3110 [ext4]
 get_tree_bdev_flags+0x132/0x1d0
 vfs_get_tree+0x26/0xd0
 vfs_cmd_create+0x59/0xe0
 __do_sys_fsconfig+0x4ed/0x6b0
 do_syscall_64+0x82/0x170
 ...

This occurs when processing a symlink inode from the orphan list. The
partial block zeroing code in the truncate path calls
ext4_dirty_journalled_data() -> folio_mark_dirty(). The latter calls
mapping->a_ops->dirty_folio(), but symlink inodes are not assigned an
a_ops vector in ext4, hence the crash.

To avoid this problem, update the ext4_dirty_journalled_data() helper to
only mark the folio dirty on regular files (for which a_ops is
assigned). This also matches the journaling logic in the ext4_symlink()
creation path, where ext4_handle_dirty_metadata() is called directly.

Fixes: d84c9ebdac1e ("ext4: Mark pages with journalled data dirty")
Signed-off-by: Brian Foster <bfoster@redhat.com>
Link: https://patch.msgid.link/20250516173800.175577-1-bfoster@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: stable@kernel.org

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 2f59036df8009da41c5901cd19d9728993891069..ce0632094c50c15608bb69a9471f34c287b56ef0 100644 (file)
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1119,7 +1119,12 @@ int ext4_walk_page_buffers(handle_t *handle, struct inode *inode,
  */
 static int ext4_dirty_journalled_data(handle_t *handle, struct buffer_head *bh)
 {
-       folio_mark_dirty(bh->b_folio);
+       struct folio *folio = bh->b_folio;
+       struct inode *inode = folio->mapping->host;
+
+       /* only regular files have a_ops */
+       if (S_ISREG(inode->i_mode))
+               folio_mark_dirty(folio);
        return ext4_handle_dirty_metadata(handle, NULL, bh);
 }