[PATCH 07/13] fsinfo: Allow mount topology and propagation info to be retrieved [ver #19]

From: David Howells
Date: Wed Mar 18 2020 - 11:09:53 EST

Add a couple of attributes to allow information about the mount topology
and propagation to be retrieved:

(1) FSINFO_ATTR_MOUNT_TOPOLOGY.

Information about a mount's parentage in the mount topology tree and
its propagation attributes.

This has to be collected with the VFS namespace lock held, so it's
separate from FSINFO_ATTR_MOUNT_INFO. The topology change counter
that a subsequent patch will export can be used to work out from the
cheaper _INFO attribute as to whether the more expensive _TOPOLOGY
attribute needs requerying.

MOUNT_PROPAGATION_* flags are added to linux/mount.h for UAPI
consumption. At some point a mount_setattr() system call needs to be
added.

(2) FSINFO_ATTR_MOUNT_CHILDREN.

Information about a mount's children in the mount topology tree.

This is formatted as an array of structures, one for each child and
capped with one for the argument mount (checked after listing all the
children). Each element contains the static IDs of the respective
mount object along with a sum of its change attributes.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

fs/fsinfo.c | 2 +
fs/internal.h | 2 +
fs/namespace.c | 91 +++++++++++++++++++++++++++++++++++++++++++
include/uapi/linux/fsinfo.h | 27 +++++++++++++
include/uapi/linux/mount.h | 10 ++++-
samples/vfs/test-fsinfo.c | 38 ++++++++++++++++++
6 files changed, 169 insertions(+), 1 deletion(-)

diff --git a/fs/fsinfo.c b/fs/fsinfo.c
index a08b172f71d2..eccea3b1579a 100644
--- a/fs/fsinfo.c
+++ b/fs/fsinfo.c
@@ -280,9 +280,11 @@ static const struct fsinfo_attribute fsinfo_common_attributes[] = {
FSINFO_VSTRUCT_N(FSINFO_ATTR_FSINFO_ATTRIBUTE_INFO, (void *)123UL),

FSINFO_VSTRUCT (FSINFO_ATTR_MOUNT_INFO, fsinfo_generic_mount_info),
+ FSINFO_VSTRUCT (FSINFO_ATTR_MOUNT_TOPOLOGY, fsinfo_generic_mount_topology),
FSINFO_STRING (FSINFO_ATTR_MOUNT_PATH, fsinfo_generic_seq_read),
FSINFO_STRING (FSINFO_ATTR_MOUNT_POINT, fsinfo_generic_mount_point),
FSINFO_STRING (FSINFO_ATTR_MOUNT_POINT_FULL, fsinfo_generic_mount_point_full),
+ FSINFO_LIST (FSINFO_ATTR_MOUNT_CHILDREN, fsinfo_generic_mount_children),
{}
};

diff --git a/fs/internal.h b/fs/internal.h
index 68e300a1e9a3..6a30320ea2f8 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -100,8 +100,10 @@ extern void dissolve_on_fput(struct vfsmount *);
extern int lookup_mount_object(struct path *, int, struct path *);
extern int fsinfo_generic_mount_source(struct path *, struct fsinfo_context *);
extern int fsinfo_generic_mount_info(struct path *, struct fsinfo_context *);
+extern int fsinfo_generic_mount_topology(struct path *, struct fsinfo_context *);
extern int fsinfo_generic_mount_point(struct path *, struct fsinfo_context *);
extern int fsinfo_generic_mount_point_full(struct path *, struct fsinfo_context *);
+extern int fsinfo_generic_mount_children(struct path *, struct fsinfo_context *);

/*
* fs_struct.c
diff --git a/fs/namespace.c b/fs/namespace.c
index 483fbbde5c28..61b110149fc5 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -4187,6 +4187,53 @@ int fsinfo_generic_mount_info(struct path *path, struct fsinfo_context *ctx)
return sizeof(*p);
}

