Re: [PATCH v2 0/4] statmount: allow to retrieve idmappings

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2025-02-04 at 12:27 +0100, Christian Brauner wrote:
> 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;
>             }
>     }
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> Changes in v2:
> - EDITME: describe what is new in this series revision.
> - EDITME: use bulletpoints and terse descriptions.
> - Link to v1: https://lore.kernel.org/r/20250130-work-mnt_idmap-statmount-v1-0-d4ced5874e14@kernel.org
> 
> ---
> Christian Brauner (4):
>       uidgid: add map_id_range_up()
>       statmount: allow to retrieve idmappings
>       samples/vfs: check whether flag was raised
>       samples/vfs: add STATMOUNT_MNT_{G,U}IDMAP
> 
>  fs/internal.h                      |  1 +
>  fs/mnt_idmapping.c                 | 51 ++++++++++++++++++++++++++++++++
>  fs/namespace.c                     | 59 +++++++++++++++++++++++++++++++++++++-
>  include/linux/mnt_idmapping.h      |  5 ++++
>  include/linux/uidgid.h             |  6 ++++
>  include/uapi/linux/mount.h         |  8 +++++-
>  kernel/user_namespace.c            | 26 +++++++++++------
>  samples/vfs/samples-vfs.h          | 14 ++++++++-
>  samples/vfs/test-list-all-mounts.c | 35 ++++++++++++++++++----
>  9 files changed, 187 insertions(+), 18 deletions(-)
> ---
> base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b
> change-id: 20250129-work-mnt_idmap-statmount-e57f258fef8e
> 

This all looks pretty sane to me.

Reviewed-by: Jeff Layton <jlayton@kernel.org>