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>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/uapi/linux/mount.h b/include/uapi/linux/mount.h
index c07008816aca..0be6ac4c1624 100644
--- a/include/uapi/linux/mount.h
+++ b/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