+/*
+ * Retrieve information about the topology at the nominated mount and
+ * its propogation attributes.
+ */
+int fsinfo_generic_mount_topology(struct path *path, struct fsinfo_context *ctx)
+{
+ struct fsinfo_mount_topology *p = ctx->buffer;
+ struct mount *m;
+ struct path root;
+
+ get_fs_root(current->fs, &root);
+
+ namespace_lock();
+
+ m = real_mount(path->mnt);
+
+ p->parent_id = m->mnt_parent->mnt_id;
+
+ if (path->mnt == root.mnt) {
+ p->parent_id = m->mnt_id;
+ } else {
+ rcu_read_lock();
+ if (!are_paths_connected(&root, path))
+ p->parent_id = m->mnt_id;
+ rcu_read_unlock();
+ }
+
+ if (IS_MNT_SHARED(m)) {
+ p->group_id = m->mnt_group_id;
+ p->propagation |= MOUNT_PROPAGATION_SHARED;
+ }
+ if (IS_MNT_SLAVE(m)) {
+ int master = m->mnt_master->mnt_group_id;
+ int dom = get_dominating_id(m, &root);
+ p->master_id = master;
+ if (dom && dom != master)
+ p->from_id = dom;
+ p->propagation |= MOUNT_PROPAGATION_SLAVE;
+ }
+ if (IS_MNT_UNBINDABLE(m))
+ p->propagation |= MOUNT_PROPAGATION_UNBINDABLE;
+
+ namespace_unlock();
+ path_put(&root);
+ return sizeof(*p);
+}
+
/*
* Return the path of this mount relative to its parent and clipped to
* the current chroot.
@@ -4260,4 +4307,48 @@ int fsinfo_generic_mount_point_full(struct path *path, struct fsinfo_context *ct
return (ctx->buffer + ctx->buf_size) - p;
}

+/*
+ * Store a mount record into the fsinfo buffer.
+ */
+static void fsinfo_store_mount(struct fsinfo_context *ctx, const struct mount *p)
+{
+ struct fsinfo_mount_child record = {};
+ unsigned int usage = ctx->usage;
+
+ if (ctx->usage >= INT_MAX)
+ return;
+ ctx->usage = usage + sizeof(record);
+
+ if (ctx->buffer && ctx->usage <= ctx->buf_size) {
+ record.mnt_unique_id = p->mnt_unique_id;
+ record.mnt_id = p->mnt_id;
+ memcpy(ctx->buffer + usage, &record, sizeof(record));
+ }
+}
+
+/*
+ * Return information about the submounts relative to path.
+ */
+int fsinfo_generic_mount_children(struct path *path, struct fsinfo_context *ctx)
+{
+ struct mount *m, *child;
+
+ m = real_mount(path->mnt);
+
+ read_seqlock_excl(&mount_lock);
+
+ list_for_each_entry_rcu(child, &m->mnt_mounts, mnt_child) {
+ if (child->mnt_parent != m)
+ continue;
+ fsinfo_store_mount(ctx, child);
+ }
+
+ /* End the list with a copy of the parameter mount's details so that
+ * userspace can quickly check for changes.
+ */
+ fsinfo_store_mount(ctx, m);
+ read_sequnlock_excl(&mount_lock);
+ return ctx->usage;
+}
+
#endif /* CONFIG_FSINFO */
diff --git a/include/uapi/linux/fsinfo.h b/include/uapi/linux/fsinfo.h
index df96301dc612..9410e320d824 100644
--- a/include/uapi/linux/fsinfo.h
+++ b/include/uapi/linux/fsinfo.h
@@ -35,6 +35,8 @@
#define FSINFO_ATTR_MOUNT_PATH 0x201 /* Bind mount/superblock path (string) */
#define FSINFO_ATTR_MOUNT_POINT 0x202 /* Relative path of mount in parent (string) */
#define FSINFO_ATTR_MOUNT_POINT_FULL 0x203 /* Absolute path of mount (string) */
+#define FSINFO_ATTR_MOUNT_TOPOLOGY 0x204 /* Mount object topology */
+#define FSINFO_ATTR_MOUNT_CHILDREN 0x205 /* Children of this mount (list) */

/*
* Optional fsinfo() parameter structure.
@@ -102,6 +104,31 @@ struct fsinfo_mount_info {

#define FSINFO_ATTR_MOUNT_INFO__STRUCT struct fsinfo_mount_info

+/*
+ * Information struct for fsinfo(FSINFO_ATTR_MOUNT_TOPOLOGY).
+ */
+struct fsinfo_mount_topology {
+ __u32 parent_id; /* Parent mount identifier */
+ __u32 group_id; /* Mount group ID */
+ __u32 master_id; /* Slave master group ID */
+ __u32 from_id; /* Slave propagated from ID */
+ __u32 propagation; /* MOUNT_PROPAGATION_* flags */
+};
+
+#define FSINFO_ATTR_MOUNT_TOPOLOGY__STRUCT struct fsinfo_mount_topology
+
+/*
+ * Information struct element for fsinfo(FSINFO_ATTR_MOUNT_CHILDREN).
+ * - An extra element is placed on the end representing the parent mount.
+ */
+struct fsinfo_mount_child {
+ __u64 mnt_unique_id; /* Kernel-lifetime unique mount ID */
+ __u32 mnt_id; /* Mount identifier (use with AT_FSINFO_MOUNTID_PATH) */
+ __u32 __padding[1];
+};
+
+#define FSINFO_ATTR_MOUNT_CHILDREN__STRUCT struct fsinfo_mount_child
+
/*
* Information struct for fsinfo(FSINFO_ATTR_STATFS).
* - This gives extended filesystem information.
diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index 96a0240f23fe..c18b21de3fdd 100644
--- a/include/uapi/linux/mount.h
+++ b/include/uapi/linux/mount.h
@@ -105,7 +105,7 @@ enum fsconfig_command {
#define FSMOUNT_CLOEXEC 0x00000001

