linux-kernel - Re: [PATCH v2 29/33] nsfs: support exhaustive file handles

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Message-ID: <qar72lx6pjoygcxefjssrbn6crytpfbpltpeuvplb3mnsle4gm@zpil2qdsn65o>
Date: Mon, 15 Sep 2025 15:26:45 +0200
From: Jan Kara <jack@...e.cz>
To: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>, 
	linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>, 
	Jeff Layton <jlayton@...nel.org>, Mike Yuan <me@...dnzj.com>, 
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>, Lennart Poettering <mzxreary@...inter.de>, 
	Daan De Meyer <daan.j.demeyer@...il.com>, Aleksa Sarai <cyphar@...har.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, 
	Johannes Weiner <hannes@...xchg.org>, Michal Koutný <mkoutny@...e.com>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Chuck Lever <chuck.lever@...cle.com>, linux-nfs@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v2 29/33] nsfs: support exhaustive file handles

On Fri 12-09-25 13:52:52, Christian Brauner 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.
> 
> Reviewed-by: Amir Goldstein <amir73il@...il.com>
> Signed-off-by: Christian Brauner <brauner@...nel.org>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  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 926e2680414e..22765fcab18e 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 = {
> @@ -598,4 +606,6 @@ 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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR