Merge tag 'drm-misc-next-fixes-2017-02-27' of git://anongit.freedesktop.org/git/drm...

[mirror_ubuntu-artful-kernel.git] / init / main.c

/*
 *  linux/init/main.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  GK 2/5/95  -  Changed to support mounting root fs via NFS
 *  Added initrd & change_root: Werner Almesberger & Hans Lermen, Feb '96
 *  Moan early if gcc is old, avoiding bogus kernels - Paul Gortmaker, May '96
 *  Simplified starting of init:  Michael A. Griffith <grif@acm.org>
 */

#define DEBUG           /* Enable initcall_debug */

#include <linux/types.h>
#include <linux/extable.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/stackprotector.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/delay.h>
#include <linux/ioport.h>
#include <linux/init.h>
#include <linux/initrd.h>
#include <linux/bootmem.h>
#include <linux/acpi.h>
#include <linux/tty.h>
#include <linux/percpu.h>
#include <linux/kmod.h>
#include <linux/vmalloc.h>
#include <linux/kernel_stat.h>
#include <linux/start_kernel.h>
#include <linux/security.h>
#include <linux/smp.h>
#include <linux/profile.h>
#include <linux/rcupdate.h>
#include <linux/moduleparam.h>
#include <linux/kallsyms.h>
#include <linux/writeback.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/cgroup.h>
#include <linux/efi.h>
#include <linux/tick.h>
#include <linux/interrupt.h>
#include <linux/taskstats_kern.h>
#include <linux/delayacct.h>
#include <linux/unistd.h>
#include <linux/rmap.h>
#include <linux/mempolicy.h>
#include <linux/key.h>
#include <linux/buffer_head.h>
#include <linux/page_ext.h>
#include <linux/debug_locks.h>
#include <linux/debugobjects.h>
#include <linux/lockdep.h>
#include <linux/kmemleak.h>
#include <linux/pid_namespace.h>
#include <linux/device.h>
#include <linux/kthread.h>
#include <linux/sched.h>
#include <linux/signal.h>
#include <linux/idr.h>
#include <linux/kgdb.h>
#include <linux/ftrace.h>
#include <linux/async.h>
#include <linux/kmemcheck.h>
#include <linux/sfi.h>
#include <linux/shmem_fs.h>
#include <linux/slab.h>
#include <linux/perf_event.h>
#include <linux/file.h>
#include <linux/ptrace.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/sched_clock.h>
#include <linux/context_tracking.h>
#include <linux/random.h>
#include <linux/list.h>
#include <linux/integrity.h>
#include <linux/proc_ns.h>
#include <linux/io.h>
#include <linux/cache.h>

#include <asm/io.h>
#include <asm/bugs.h>
#include <asm/setup.h>
#include <asm/sections.h>
#include <asm/cacheflush.h>

static int kernel_init(void *);

extern void init_IRQ(void);
extern void fork_init(void);
extern void radix_tree_init(void);

/*
 * Debug helper: via this flag we know that we are in 'early bootup code'
 * where only the boot processor is running with IRQ disabled.  This means
 * two things - IRQ must not be enabled before the flag is cleared and some
 * operations which are not allowed with IRQ disabled are allowed while the
 * flag is set.
 */
bool early_boot_irqs_disabled __read_mostly;

enum system_states system_state __read_mostly;
EXPORT_SYMBOL(system_state);

/*
 * Boot command-line arguments
 */
#define MAX_INIT_ARGS CONFIG_INIT_ENV_ARG_LIMIT
#define MAX_INIT_ENVS CONFIG_INIT_ENV_ARG_LIMIT

extern void time_init(void);
/* Default late time init is NULL. archs can override this later. */
void (*__initdata late_time_init)(void);

/* Untouched command line saved by arch-specific code. */
char __initdata boot_command_line[COMMAND_LINE_SIZE];
/* Untouched saved command line (eg. for /proc) */
char *saved_command_line;
/* Command line for parameter parsing */
static char *static_command_line;
/* Command line for per-initcall parameter parsing */
static char *initcall_command_line;

static char *execute_command;
static char *ramdisk_execute_command;

/*
 * Used to generate warnings if static_key manipulation functions are used
 * before jump_label_init is called.
 */
bool static_key_initialized __read_mostly;
EXPORT_SYMBOL_GPL(static_key_initialized);

/*
 * If set, this is an indication to the drivers that reset the underlying
 * device before going ahead with the initialization otherwise driver might
 * rely on the BIOS and skip the reset operation.
 *
 * This is useful if kernel is booting in an unreliable environment.
 * For ex. kdump situation where previous kernel has crashed, BIOS has been
 * skipped and devices will be in unknown state.
 */
unsigned int reset_devices;
EXPORT_SYMBOL(reset_devices);

static int __init set_reset_devices(char *str)
{
        reset_devices = 1;
        return 1;
}

