Merge patch series "statmount: allow to retrieve idmappings"

Christian Brauner <brauner@kernel.org> says:

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;
            }
    }

* patches from https://lore.kernel.org/r/20250204-work-mnt_idmap-statmount-v2-0-007720f39f2e@kernel.org:
  samples/vfs: add STATMOUNT_MNT_{G,U}IDMAP
  samples/vfs: check whether flag was raised
  statmount: allow to retrieve idmappings
  uidgid: add map_id_range_up()

Link: https://lore.kernel.org/r/20250204-work-mnt_idmap-statmount-v2-0-007720f39f2e@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/internal.h

@@ -338,3 +338,4 @@ static inline bool path_mounted(const struct path *path)
 	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

@@ -6,6 +6,7 @@
 #include <linux/mnt_idmapping.h>
 #include <linux/slab.h>
 #include <linux/user_namespace.h>
+#include <linux/seq_file.h>
 
 #include "internal.h"
 
@@ -334,3 +335,53 @@ void mnt_idmap_put(struct mnt_idmap *idmap)
 		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

@@ -5008,6 +5008,7 @@ struct kstatmount {
 	struct statmount __user *buf;
 	size_t bufsize;
 	struct vfsmount *mnt;
+	struct mnt_idmap *idmap;
 	u64 mask;
 	struct path root;
 	struct statmount sm;
@@ -5278,6 +5279,46 @@ static int statmount_opt_sec_array(struct kstatmount *s, struct seq_file *seq)
 	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;
@@ -5319,6 +5360,14 @@ static int statmount_string(struct kstatmount *s, u64 flag)
 		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 -EINVAL;
@@ -5443,6 +5492,7 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 		return err;
 
 	s->root = root;
+	s->idmap = mnt_idmap(s->mnt);
 	if (s->mask & STATMOUNT_SB_BASIC)
 		statmount_sb_basic(s);
 
@@ -5476,6 +5526,12 @@ static int do_statmount(struct kstatmount *s, u64 mnt_id, u64 mnt_ns_id,
 	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);
 
@@ -5499,7 +5555,8 @@ 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_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

@@ -25,6 +25,11 @@ static_assert(sizeof(vfsgid_t) == sizeof(kgid_t));
 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/linux/uidgid.h

@@ -132,6 +132,7 @@ static inline bool kgid_has_mapping(struct user_namespace *ns, kgid_t gid)
 
 u32 map_id_down(struct uid_gid_map *map, u32 id);
 u32 map_id_up(struct uid_gid_map *map, u32 id);
+u32 map_id_range_up(struct uid_gid_map *map, u32 id, u32 count);
 
 #else
 
@@ -186,6 +187,11 @@ static inline u32 map_id_down(struct uid_gid_map *map, u32 id)
 	return id;
 }
 
+static inline u32 map_id_range_up(struct uid_gid_map *map, u32 id, u32 count)
+{
+	return id;
+}
+
 static inline u32 map_id_up(struct uid_gid_map *map, u32 id)
 {
 	return id;

include/uapi/linux/mount.h

@@ -179,7 +179,11 @@ 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];
+	__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 */
 };
 
@@ -217,6 +221,8 @@ 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_MNT_UIDMAP		0x00001000U	/* Want/got uidmap... */
+#define STATMOUNT_MNT_GIDMAP		0x00002000U	/* Want/got gidmap... */
 
 /*
  * Special @mnt_id values that can be passed to listmount

kernel/user_namespace.c

@@ -238,7 +238,7 @@ EXPORT_SYMBOL(__put_user_ns);
 struct idmap_key {
 	bool map_up; /* true  -> id from kid; false -> kid from id */
 	u32 id; /* id to find */
-	u32 count; /* == 0 unless used with map_id_range_down() */
+	u32 count;
 };
 
 /*
@@ -343,16 +343,19 @@ u32 map_id_down(struct uid_gid_map *map, u32 id)
  * UID_GID_MAP_MAX_BASE_EXTENTS.
  */
 static struct uid_gid_extent *
-map_id_up_base(unsigned extents, struct uid_gid_map *map, u32 id)
+map_id_range_up_base(unsigned extents, struct uid_gid_map *map, u32 id, u32 count)
 {
 	unsigned idx;
-	u32 first, last;
+	u32 first, last, id2;
+
+	id2 = id + count - 1;
 
 	/* Find the matching extent */
 	for (idx = 0; idx < extents; idx++) {
 		first = map->extent[idx].lower_first;
 		last = first + map->extent[idx].count - 1;
-		if (id >= first && id <= last)
+		if (id >= first && id <= last &&
+		    (id2 >= first && id2 <= last))
 			return &map->extent[idx];
 	}
 	return NULL;
@@ -363,28 +366,28 @@ map_id_up_base(unsigned extents, struct uid_gid_map *map, u32 id)
  * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
  */
 static struct uid_gid_extent *
-map_id_up_max(unsigned extents, struct uid_gid_map *map, u32 id)
+map_id_range_up_max(unsigned extents, struct uid_gid_map *map, u32 id, u32 count)
 {
 	struct idmap_key key;
 
 	key.map_up = true;
-	key.count = 1;
+	key.count = count;
 	key.id = id;
 
 	return bsearch(&key, map->reverse, extents,
 		       sizeof(struct uid_gid_extent), cmp_map_id);
 }
 
