[PATCH 1/1] squashfs: enable idmapped mounts

[Michael Weiß]

For squashfs all needed functionality for idmapped mounts is already
implemented by the generic handlers in the VFS. Thus, it is sufficient
to just enable the corresponding FS_ALLOW_IDMAP flag to support
idmapped mounts.

We use this for unprivileged (user namespaced) containers based on
squashfs images as rootfs in GyroidOS.

A simple test using the mount-idmapped tool executed as user with
uid=1000 looks as follows:

$ mkdir test
$ echo "test" > test/test_file
$ mksquashfs test/ fs.img
$ sudo mkdir /mnt/test
$ sudo mkdir /mnt/mapped
$ sudo mount fs.img -o loop /mnt/test/
$ sudo ./mount-idmapped --map-mount b:1000:2000:1 /mnt/test/ /mnt/mapped/

$ mount | tail -n2
fs.img on /mnt/test type squashfs (ro,relatime,errors=continue)
fs.img on /mnt/mapped type squashfs (ro,relatime,idmapped,errors=continue)

$ ls -lan /mnt/test/
total 5
drwxr-xr-x 2 1000 1000   32 Okt 24 13:36 .
drwxr-xr-x 6    0    0 4096 Okt 24 13:38 ..
-rw-r--r-- 1 1000 1000    5 Okt 24 13:36 test_file

$ ls -lan /mnt/mapped/
total 5
drwxr-xr-x 2 2000 2000   32 Okt 24 13:36 .
drwxr-xr-x 6    0    0 4096 Okt 24 13:38 ..
-rw-r--r-- 1 2000 2000    5 Okt 24 13:36 test_file

Signed-off-by: Michael Weiß <michael.weiss@xxxxxxxxxxxxxxxxxxx>
---
 fs/squashfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index 32565dafa7f3..2636cb354435 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -568,7 +568,7 @@ static struct file_system_type squashfs_fs_type = {
 	.init_fs_context = squashfs_init_fs_context,
 	.parameters = squashfs_fs_parameters,
 	.kill_sb = kill_block_super,
-	.fs_flags = FS_REQUIRES_DEV
+	.fs_flags = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
 };
 MODULE_ALIAS_FS("squashfs");
 
-- 
2.30.2