__setup("reset_devices", set_reset_devices);

static const char *argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
static const char *panic_later, *panic_param;

extern const struct obs_kernel_param __setup_start[], __setup_end[];

static bool __init obsolete_checksetup(char *line)
{
        const struct obs_kernel_param *p;
        bool had_early_param = false;

        p = __setup_start;
        do {
                int n = strlen(p->str);
                if (parameqn(line, p->str, n)) {
                        if (p->early) {
                                /* Already done in parse_early_param?
                                 * (Needs exact match on param part).
                                 * Keep iterating, as we can have early
                                 * params and __setups of same names 8( */
                                if (line[n] == '\0' || line[n] == '=')
                                        had_early_param = true;
                        } else if (!p->setup_func) {
                                pr_warn("Parameter %s is obsolete, ignored\n",
                                        p->str);
                                return true;
                        } else if (p->setup_func(line + n))
                                return true;
                }
                p++;
        } while (p < __setup_end);

        return had_early_param;
}

/*
 * This should be approx 2 Bo*oMips to start (note initial shift), and will
 * still work even if initially too large, it will just take slightly longer
 */
unsigned long loops_per_jiffy = (1<<12);
EXPORT_SYMBOL(loops_per_jiffy);

static int __init debug_kernel(char *str)
{
        console_loglevel = CONSOLE_LOGLEVEL_DEBUG;
        return 0;
}

static int __init quiet_kernel(char *str)
{
        console_loglevel = CONSOLE_LOGLEVEL_QUIET;
        return 0;
}

early_param("debug", debug_kernel);
early_param("quiet", quiet_kernel);

static int __init loglevel(char *str)
{
        int newlevel;

        /*
         * Only update loglevel value when a correct setting was passed,
         * to prevent blind crashes (when loglevel being set to 0) that
         * are quite hard to debug
         */
        if (get_option(&str, &newlevel)) {
                console_loglevel = newlevel;
                return 0;
        }

        return -EINVAL;
}

early_param("loglevel", loglevel);

/* Change NUL term back to "=", to make "param" the whole string. */
static int __init repair_env_string(char *param, char *val,
                                    const char *unused, void *arg)
{
        if (val) {
                /* param=val or param="val"? */
                if (val == param+strlen(param)+1)
                        val[-1] = '=';
                else if (val == param+strlen(param)+2) {
                        val[-2] = '=';
                        memmove(val-1, val, strlen(val)+1);
                        val--;
                } else
                        BUG();
        }
        return 0;
}

/* Anything after -- gets handed straight to init. */
static int __init set_init_arg(char *param, char *val,
                               const char *unused, void *arg)
{
        unsigned int i;

        if (panic_later)
                return 0;

        repair_env_string(param, val, unused, NULL);

        for (i = 0; argv_init[i]; i++) {
                if (i == MAX_INIT_ARGS) {
                        panic_later = "init";
                        panic_param = param;
                        return 0;
                }
        }
        argv_init[i] = param;
        return 0;
}

/*
 * Unknown boot options get handed to init, unless they look like
 * unused parameters (modprobe will find them in /proc/cmdline).
 */
static int __init unknown_bootoption(char *param, char *val,
                                     const char *unused, void *arg)
{
        repair_env_string(param, val, unused, NULL);

        /* Handle obsolete-style parameters */
        if (obsolete_checksetup(param))
                return 0;

        /* Unused module parameter. */
        if (strchr(param, '.') && (!val || strchr(param, '.') < val))
                return 0;

        if (panic_later)
                return 0;

        if (val) {
                /* Environment option */
                unsigned int i;
                for (i = 0; envp_init[i]; i++) {
                        if (i == MAX_INIT_ENVS) {
                                panic_later = "env";
                                panic_param = param;
                        }
                        if (!strncmp(param, envp_init[i], val - param))
                                break;
                }
                envp_init[i] = param;
        } else {
                /* Command line option */
                unsigned int i;
                for (i = 0; argv_init[i]; i++) {
                        if (i == MAX_INIT_ARGS) {
                                panic_later = "init";
                                panic_param = param;
                        }
                }
                argv_init[i] = param;
        }
        return 0;
}

static int __init init_setup(char *str)
{
        unsigned int i;

        execute_command = str;
        /*
         * In case LILO is going to boot us with default command line,
         * it prepends "auto" before the whole cmdline which makes
         * the shell think it should execute a script with such name.
         * So we ignore all arguments entered _before_ init=... [MJ]
         */
        for (i = 1; i < MAX_INIT_ARGS; i++)
                argv_init[i] = NULL;
        return 1;
}
__setup("init=", init_setup);

static int __init rdinit_setup(char *str)
{
        unsigned int i;

        ramdisk_execute_command = str;
        /* See "auto" comment in init_setup */
        for (i = 1; i < MAX_INIT_ARGS; i++)
                argv_init[i] = NULL;
        return 1;
}
__setup("rdinit=", rdinit_setup);

