statmount: allow to retrieve idmappings

This adds the STATMOUNT_MNT_UIDMAP and STATMOUNT_MNT_GIDMAP options.
It allows the retrieval of idmappings via statmount().

Currently it isn't possible to figure out what idmappings are applied to
an idmapped mount. This information is often crucial. Before statmount()
the only realistic options for an interface like this would have been to
add it to /proc/<pid>/fdinfo/<nr> or to expose it in
/proc/<pid>/mountinfo. Both solution would have been pretty ugly and
would've shown information that is of strong interest to some
application but not all. statmount() is perfect for this.

The idmappings applied to an idmapped mount are shown relative to the
caller's user namespace. This is the most useful solution that doesn't
risk leaking information or confuse the caller.

For example, an idmapped mount might have been created with the
following idmappings:

mount --bind -o X-mount.idmap="0:10000:1000 2000:2000:1 3000:3000:1" /srv /opt

Listing the idmappings through statmount() in the same context shows:

mnt_id: 2147485088
mnt_parent_id: 2147484816
fs_type: btrfs
mnt_root: /srv
mnt_point: /opt
mnt_opts: ssd,discard=async,space_cache=v2,subvolid=5,subvol=/
mnt_uidmap[0]: 0 10000 1000
mnt_uidmap[1]: 2000 2000 1
mnt_uidmap[2]: 3000 3000 1
mnt_gidmap[0]: 0 10000 1000
mnt_gidmap[1]: 2000 2000 1
mnt_gidmap[2]: 3000 3000 1

But the idmappings might not always be resolvable in the caller's user
namespace. For example:

unshare --user --map-root

In this case statmount() will skip any mappings that fil to resolve in
the caller's idmapping:

mnt_id: 2147485087
mnt_parent_id: 2147484016
fs_type: btrfs
mnt_root: /srv
mnt_point: /opt
mnt_opts: ssd,discard=async,space_cache=v2,subvolid=5,subvol=/

The caller can differentiate between a mount not being idmapped and a
mount that is idmapped but where all mappings fail to resolve in the
caller's idmapping by check for the STATMOUNT_MNT_{G,U}IDMAP flag being
raised but the number of mappings in ->mnt_{g,u}idmap_num being zero.

Note that statmount() requires that the whole range must be resolvable
in the caller's user namespace. If a subrange fails to map it will still
list the map as not resolvable. This is a practical compromise to avoid
having to find which subranges are resovable and wich aren't.

Idmappings are listed as a string array with each mapping separated by
zero bytes. This allows to retrieve the idmappings and immediately use
them for writing to e.g., /proc/<pid>/{g,u}id_map and it also allow for
simple iteration like:

if (stmnt->mask & STATMOUNT_MNT_UIDMAP) {
const char *idmap = stmnt->str + stmnt->mnt_uidmap;

for (size_t idx = 0; idx < stmnt->mnt_uidmap_nr; idx++) {
printf("mnt_uidmap[%lu]: %s\n", idx, idmap);
idmap += strlen(idmap) + 1;
}
}

Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/internal.h

···
 	return path->mnt->mnt_root == path->dentry;
 }
 void file_f_owner_release(struct file *file);
+int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map);

fs/mnt_idmapping.c

···
 #include <linux/mnt_idmapping.h>
 #include <linux/slab.h>
 #include <linux/user_namespace.h>
+#include <linux/seq_file.h>
 
 #include "internal.h"
 
···
 		free_mnt_idmap(idmap);
 }
 EXPORT_SYMBOL_GPL(mnt_idmap_put);
+
+int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map)
+{
+	struct uid_gid_map *map, *map_up;
+	u32 idx, nr_mappings;
+
+	if (!is_valid_mnt_idmap(idmap))
+		return 0;
+
+	/*
+	 * Idmappings are shown relative to the caller's idmapping.
+	 * This is both the most intuitive and most useful solution.
+	 */
+	if (uid_map) {
+		map = &idmap->uid_map;
+		map_up = &current_user_ns()->uid_map;
+	} else {
+		map = &idmap->gid_map;
+		map_up = &current_user_ns()->gid_map;
+	}
+
+	for (idx = 0, nr_mappings = 0; idx < map->nr_extents; idx++) {
+		uid_t lower;
+		struct uid_gid_extent *extent;
+
+		if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+			extent = &map->extent[idx];
+		else
+			extent = &map->forward[idx];
+
+		/*
+		 * Verify that the whole range of the mapping can be
+		 * resolved in the caller's idmapping. If it cannot be
+		 * resolved skip the mapping.
+		 */
+		lower = map_id_range_up(map_up, extent->lower_first, extent->count);
+		if (lower == (uid_t) -1)
+			continue;
+
+		seq_printf(seq, "%u %u %u", extent->first, lower, extent->count);
+
+		seq->count++; /* mappings are separated by \0 */
+		if (seq_has_overflowed(seq))
+			return -EAGAIN;
+
+		nr_mappings++;
+	}
+
+	return nr_mappings;
+}

fs/namespace.c

···
 	struct statmount __user *buf;
 	size_t bufsize;
 	struct vfsmount *mnt;
+	struct mnt_idmap *idmap;
 	u64 mask;
 	struct path root;
 	struct statmount sm;
···
 	return 0;
 }
 