-u32 map_id_up(struct uid_gid_map *map, u32 id)
+u32 map_id_range_up(struct uid_gid_map *map, u32 id, u32 count)
 {
 	struct uid_gid_extent *extent;
 	unsigned extents = map->nr_extents;
 	smp_rmb();
 
 	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
-		extent = map_id_up_base(extents, map, id);
+		extent = map_id_range_up_base(extents, map, id, count);
 	else
-		extent = map_id_up_max(extents, map, id);
+		extent = map_id_range_up_max(extents, map, id, count);
 
 	/* Map the id or note failure */
 	if (extent)
@@ -395,6 +398,11 @@ u32 map_id_up(struct uid_gid_map *map, u32 id)
 	return id;
 }
 
+u32 map_id_up(struct uid_gid_map *map, u32 id)
+{
+	return map_id_range_up(map, id, 1);
+}
+
 /**
  *	make_kuid - Map a user-namespace uid pair into a kuid.
  *	@ns:  User namespace that the uid is in

samples/vfs/samples-vfs.h

@@ -42,7 +42,11 @@ 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];
+	__u32 mnt_uidmap_num;	/* Number of uid mappings */
+	__u32 mnt_uidmap;	/* [str] Array of uid mappings */
+	__u32 mnt_gidmap_num;	/* Number of gid mappings */
+	__u32 mnt_gidmap;	/* [str] Array of gid mappings */
+	__u64 __spare2[44];
 	char str[];		/* Variable size part containing strings */
 };
 
@@ -158,6 +162,14 @@ struct mnt_ns_info {
 #define STATX_MNT_ID_UNIQUE 0x00004000U /* Want/got extended stx_mount_id */
 #endif
 
+#ifndef STATMOUNT_MNT_UIDMAP
+#define STATMOUNT_MNT_UIDMAP		0x00002000U	/* Want/got uidmap... */
+#endif
+
+#ifndef STATMOUNT_MNT_GIDMAP
+#define STATMOUNT_MNT_GIDMAP		0x00004000U	/* Want/got gidmap... */
+#endif
+
 #ifndef MOUNT_ATTR_RDONLY
 #define MOUNT_ATTR_RDONLY	0x00000001 /* Mount read-only */
 #endif

samples/vfs/test-list-all-mounts.c

@@ -128,20 +128,43 @@ int main(int argc, char *argv[])
 					      STATMOUNT_MNT_POINT |
 					      STATMOUNT_MNT_NS_ID |
 					      STATMOUNT_MNT_OPTS |
-					      STATMOUNT_FS_TYPE, 0);
+					      STATMOUNT_FS_TYPE |
+					      STATMOUNT_MNT_UIDMAP |
+					      STATMOUNT_MNT_GIDMAP, 0);
 			if (!stmnt) {
 				printf("Failed to statmount(%" PRIu64 ") in mount namespace(%" PRIu64 ")\n",
 				       (uint64_t)last_mnt_id, (uint64_t)info.mnt_ns_id);
 				continue;
 			}
 
-			printf("mnt_id:\t\t%" PRIu64 "\nmnt_parent_id:\t%" PRIu64 "\nfs_type:\t%s\nmnt_root:\t%s\nmnt_point:\t%s\nmnt_opts:\t%s\n\n",
+			printf("mnt_id:\t\t%" PRIu64 "\nmnt_parent_id:\t%" PRIu64 "\nfs_type:\t%s\nmnt_root:\t%s\nmnt_point:\t%s\nmnt_opts:\t%s\n",
 			       (uint64_t)stmnt->mnt_id,
 			       (uint64_t)stmnt->mnt_parent_id,
-			       stmnt->str + stmnt->fs_type,
-			       stmnt->str + stmnt->mnt_root,
-			       stmnt->str + stmnt->mnt_point,
-			       stmnt->str + stmnt->mnt_opts);
+			       (stmnt->mask & STATMOUNT_FS_TYPE)   ? stmnt->str + stmnt->fs_type   : "",
+			       (stmnt->mask & STATMOUNT_MNT_ROOT)  ? stmnt->str + stmnt->mnt_root  : "",
+			       (stmnt->mask & STATMOUNT_MNT_POINT) ? stmnt->str + stmnt->mnt_point : "",
+			       (stmnt->mask & STATMOUNT_MNT_OPTS)  ? stmnt->str + stmnt->mnt_opts  : "");
+
+			if (stmnt->mask & STATMOUNT_MNT_UIDMAP) {
+				const char *idmap = stmnt->str + stmnt->mnt_uidmap;
+
+				for (size_t idx = 0; idx < stmnt->mnt_uidmap_num; idx++) {
+					printf("mnt_uidmap[%lu]:\t%s\n", idx, idmap);
+					idmap += strlen(idmap) + 1;
+				}
+			}
+
+			if (stmnt->mask & STATMOUNT_MNT_GIDMAP) {
+				const char *idmap = stmnt->str + stmnt->mnt_gidmap;
+
+				for (size_t idx = 0; idx < stmnt->mnt_gidmap_num; idx++) {
+					printf("mnt_gidmap[%lu]:\t%s\n", idx, idmap);
+					idmap += strlen(idmap) + 1;
+				}
+			}
+
+			printf("\n");
+
 			free(stmnt);
 		}
 	}