#ifndef CONFIG_SMP
static const unsigned int setup_max_cpus = NR_CPUS;
static inline void setup_nr_cpu_ids(void) { }
static inline void smp_prepare_cpus(unsigned int maxcpus) { }
#endif

/*
 * We need to store the untouched command line for future reference.
 * We also need to store the touched command line since the parameter
 * parsing is performed in place, and we should allow a component to
 * store reference of name/value for future reference.
 */
static void __init setup_command_line(char *command_line)
{
        saved_command_line =
                memblock_virt_alloc(strlen(boot_command_line) + 1, 0);
        initcall_command_line =
                memblock_virt_alloc(strlen(boot_command_line) + 1, 0);
        static_command_line = memblock_virt_alloc(strlen(command_line) + 1, 0);
        strcpy(saved_command_line, boot_command_line);
        strcpy(static_command_line, command_line);
}

/*
 * We need to finalize in a non-__init function or else race conditions
 * between the root thread and the init thread may cause start_kernel to
 * be reaped by free_initmem before the root thread has proceeded to
 * cpu_idle.
 *
 * gcc-3.4 accidentally inlines this function, so use noinline.
 */

static __initdata DECLARE_COMPLETION(kthreadd_done);

static noinline void __ref rest_init(void)
{
        int pid;

        rcu_scheduler_starting();
        /*
         * We need to spawn init first so that it obtains pid 1, however
         * the init task will end up wanting to create kthreads, which, if
         * we schedule it before we create kthreadd, will OOPS.
         */
        kernel_thread(kernel_init, NULL, CLONE_FS);
        numa_default_policy();
        pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
        rcu_read_lock();
        kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
        rcu_read_unlock();
        complete(&kthreadd_done);

        /*
         * The boot idle thread must execute schedule()
         * at least once to get things moving:
         */
        init_idle_bootup_task(current);
        schedule_preempt_disabled();
        /* Call into cpu_idle with preempt disabled */
        cpu_startup_entry(CPUHP_ONLINE);
}

/* Check for early params. */
static int __init do_early_param(char *param, char *val,
                                 const char *unused, void *arg)
{
        const struct obs_kernel_param *p;

        for (p = __setup_start; p < __setup_end; p++) {
                if ((p->early && parameq(param, p->str)) ||
                    (strcmp(param, "console") == 0 &&
                     strcmp(p->str, "earlycon") == 0)
                ) {
                        if (p->setup_func(val) != 0)
                                pr_warn("Malformed early option '%s'\n", param);
                }
        }
        /* We accept everything at this stage. */
        return 0;
}

void __init parse_early_options(char *cmdline)
{
        parse_args("early options", cmdline, NULL, 0, 0, 0, NULL,
                   do_early_param);
}

/* Arch code calls this early on, or if not, just before other parsing. */
void __init parse_early_param(void)
{
        static int done __initdata;
        static char tmp_cmdline[COMMAND_LINE_SIZE] __initdata;

        if (done)
                return;

        /* All fall through to do_early_param. */
        strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE);
        parse_early_options(tmp_cmdline);
        done = 1;
}

void __init __weak arch_post_acpi_subsys_init(void) { }

void __init __weak smp_setup_processor_id(void)
{
}

# if THREAD_SIZE >= PAGE_SIZE
void __init __weak thread_stack_cache_init(void)
{
}
#endif

/*
 * Set up kernel memory allocators
 */
static void __init mm_init(void)
{
        /*
         * page_ext requires contiguous pages,
         * bigger than MAX_ORDER unless SPARSEMEM.
         */
        page_ext_init_flatmem();
        mem_init();
        kmem_cache_init();
        percpu_init_late();
        pgtable_init();
        vmalloc_init();
        ioremap_huge_init();
}

asmlinkage __visible void __init start_kernel(void)
{
        char *command_line;
        char *after_dashes;

        set_task_stack_end_magic(&init_task);
        smp_setup_processor_id();
        debug_objects_early_init();

        /*
         * Set up the the initial canary ASAP:
         */
        boot_init_stack_canary();

        cgroup_init_early();

        local_irq_disable();
        early_boot_irqs_disabled = true;

