Re: [PATCH] statmount: add flag to retrieve unescaped options

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2024-11-12 at 11:10 +0100, Miklos Szeredi wrote:
> Filesystem options can be retrieved with STATMOUNT_MNT_OPTS, which
> returns a string of comma separated options, where some characters are
> escaped using the \OOO notation.
> 
> Add a new flag, STATMOUNT_OPT_ARRAY, which instead returns the raw
> option values separated with '\0' charaters.
> 
> Since escaped charaters are rare, this inteface is preferable for
> non-libmount users which likley don't want to deal with option
> de-escaping.
> 
> Example code:
> 
> 	if (st->mask & STATMOUNT_OPT_ARRAY) {
> 		const char *opt = st->str + st->opt_array;
> 
> 		for (unsigned int i = 0; i < st->opt_num; i++) {
> 			printf("opt_array[%i]: <%s>\n", i, opt);
> 			opt += strlen(opt) + 1;
> 		}
> 	}
> 

If the options are separated by NULs, how does userland know where to
stop?

At some point we will probably end up adding a new string value that
would go after the opt array, and userland will need some way to
clearly tell where that new string begins and the NUL-terminated
options array ends.

> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> ---
>  fs/namespace.c             | 42 ++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/mount.h |  7 +++++--
>  2 files changed, 47 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 9a4ab1bc8b94..a16f75011610 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -5074,6 +5074,41 @@ static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
>  	return 0;
>  }
>  
> +static int statmount_opt_array(struct kstatmount *s, struct seq_file *seq)
> +{
> +	struct vfsmount *mnt = s->mnt;
> +	struct super_block *sb = mnt->mnt_sb;
> +	size_t start = seq->count;
> +	u32 count = 0;
> +	char *p, *end, *next, *u = seq->buf + start;
> +	int err;
> +
> +       if (!sb->s_op->show_options)
> +               return 0;
> +
> +       err = sb->s_op->show_options(seq, mnt->mnt_root);
> +       if (err)
> +	       return err;
> +
> +       if (unlikely(seq_has_overflowed(seq)))
> +	       return -EAGAIN;
> +
> +       end = seq->buf + seq->count;
> +       *end = '\0';
> +       for (p = u + 1; p < end; p = next + 1) {
> +               next = strchrnul(p, ',');
> +               *next = '\0';
> +               u += string_unescape(p, u, 0, UNESCAPE_OCTAL) + 1;
> +	       count++;
> +	       if (!count)
> +		       return -EOVERFLOW;
> +       }
> +       seq->count = u - 1 - seq->buf;
> +       s->sm.opt_num = count;
> +
> +       return 0;
> +}
> +
>  static int statmount_string(struct kstatmount *s, u64 flag)
>  {
>  	int ret = 0;
> @@ -5099,6 +5134,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
>  		sm->mnt_opts = start;
>  		ret = statmount_mnt_opts(s, seq);
>  		break;
> +	case STATMOUNT_OPT_ARRAY:
> +		sm->opt_array = start;
> +		ret = statmount_opt_array(s, seq);
> +		break;
>  	case STATMOUNT_FS_SUBTYPE:
>  		sm->fs_subtype = start;
>  		statmount_fs_subtype(s, seq);
> @@ -5252,6 +5291,9 @@ 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_OPT_ARRAY)
> +		err = statmount_string(s, STATMOUNT_OPT_ARRAY);
> +
>  	if (!err && s->mask & STATMOUNT_FS_SUBTYPE)
>  		err = statmount_string(s, STATMOUNT_FS_SUBTYPE);
>  
> diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
> index 2b49e9131d77..c0fda4604187 100644
> --- a/include/uapi/linux/mount.h
> +++ b/include/uapi/linux/mount.h
> @@ -154,7 +154,7 @@ struct mount_attr {
>   */
>  struct statmount {
>  	__u32 size;		/* Total size, including strings */
> -	__u32 mnt_opts;		/* [str] Mount options of the mount */
> +	__u32 mnt_opts;		/* [str] Options (comma separated, escaped) */
>  	__u64 mask;		/* What results were written */
>  	__u32 sb_dev_major;	/* Device ID */
>  	__u32 sb_dev_minor;
> @@ -175,7 +175,9 @@ struct statmount {
>  	__u64 mnt_ns_id;	/* ID of the mount namespace */
>  	__u32 fs_subtype;	/* [str] Subtype of fs_type (if any) */
>  	__u32 sb_source;	/* [str] Source string of the mount */
> -	__u64 __spare2[48];
> +	__u32 opt_num;		/* Number of fs options */
> +	__u32 opt_array;	/* [str] Array of nul terminated fs options */
> +	__u64 __spare2[47];
>  	char str[];		/* Variable size part containing strings */
>  };
>  
> @@ -211,6 +213,7 @@ struct mnt_id_req {
>  #define STATMOUNT_MNT_OPTS		0x00000080U	/* Want/got mnt_opts */
>  #define STATMOUNT_FS_SUBTYPE		0x00000100U	/* Want/got fs_subtype */
>  #define STATMOUNT_SB_SOURCE		0x00000200U	/* Want/got sb_source */
> +#define STATMOUNT_OPT_ARRAY		0x00000400U	/* Want/got opt_... */
>  
>  /*
>   * Special @mnt_id values that can be passed to listmount

-- 
Jeff Layton <jlayton@kernel.org>