On Wed, Sep 10, 2025 at 4:39 PM Christian Brauner <brau...@kernel.org> wrote:
>
> Pidfd file handles are exhaustive meaning they don't require a handle on
> another pidfd to pass to open_by_handle_at() so it can derive the
> filesystem to decode in. Instead it can be derived from the file
> handle itself. The same is possible for namespace file handles.
>
> Signed-off-by: Christian Brauner <brau...@kernel.org>
Reviewed-by: Amir Goldstein <amir7...@gmail.com>
> ---
> fs/fhandle.c | 6 ++++++
> fs/internal.h | 1 +
> fs/nsfs.c | 10 ++++++++++
> include/uapi/linux/fcntl.h | 1 +
> 4 files changed, 18 insertions(+)
>
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 7c236f64cdea..f18c855bb0c2 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -11,6 +11,7 @@
> #include <linux/personality.h>
> #include <linux/uaccess.h>
> #include <linux/compat.h>
> +#include <linux/nsfs.h>
> #include "internal.h"
> #include "mount.h"
>
> @@ -189,6 +190,11 @@ static int get_path_anchor(int fd, struct path *root)
> return 0;
> }
>
> + if (fd == FD_NSFS_ROOT) {
> + nsfs_get_root(root);
> + return 0;
> + }
> +
> return -EBADF;
> }
>
> diff --git a/fs/internal.h b/fs/internal.h
> index 38e8aab27bbd..a33d18ee5b74 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -355,3 +355,4 @@ int anon_inode_getattr(struct mnt_idmap *idmap, const
> struct path *path,
> int anon_inode_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
> struct iattr *attr);
> void pidfs_get_root(struct path *path);
> +void nsfs_get_root(struct path *path);
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index a1585a2f4f03..3c6fcf652633 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -25,6 +25,14 @@
>
> static struct vfsmount *nsfs_mnt;
>
> +static struct path nsfs_root_path = {};
> +
> +void nsfs_get_root(struct path *path)
> +{
> + *path = nsfs_root_path;
> + path_get(path);
> +}
> +
> static long ns_ioctl(struct file *filp, unsigned int ioctl,
> unsigned long arg);
> static const struct file_operations ns_file_operations = {
> @@ -616,4 +624,6 @@ void __init nsfs_init(void)
> if (IS_ERR(nsfs_mnt))
> panic("can't set nsfs up\n");
> nsfs_mnt->mnt_sb->s_flags &= ~SB_NOUSER;
> + nsfs_root_path.mnt = nsfs_mnt;
> + nsfs_root_path.dentry = nsfs_mnt->mnt_root;
> }
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index f291ab4f94eb..3741ea1b73d8 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -111,6 +111,7 @@
> #define PIDFD_SELF_THREAD_GROUP -10001 /* Current thread
> group leader. */
>
> #define FD_PIDFS_ROOT -10002 /* Root of the pidfs
> filesystem */
> +#define FD_NSFS_ROOT -10003 /* Root of the nsfs filesystem
> */
> #define FD_INVALID -10009 /* Invalid file descriptor:
> -10000 - EBADF = -10009 */
>
> /* Generic flags for the *at(2) family of syscalls. */
>
> --
> 2.47.3
>