/*
- * Mount attributes.
+ * Mount object attributes (these are separate to filesystem attributes).
*/
#define MOUNT_ATTR_RDONLY 0x00000001 /* Mount read-only */
#define MOUNT_ATTR_NOSUID 0x00000002 /* Ignore suid and sgid bits */
@@ -117,4 +117,12 @@ enum fsconfig_command {
#define MOUNT_ATTR_STRICTATIME 0x00000020 /* - Always perform atime updates */
#define MOUNT_ATTR_NODIRATIME 0x00000080 /* Do not update directory access times */

+/*
+ * Mount object propagation attributes.
+ */
+#define MOUNT_PROPAGATION_UNBINDABLE 0x00000001 /* Mount is unbindable */
+#define MOUNT_PROPAGATION_SLAVE 0x00000002 /* Mount is slave */
+#define MOUNT_PROPAGATION_PRIVATE 0x00000000 /* Mount is private (ie. not shared) */
+#define MOUNT_PROPAGATION_SHARED 0x00000004 /* Mount is shared */
+
#endif /* _UAPI_LINUX_MOUNT_H */
diff --git a/samples/vfs/test-fsinfo.c b/samples/vfs/test-fsinfo.c
index b23d0d56988f..762ab4517cd9 100644
--- a/samples/vfs/test-fsinfo.c
+++ b/samples/vfs/test-fsinfo.c
@@ -299,6 +299,42 @@ static void dump_fsinfo_generic_mount_info(void *reply, unsigned int size)
printf("\tattr : %x\n", r->attr);
}

+static void dump_fsinfo_generic_mount_topology(void *reply, unsigned int size)
+{
+ struct fsinfo_mount_topology *r = reply;
+
+ printf("\n");
+ printf("\tparent : %x\n", r->parent_id);
+ printf("\tgroup : %x\n", r->group_id);
+ printf("\tmaster : %x\n", r->master_id);
+ printf("\tfrom : %x\n", r->from_id);
+ printf("\tpropag : %x\n", r->propagation);
+}
+
+static void dump_fsinfo_generic_mount_children(void *reply, unsigned int size)
+{
+ struct fsinfo_mount_child *r = reply;
+ ssize_t mplen;
+ char path[32], *mp;
+
+ struct fsinfo_params params = {
+ .flags = FSINFO_FLAGS_QUERY_MOUNT,
+ .request = FSINFO_ATTR_MOUNT_POINT,
+ };
+
+ if (!list_last) {
+ sprintf(path, "%u", r->mnt_id);
+ mplen = get_fsinfo(path, "FSINFO_ATTR_MOUNT_POINT", &params, (void **)&mp);
+ if (mplen < 0)
+ mp = "-";
+ } else {
+ mp = "<this>";
+ }
+
+ printf("%8x %16llx %s\n",
+ r->mnt_id, (unsigned long long)r->mnt_unique_id, mp);
+}
+
static void dump_string(void *reply, unsigned int size)
{
char *s = reply, *p;
@@ -377,9 +413,11 @@ static const struct fsinfo_attribute fsinfo_attributes[] = {
FSINFO_LIST (FSINFO_ATTR_FSINFO_ATTRIBUTES, fsinfo_meta_attributes),

FSINFO_VSTRUCT (FSINFO_ATTR_MOUNT_INFO, fsinfo_generic_mount_info),
+ FSINFO_VSTRUCT (FSINFO_ATTR_MOUNT_TOPOLOGY, fsinfo_generic_mount_topology),
FSINFO_STRING (FSINFO_ATTR_MOUNT_PATH, string),
FSINFO_STRING_N (FSINFO_ATTR_MOUNT_POINT, string),
FSINFO_STRING_N (FSINFO_ATTR_MOUNT_POINT_FULL, string),
+ FSINFO_LIST (FSINFO_ATTR_MOUNT_CHILDREN, fsinfo_generic_mount_children),
{}
};