Re: [PATCH 6/7] tomoyo: Convert from sb_mount to granular mount hooks

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2026/03/22 10:06, Song Liu wrote:
>>> @@ -70,6 +70,7 @@ static bool tomoyo_check_mount_acl(struct tomoyo_request_info *r,
>>>  * @dir:      Pointer to "struct path".
>>>  * @type:     Name of filesystem type.
>>>  * @flags:    Mount options.
>>> + * @dev_path: Pre-resolved device/source path. Maybe NULL.
>>
>> I guess that we can avoid passing maybe-NULL dev_name if Al Viro can accept
>> resolving maybe-NULL dev_path argument before calling LSM hooks.
> 
> If I understand the code correctly, tomoyo records requested_dev_name for
> new mount. namespace.c, OTOH, does NOT do kern_path() for new mount. This 
> is why we keep the maybe-NULL dev_name here. I personally think this is 
> the best approach without changing tomoyo behavior. 

Well, namespace.c does kern_path() for new mount, but it happens a bit later after
security_mount_new() was called.

  do_new_mount_fc() => fc_mount() => vfs_get_tree() => fc->ops->get_tree() == e.g. ext4_get_tree()
  => get_tree_bdev() => get_tree_bdev_flags() => lookup_bdev() => kern_path()

@@ -3835,6 +3855,9 @@ static int do_new_mount(const struct path *path, const char *fstype,
 		err = parse_monolithic_mount_data(fc, data);
 	if (!err && !mount_capable(fc))
 		err = -EPERM;
+
+	if (!err)
+		err = security_mount_new(fc, path, mnt_flags, flags, data);
 	if (!err)
 		err = do_new_mount_fc(fc, path, mnt_flags);
 

Since not all filesystems need to resolve dev_name argument, conversion from dev_name
to "struct path" is up to individual filesystem. If we can use a flag like FS_REQUIRES_DEV
that tells whether that filesystem will resolve dev_name argument, I think we can resolve
the dev_name argument before security_mount_new() is called (and can avoid TOCTOU).