cgroup: fix parsing empty mount option string

[ Upstream commit e250d91d65750a0c0c62483ac4f9f357e7317617 ]

This fixes the case where all mount options specified are consumed by an
LSM and all that's left is an empty string. In this case cgroupfs should
accept the string and not fail.

How to reproduce (with SELinux enabled):

    # umount /sys/fs/cgroup/unified
    # mount -o context=system_u:object_r:cgroup_t:s0 -t cgroup2 cgroup2 /sys/fs/cgroup/unified
    mount: /sys/fs/cgroup/unified: wrong fs type, bad option, bad superblock on cgroup2, missing codepage or helper program, or other error.
    # dmesg | tail -n 1
    [   31.575952] cgroup: cgroup2: unknown option ""

Fixes: 67e9c74b8a87 ("cgroup: replace __DEVEL__sane_behavior with cgroup2 fs type")
[NOTE: should apply on top of commit 5136f6365ce3 ("cgroup: implement "nsdelegate" mount option"), older versions need manual rebase]
Suggested-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 109c32c56de7e9d24e2f2d84264aee8a7848ed0d..21bbfc09e3959dd423d3e897ad19c438aef19651 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1692,7 +1692,7 @@ static int parse_cgroup_root_flags(char *data, unsigned int *root_flags)
 
        *root_flags = 0;
 
-       if (!data)
+       if (!data || *data == '\0')
                return 0;
 
        while ((token = strsep(&data, ",")) != NULL) {