Re: [PATCH RFC v2 15/23] fs: add real_fs to track task's actual fs_struct

[Jann Horn]

On Fri, Mar 6, 2026 at 12:31 AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
> Add a real_fs field to task_struct that always mirrors the fs field.
> This lays the groundwork for distinguishing between a task's permanent
> fs_struct and one that is temporarily overridden via scoped_with_init_fs().
>
> When a kthread temporarily overrides current->fs for path lookup, we
> need to know the original fs_struct for operations like exit_fs() and
> unshare_fs_struct() that must operate on the real, permanent fs.

Note that there are remote accesses to ->fs from procfs, including
(idk if there are more, I didn't look closely):

 - mounts_open_common
 - get_task_root
 - proc_cwd_link

These expect that task_lock() keeps the task_struct::fs pointer
stable, and I don't see anything that prevents them operating on
kthreads.

You should probably ensure that remote accesses to task_struct::fs all
use task_struct::real_fs, just like how there are no remote accesses
to task_struct::cred - that makes logical sense since when userspace
queries a task's file system root/cwd/mounts, what the task is
currently doing probably shouldn't affect the result.

Then you could also change the locking rules such that task_struct::fs
has no locking while task_lock() protects modifications of, and remote
access to, task_struct::real_fs.