+static inline int statmount_mnt_uidmap(struct kstatmount *s, struct seq_file *seq)
+{
+	int ret;
+
+	ret = statmount_mnt_idmap(s->idmap, seq, true);
+	if (ret < 0)
+		return ret;
+
+	s->sm.mnt_uidmap_num = ret;
+	/*
+	 * Always raise STATMOUNT_MNT_UIDMAP even if there are no valid
+	 * mappings. This allows userspace to distinguish between a
+	 * non-idmapped mount and an idmapped mount where none of the
+	 * individual mappings are valid in the caller's idmapping.
+	 */
+	if (is_valid_mnt_idmap(s->idmap))
+		s->sm.mask |= STATMOUNT_MNT_UIDMAP;
+	return 0;
+}
+
+static inline int statmount_mnt_gidmap(struct kstatmount *s, struct seq_file *seq)
+{
+	int ret;
+
+	ret = statmount_mnt_idmap(s->idmap, seq, false);
+	if (ret < 0)
+		return ret;
+
+	s->sm.mnt_gidmap_num = ret;
+	/*
+	 * Always raise STATMOUNT_MNT_GIDMAP even if there are no valid
+	 * mappings. This allows userspace to distinguish between a
+	 * non-idmapped mount and an idmapped mount where none of the
+	 * individual mappings are valid in the caller's idmapping.
+	 */
+	if (is_valid_mnt_idmap(s->idmap))
+		s->sm.mask |= STATMOUNT_MNT_GIDMAP;
+	return 0;
+}
+
 static int statmount_string(struct kstatmount *s, u64 flag)
 {
 	int ret = 0;
···
 	case STATMOUNT_SB_SOURCE:
 		sm->sb_source = start;
 		ret = statmount_sb_source(s, seq);
+		break;
+	case STATMOUNT_MNT_UIDMAP:
+		sm->mnt_uidmap = start;
+		ret = statmount_mnt_uidmap(s, seq);
+		break;
+	case STATMOUNT_MNT_GIDMAP:
+		sm->mnt_gidmap = start;
+		ret = statmount_mnt_gidmap(s, seq);
 		break;
 	default:
 		WARN_ON_ONCE(true);
···
 		return err;
 
 	s->root = root;
+	s->idmap = mnt_idmap(s->mnt);
 	if (s->mask & STATMOUNT_SB_BASIC)
 		statmount_sb_basic(s);
 
···
 	if (!err && s->mask & STATMOUNT_SB_SOURCE)
 		err = statmount_string(s, STATMOUNT_SB_SOURCE);
 
+	if (!err && s->mask & STATMOUNT_MNT_UIDMAP)
+		err = statmount_string(s, STATMOUNT_MNT_UIDMAP);
+
+	if (!err && s->mask & STATMOUNT_MNT_GIDMAP)
+		err = statmount_string(s, STATMOUNT_MNT_GIDMAP);
+
 	if (!err && s->mask & STATMOUNT_MNT_NS_ID)
 		statmount_mnt_ns_id(s, ns);
 
···
 #define STATMOUNT_STRING_REQ (STATMOUNT_MNT_ROOT | STATMOUNT_MNT_POINT | \
 			      STATMOUNT_FS_TYPE | STATMOUNT_MNT_OPTS | \
 			      STATMOUNT_FS_SUBTYPE | STATMOUNT_SB_SOURCE | \
-			      STATMOUNT_OPT_ARRAY | STATMOUNT_OPT_SEC_ARRAY)
+			      STATMOUNT_OPT_ARRAY | STATMOUNT_OPT_SEC_ARRAY | \
+			      STATMOUNT_MNT_UIDMAP | STATMOUNT_MNT_GIDMAP)
 
 static int prepare_kstatmount(struct kstatmount *ks, struct mnt_id_req *kreq,
 			      struct statmount __user *buf, size_t bufsize,

include/linux/mnt_idmapping.h

···
 static_assert(offsetof(vfsuid_t, val) == offsetof(kuid_t, val));
 static_assert(offsetof(vfsgid_t, val) == offsetof(kgid_t, val));
 
+static inline bool is_valid_mnt_idmap(const struct mnt_idmap *idmap)
+{
+	return idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap;
+}
+
 #ifdef CONFIG_MULTIUSER
 static inline uid_t __vfsuid_val(vfsuid_t uid)
 {

include/uapi/linux/mount.h

···
 	__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];
+	__u32 mnt_uidmap_num;	/* Number of uid mappings */
+	__u32 mnt_uidmap;	/* [str] Array of uid mappings (as seen from callers namespace) */
+	__u32 mnt_gidmap_num;	/* Number of gid mappings */
+	__u32 mnt_gidmap;	/* [str] Array of gid mappings (as seen from callers namespace) */
+	__u64 __spare2[44];
 	char str[];		/* Variable size part containing strings */
 };
 
···
 #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_MNT_UIDMAP		0x00001000U	/* Want/got uidmap... */
+#define STATMOUNT_MNT_GIDMAP		0x00002000U	/* Want/got gidmap... */
 
 /*
  * Special @mnt_id values that can be passed to listmount