fs: add the ability for statmount() to report the fs_subtype

/proc/self/mountinfo prints out the sb->s_subtype after the type. This
is particularly useful for disambiguating FUSE mounts (at least when the
userland driver bothers to set it). Add STATMOUNT_FS_SUBTYPE and claim
one of the __spare2 fields to point to the offset into the str[] array.

Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ian Kent <raven@themaw.net>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20241111-statmount-v4-2-2eaf35d07a80@kernel.org
Acked-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index 23187a41475460eae6c9955f2b050ca90685a016..dbd89fffd919be26e3570e3dbc46f120cacd96bb 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5004,6 +5004,14 @@ static int statmount_fs_type(struct kstatmount *s, struct seq_file *seq)
        return 0;
 }
 
+static void statmount_fs_subtype(struct kstatmount *s, struct seq_file *seq)
+{
+       struct super_block *sb = s->mnt->mnt_sb;
+
+       if (sb->s_subtype)
+               seq_puts(seq, sb->s_subtype);
+}
+
 static void statmount_mnt_ns_id(struct kstatmount *s, struct mnt_namespace *ns)
 {
        s->sm.mask |= STATMOUNT_MNT_NS_ID;
@@ -5040,7 +5048,7 @@ static int statmount_mnt_opts(struct kstatmount *s, struct seq_file *seq)
 
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
-       int ret;
+       int ret = 0;
        size_t kbufsize;
        struct seq_file *seq = &s->seq;
        struct statmount *sm = &s->sm;
@@ -5063,6 +5071,10 @@ static int statmount_string(struct kstatmount *s, u64 flag)
                sm->mnt_opts = start;
                ret = statmount_mnt_opts(s, seq);
                break;
+       case STATMOUNT_FS_SUBTYPE:
+               sm->fs_subtype = start;
+               statmount_fs_subtype(s, seq);
+               break;
        default:
                WARN_ON_ONCE(true);
                return -EINVAL;
@@ -5208,6 +5220,9 @@ 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);
 
@@ -5229,7 +5244,7 @@ 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..2e939dddf9cbabe574dafdb6cff9ad4cf9298a74 100644 (file)
--- 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 fs_subtype */
 
 /*
  * Special @mnt_id values that can be passed to listmount