idmapped-mounts: add more explanations to setgid tests

The explanations before were a bit thin and people not familiar with
setgid inheritance might get confused. Make it easier to understand the
tests.

Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Eryu Guan <guaneryu@gmail.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: fstests@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
index 56b26b0ce2d1e15ada70cfc326b45a9ffe7e5b84..c53e194207bfd1f89d4c56905e0439664e47d653 100644 (file)
--- a/src/idmapped-mounts/idmapped-mounts.c
+++ b/src/idmapped-mounts/idmapped-mounts.c
@@ -8128,6 +8128,14 @@ static int setgid_create_idmapped_in_userns(void)
        if (wait_for_pid(pid))
                goto out;
 
+       /*
+        * Below we verify that setgid inheritance for a newly created file or
+        * directory works correctly. As part of this we need to verify that
+        * newly created files or directories inherit their gid from their
+        * parent directory. So we change the parent directorie's gid to 1000
+        * and create a file with fs{g,u}id 0 and verify that the newly created
+        * file and directory inherit gid 1000, not 0.
+        */
        if (fchownat(t_dir1_fd, "", -1, 1000, AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) {
                log_stderr("failure: fchownat");
                goto out;
@@ -8172,12 +8180,19 @@ static int setgid_create_idmapped_in_userns(void)
                                die("failure: is_setgid");
                }
 
-               /* Files and directories created in setgid directories inherit
-                * the i_gid of the parent directory.
+               /*
+                * In setgid directories newly created files always inherit the
+                * gid from the parent directory. Verify that the file is owned
+                * by gid 1000, not by gid 0.
                 */
                if (!expected_uid_gid(open_tree_fd, FILE1, 0, 0, 1000))
                        die("failure: check ownership");
 
+               /*
+                * In setgid directories newly created directories always
+                * inherit the gid from the parent directory. Verify that the
+                * directory is owned by gid 1000, not by gid 0.
+                */
                if (!expected_uid_gid(open_tree_fd, DIR1, 0, 0, 1000))
                        die("failure: check ownership");