[PATCH] tracefs: fix default permissions not being applied on initial mount

[David Carlier]

Commit e4d32142d1de ("tracing: Fix tracefs mount options") moved the
option application from tracefs_fill_super() to tracefs_reconfigure()
called from tracefs_get_tree(). This fixed mount options being ignored
on user-space mounts when the superblock already exists, but introduced
a regression for the initial kernel-internal mount.

On the first mount (via simple_pin_fs during init), sget_fc() transfers
fc->s_fs_info to sb->s_fs_info and sets fc->s_fs_info to NULL. When
tracefs_get_tree() then calls tracefs_reconfigure(), it sees a NULL
fc->s_fs_info and returns early without applying any options. The root
inode keeps mode 0755 from simple_fill_super() instead of the intended
TRACEFS_DEFAULT_MODE (0700).

Furthermore, even on subsequent user-space mounts without an explicit
mode= option, tracefs_apply_options(sb, true) gates the mode behind
fsi->opts & BIT(Opt_mode), which is unset for the defaults. So the
mode is never corrected unless the user explicitly passes mode=0700.

Restore the tracefs_apply_options(sb, false) call in tracefs_fill_super()
to apply default permissions on initial superblock creation, matching
what debugfs does in debugfs_fill_super().

Fixes: e4d32142d1de ("tracing: Fix tracefs mount options")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
---
 fs/tracefs/inode.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
index edfdf139cb02..36058ba5ee48 100644
--- a/fs/tracefs/inode.c
+++ b/fs/tracefs/inode.c
@@ -491,6 +491,7 @@ static int tracefs_fill_super(struct super_block *sb, struct fs_context *fc)
 		return err;
 
 	sb->s_op = &tracefs_super_operations;
+	tracefs_apply_options(sb, false);
 	set_default_d_op(sb, &tracefs_dentry_operations);
 
 	return 0;
-- 
2.53.0