[bug report] selinux: kill selinux_sb_get_mnt_opts()

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Al Viro,

The patch e3489f8974e1: "selinux: kill selinux_sb_get_mnt_opts()"
from Dec 13, 2018, leads to the following static checker warning:

	security/selinux/hooks.c:1075 show_sid()
	error: we previously assumed 'context' could be null (see line 1070)

security/selinux/hooks.c
    1061 static int show_sid(struct seq_file *m, u32 sid)
    1062 {
    1063 	char *context = NULL;
    1064 	u32 len;
    1065 	int rc;
    1066 
    1067 	rc = security_sid_to_context(&selinux_state, sid,
    1068 					     &context, &len);
    1069 	if (!rc) {
    1070 		bool has_comma = context && strchr(context, ',');
                                         ^^^^^^^
Can context be NULL?

    1071 
    1072 		seq_putc(m, '=');
    1073 		if (has_comma)
    1074 			seq_putc(m, '\"');
--> 1075 		seq_escape(m, context, "\"\n\\");
                                      ^^^^^^^
Because if it is, then we're toasted when we pass it to seq_escape().

    1076 		if (has_comma)
    1077 			seq_putc(m, '\"');
    1078 	}
    1079 	kfree(context);
    1080 	return rc;
    1081 }

regards,
dan carpenter