linux-kernel - Re: [PATCH] fs: add the ability for statmount() to report the fs

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Message-ID: <20241106133715.ellv3pf7ekz34fmi@quack3>
Date: Wed, 6 Nov 2024 14:37:15 +0100
From: Jan Kara <jack@...e.cz>
To: Jeff Layton <jlayton@...nel.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
	Josef Bacik <josef@...icpanda.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: add the ability for statmount() to report the
 fs_subtype

On Wed 06-11-24 08:29:19, Jeff Layton wrote:
> /proc/self/mountinfo prints out the sb->s_subtype after the type. In
> particular, FUSE makes use of this to display the fstype as
> fuse.<subtype>.
> 
> Add STATMOUNT_FS_SUBTYPE and claim one of the __spare2 fields to point
> to the offset into the str[] array. The STATMOUNT_FS_SUBTYPE will only
> be set in the return mask if there is a subtype associated with the
> mount.
> 
> Signed-off-by: Jeff Layton <jlayton@...nel.org>

Looks good to me. I'm just curious: Do you have any particular user that is
interested in getting subtype from statmount(2)?

								Honza

> ---
>  fs/namespace.c             | 20 +++++++++++++++++++-
>  include/uapi/linux/mount.h |  5 ++++-
>  2 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ba77ce1c6788dfe461814b5826fcbb3aab68fad4..5f2fb692449a9c0a15b60549fb9f7bedd10f1f3d 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -5006,6 +5006,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
>  	return 0;
>  }
>  
> +static int statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
> +{
> +	struct super_block *sb = s->mnt->mnt_sb;
> +
> +	seq_puts(seq, sb->s_subtype);
> +	return 0;
> +}
> +
>  static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
>  {
>  	s->sm.mask |= STATMOUNT_MNT_NS_ID;
> @@ -5064,6 +5072,13 @@ static int statmount_string(struct kstatmount *s, u64 flag)
>  		sm->mnt_opts = seq->count;
>  		ret = statmount_mnt_opts(s, seq);
>  		break;
> +	case STATMOUNT_FS_SUBTYPE:
> +		/* ignore if no s_subtype */
> +		if (!s->mnt->mnt_sb->s_subtype)
> +			return 0;
> +		sm->fs_subtype = seq->count;
> +		ret = statmount_fs_subtype(s, seq);
> +		break;
>  	default:
>  		WARN_ON_ONCE(true);
>  		return -EINVAL;
> @@ -5203,6 +5218,9 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
>  	if (!err && s->mask & STATMOUNT_MNT_OPTS)
>  		err = statmount_string(s, STATMOUNT_MNT_OPTS);
>  
> +	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
> +		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
> +
>  	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
>  		statmount_mnt_ns_id(s, ns);
>  
> @@ -5224,7 +5242,7 @@ static inline bool retry_statmount(const long ret, size_t *seq_size)
>  }
>  
>  #define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT | \
> -			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS)
> +			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | STATMOUNT_FS_SUBTYPE)
>  
>  static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
>  			      struct statmount __user *buf, size_t bufsize,
> diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> index 225bc366ffcbf0319929e2f55f1fbea88e4d7b81..fa206fb56b3b25cf80f7d430e1b6bab19c3220e4 100644
> --- a/include/uapi/linux/mount.h
> +++ b/include/uapi/linux/mount.h
> @@ -173,7 +173,9 @@ struct statmount {
>  	__u32 mnt_root;		/* [str] Root of mount relative to root of fs */
>  	__u32 mnt_point;	/* [str] Mountpoint relative to current root */
>  	__u64 mnt_ns_id;	/* ID of the mount namespace */
> -	__u64 __spare2[49];
> +	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
> +	__u32 __spare1[1];
> +	__u64 __spare2[48];
>  	char str[];		/* Variable size part containing strings */
>  };
>  
> @@ -207,6 +209,7 @@ struct mnt_id_req {
>  #define STATMOUNT_FS_TYPE		0x00000020U	/* Want/got fs_type */
>  #define STATMOUNT_MNT_NS_ID		0x00000040U	/* Want/got mnt_ns_id */
>  #define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
> +#define STATMOUNT_FS_SUBTYPE		0x00000100U	/* Want/got subtype */
>  
>  /*
>   * Special @mnt_id values that can be passed to listmount
> 
> ---
> base-commit: 26213e1a6caa5a7f508b919059b0122b451f4dfe
> change-id: 20241106-statmount-3f91a7ed75fa
> 
> Best regards,
> -- 
> Jeff Layton <jlayton@...nel.org>
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR