In case of idmapped mounts and statmount(), three cases can occur:

1. The mount is not an idmapped mount. In this case, smbuf->mask will
   *not* have STATMOUNT_MNT_{UID,GID}MAP set.
2. The mount is an idmapped mount but *all* its mappings are *not*
   resolvable in the user namespace of the caller.
   In this case, smbuf->mask will have STATMOUNT_MNT_{UID,GID}MAP set
   but smbuf->mnt_{uid,gid}map_num will be 0.
3. The mount is an idmapped mount and *all* its mappings are resolvable
   in the user namespace of the caller.
   In this case, smbuf->mask will have STATMOUNT_MNT_{UID,GID}MAP set
   and mbuf->mnt_{uid,gid}map_num will be greater than 0.

The current documentation fails to differentiate between case 1 and 2
and incorrectly states that STATMOUNT_MNT_{UID,GID}MAP will be set for
non-idmapped mounts.

We can verify that the above is the case by looking at [1] and is made
explicitly clear by the comment in the implementation [2]. The case for
STATMOUNT_MNT_{UID,GID}MAP not being raised for a non-idmapped mount can
be verified by running this program [3].

[1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=37c4a9590e1efcae7749682239fc22a330d2d325>
[2]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/namespace.c#n5489>
[3]: <https://gist.github.com/bsach64/674264ec69e592f906b2713c9f95060b>

Signed-off-by: Bhavik Sachdev <redacted>
---
Hey Alex!

My understanding of how statmount() differentiated between idmapped
mounts and non-idmapped mounts was incorrect. This patch fixes the
incorrect documentation introduced as a result.

Thanks,
Bhavik

 man/man2/statmount.2 | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/man/man2/statmount.2 b/man/man2/statmount.2
index 42ca902d9..40a07181b 100644
--- a/man/man2/statmount.2
+++ b/man/man2/statmount.2

@@ -356,7 +356,8 @@ .SS The returned information
 If
 .I smbuf.mask
 has STATMOUNT_UIDMAP set and this field is 0,
-the mount is not an idmapped mount.
+then uid mappings applied on the mount cannot be resolved in the user namespace
+of the caller.
 .TP
 .IR smbuf.mnt_uidmap " (since Linux 6.15)"
 The offset to the location in the

@@ -372,7 +373,8 @@ .SS The returned information
 If
 .I smbuf.mask
 has STATMOUNT_GIDMAP set and this field is 0,
-the mount is not an idmapped mount.
+then gid mappings applied on the mount cannot be resolved in the user namespace
+of the caller.
 .TP
 .IR smbuf.mnt_gidmap " (since Linux 6.15)"
 The offset to the location in the

-- 
2.53.0