statmount: add a new supported_mask field

Some of the fields in the statmount() reply can be optional. If the
kernel has nothing to emit in that field, then it doesn't set the flag
in the reply. This presents a problem: There is currently no way to
know what mask flags the kernel supports since you can't always count on
them being in the reply.

Add a new STATMOUNT_SUPPORTED_MASK flag and field that the kernel can
set in the reply. Userland can use this to determine if the fields it
requires from the kernel are supported. This also gives us a way to
deprecate fields in the future, if that should become necessary.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20250206-statmount-v2-1-6ae70a21c2ab@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/namespace.c b/fs/namespace.c
index d470a369d42bc589c7f41e8acf084815a7f0389e..c5ada826d48899b43c6952ddac552c464f7b46da 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -5410,6 +5410,21 @@ static int grab_requested_root(struct mnt_namespace *ns, struct path *root)
        return 0;
 }
 
+/* This must be updated whenever a new flag is added */
+#define STATMOUNT_SUPPORTED (STATMOUNT_SB_BASIC | \
+                            STATMOUNT_MNT_BASIC | \
+                            STATMOUNT_PROPAGATE_FROM | \
+                            STATMOUNT_MNT_ROOT | \
+                            STATMOUNT_MNT_POINT | \
+                            STATMOUNT_FS_TYPE | \
+                            STATMOUNT_MNT_NS_ID | \
+                            STATMOUNT_MNT_OPTS | \
+                            STATMOUNT_FS_SUBTYPE | \
+                            STATMOUNT_SB_SOURCE | \
+                            STATMOUNT_OPT_ARRAY | \
+                            STATMOUNT_OPT_SEC_ARRAY | \
+                            STATMOUNT_SUPPORTED_MASK)
+
 static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
                        struct mnt_namespace *ns)
 {
@@ -5479,9 +5494,17 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
        if (!err && s->mask & STATMOUNT_MNT_NS_ID)
                statmount_mnt_ns_id(s, ns);
 
+       if (!err && s->mask & STATMOUNT_SUPPORTED_MASK) {
+               s->sm.mask |= STATMOUNT_SUPPORTED_MASK;
+               s->sm.supported_mask = STATMOUNT_SUPPORTED;
+       }
+
        if (err)
                return err;
 
+       /* Are there bits in the return mask not present in STATMOUNT_SUPPORTED? */
+       WARN_ON_ONCE(~STATMOUNT_SUPPORTED & s->sm.mask);
+
        return 0;
 }
 

diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index c07008816acae89cbea3087caf50d537d4e78298..c553dc4ba68407ee38c27238e9bdec2ebf5e2457 100644 (file)
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -179,7 +179,8 @@ struct statmount {
        __u32 opt_array;        /* [str] Array of nul terminated fs options */
        __u32 opt_sec_num;      /* Number of security options */
        __u32 opt_sec_array;    /* [str] Array of nul terminated security options */
-       __u64 __spare2[46];
+       __u64 supported_mask;   /* Mask flags that this kernel supports */
+       __u64 __spare2[45];
        char str[];             /* Variable size part containing strings */
 };
 
@@ -217,6 +218,7 @@ struct mnt_id_req {
 #define STATMOUNT_SB_SOURCE            0x00000200U     /* Want/got sb_source */
 #define STATMOUNT_OPT_ARRAY            0x00000400U     /* Want/got opt_... */
 #define STATMOUNT_OPT_SEC_ARRAY                0x00000800U     /* Want/got opt_sec... */
+#define STATMOUNT_SUPPORTED_MASK       0x00001000U     /* Want/got supported mask flags */
 
 /*
  * Special @mnt_id values that can be passed to listmount