selinux状态获取过程

以Android8.0为例

system/core/init/init.cpp

static void selinux_initialize(bool in_kernel_domain) {
   
        bool kernel_enforcing = (security_getenforce() == 1);
        bool is_enforcing = selinux_is_enforcing();
        if (kernel_enforcing != is_enforcing) {
            if (security_setenforce(is_enforcing)) {
                PLOG(ERROR) << "security_setenforce(%s) failed" << (is_enforcing ? "true" : "false");
                security_failure();
            }
        }
}

is_enforcing的值默认是1(cmdline默认不会设置androidboot.selinux) 

enum selinux_enforcing_status { SELINUX_PERMISSIVE, SELINUX_ENFORCING };

static selinux_enforcing_status selinux_status_from_cmdline() {
    selinux_enforcing_status status = SELINUX_ENFORCING;

    import_kernel_cmdline(false, [&](const std::string& key, const std::string& value, bool in_qemu) {
        if (key == "androidboot.selinux" && value == "permissive") {
            status = SELINUX_PERMISSIVE;
        }
    });

    return status;
}

static bool selinux_is_enforcing(void)
{
    if (ALLOW_PERMISSIVE_SELINUX) {
        return selinux_status_from_cmdline() == SELINUX_ENFORCING;
    }
    return true;
}

通过节点判断selinux状态

external/selinux/libselinux/src/getenforce.c

int security_getenforce(void)
{
	int fd, ret, enforce = 0;
	char path[PATH_MAX];
	char buf[20];

	if (!selinux_mnt) {
		errno = ENOENT;
		return -1;
	}

	snprintf(path, sizeof path, "%s/enforce", selinux_mnt);
	fd = open(path, O_RDONLY | O_CLOEXEC);
	if (fd < 0)
		return -1;

	memset(buf, 0, sizeof buf);
	ret = read(fd, buf, sizeof buf - 1);
	close(fd);
	if (ret < 0)
		return -1;

	if (sscanf(buf, "%d", &enforce) != 1)
		return -1;

	return !!enforce;
}

kernel/security/selinux/selinuxfs.c

static int __init init_sel_fs(void)
{
	if (!selinux_enabled)
		return 0;
	selinuxfs_kobj = kobject_create_and_add("selinux", fs_kobj);
	err = register_filesystem(&sel_fs_type)
}

所以可以在cmdline上加上androidboot.selinux=0来关闭selinux 

kernel_enforcing默认是１，有设置androidboot.selinux=0，kernel_enforcing是０

#ifdef CONFIG_SECURITY_SELINUX_BOOTPARAM
int selinux_enabled = CONFIG_SECURITY_SELINUX_BOOTPARAM_VALUE;

static int __init selinux_enabled_setup(char *str)
{
	unsigned long enabled;
	if (!strict_strtoul(str, 0, &enabled))
		selinux_enabled = enabled ? 1 : 0;
	return 1;
}
__setup("selinux=", selinux_enabled_setup);
#else
int selinux_enabled = 1;
#endif

 linux创建selinux节点

static struct dentry *sel_mount(struct file_system_type *fs_type,
		      int flags, const char *dev_name, void *data)
{
	return mount_single(fs_type, flags, data, sel_fill_super);
}

static struct file_system_type sel_fs_type = {
	.name		= "selinuxfs",
	.mount		= sel_mount,
	
};

static int sel_fill_super(struct super_block *sb, void *data, int silent)
{
	static struct tree_descr selinux_files[] = {
		[SEL_LOAD] = {"load", &sel_load_ops, S_IRUSR|S_IWUSR},
		[SEL_ENFORCE] = {"enforce", &sel_enforce_ops, S_IRUGO|S_IWUSR},
		[SEL_CONTEXT] = {"context", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_ACCESS] = {"access", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_CREATE] = {"create", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_RELABEL] = {"relabel", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_USER] = {"user", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_POLICYVERS] = {"policyvers", &sel_policyvers_ops, S_IRUGO},
		[SEL_COMMIT_BOOLS] = {"commit_pending_bools", &sel_commit_bools_ops, S_IWUSR},
		[SEL_MLS] = {"mls", &sel_mls_ops, S_IRUGO},
		[SEL_DISABLE] = {"disable", &sel_disable_ops, S_IWUSR},
		[SEL_MEMBER] = {"member", &transaction_ops, S_IRUGO|S_IWUGO},
		[SEL_CHECKREQPROT] = {"checkreqprot", &sel_checkreqprot_ops, S_IRUGO|S_IWUSR},
		[SEL_REJECT_UNKNOWN] = {"reject_unknown", &sel_handle_unknown_ops, S_IRUGO},
		[SEL_DENY_UNKNOWN] = {"deny_unknown", &sel_handle_unknown_ops, S_IRUGO},
		[SEL_STATUS] = {"status", &sel_handle_status_ops, S_IRUGO},
		[SEL_POLICY] = {"policy", &sel_policy_ops, S_IRUGO},
		/* last one */ {""}
	};
	
}