/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
        boot_cpu_init();
        page_address_init();
        pr_notice("%s", linux_banner);
        setup_arch(&command_line);
        mm_init_cpumask(&init_mm);
        setup_command_line(command_line);
        setup_nr_cpu_ids();
        setup_per_cpu_areas();
        boot_cpu_state_init();
        smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */

        build_all_zonelists(NULL, NULL);
        page_alloc_init();

        pr_notice("Kernel command line: %s\n", boot_command_line);
        parse_early_param();
        after_dashes = parse_args("Booting kernel",
                                  static_command_line, __start___param,
                                  __stop___param - __start___param,
                                  -1, -1, NULL, &unknown_bootoption);
        if (!IS_ERR_OR_NULL(after_dashes))
                parse_args("Setting init args", after_dashes, NULL, 0, -1, -1,
                           NULL, set_init_arg);

        jump_label_init();

        /*
         * These use large bootmem allocations and must precede
         * kmem_cache_init()
         */
        setup_log_buf(0);
        pidhash_init();
        vfs_caches_init_early();
        sort_main_extable();
        trap_init();
        mm_init();

        /*
         * Set up the scheduler prior starting any interrupts (such as the
         * timer interrupt). Full topology setup happens at smp_init()
         * time - but meanwhile we still have a functioning scheduler.
         */
        sched_init();
        /*
         * Disable preemption - early bootup scheduling is extremely
         * fragile until we cpu_idle() for the first time.
         */
        preempt_disable();
        if (WARN(!irqs_disabled(),
                 "Interrupts were enabled *very* early, fixing it\n"))
                local_irq_disable();
        idr_init_cache();

        /*
         * Allow workqueue creation and work item queueing/cancelling
         * early.  Work item execution depends on kthreads and starts after
         * workqueue_init().
         */
        workqueue_init_early();

        rcu_init();

        /* trace_printk() and trace points may be used after this */
        trace_init();

        context_tracking_init();
        radix_tree_init();
        /* init some links before init_ISA_irqs() */
        early_irq_init();
        init_IRQ();
        tick_init();
        rcu_init_nohz();
        init_timers();
        hrtimers_init();
        softirq_init();
        timekeeping_init();
        time_init();
        sched_clock_postinit();
        printk_safe_init();
        perf_event_init();
        profile_init();
        call_function_init();
        WARN(!irqs_disabled(), "Interrupts were enabled early\n");
        early_boot_irqs_disabled = false;
        local_irq_enable();

        kmem_cache_init_late();

        /*
         * HACK ALERT! This is early. We're enabling the console before
         * we've done PCI setups etc, and console_init() must be aware of
         * this. But we do want output early, in case something goes wrong.
         */
        console_init();
        if (panic_later)
                panic("Too many boot %s vars at `%s'", panic_later,
                      panic_param);

        lockdep_info();

        /*
         * Need to run this when irqs are enabled, because it wants
         * to self-test [hard/soft]-irqs on/off lock inversion bugs
         * too:
         */
        locking_selftest();

#ifdef CONFIG_BLK_DEV_INITRD
        if (initrd_start && !initrd_below_start_ok &&
            page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
                pr_crit("initrd overwritten (0x%08lx < 0x%08lx) - disabling it.\n",
                    page_to_pfn(virt_to_page((void *)initrd_start)),
                    min_low_pfn);
                initrd_start = 0;
        }
#endif
        page_ext_init();
        debug_objects_mem_init();
        kmemleak_init();
        setup_per_cpu_pageset();
        numa_policy_init();
        if (late_time_init)
                late_time_init();
        calibrate_delay();
        pidmap_init();
        anon_vma_init();
        acpi_early_init();
#ifdef CONFIG_X86
        if (efi_enabled(EFI_RUNTIME_SERVICES))
                efi_enter_virtual_mode();
#endif
#ifdef CONFIG_X86_ESPFIX64
        /* Should be run before the first non-init thread is created */
        init_espfix_bsp();
#endif
        thread_stack_cache_init();
        cred_init();
        fork_init();
        proc_caches_init();
        buffer_init();
        key_init();
        security_init();
        dbg_late_init();
        vfs_caches_init();
        pagecache_init();
        signals_init();
        proc_root_init();
        nsfs_init();
        cpuset_init();
        cgroup_init();
        taskstats_init_early();
        delayacct_init();

        check_bugs();

        acpi_subsystem_init();
        arch_post_acpi_subsys_init();
        sfi_init_late();

        if (efi_enabled(EFI_RUNTIME_SERVICES)) {
                efi_free_boot_services();
        }

        ftrace_init();

        /* Do the rest non-__init'ed, we're now alive */
        rest_init();
}

/* Call all constructor functions linked into the kernel. */
static void __init do_ctors(void)
{
#ifdef CONFIG_CONSTRUCTORS
        ctor_fn_t *fn = (ctor_fn_t *) __ctors_start;

        for (; fn < (ctor_fn_t *) __ctors_end; fn++)
                (*fn)();
#endif
}

bool initcall_debug;
core_param(initcall_debug, initcall_debug, bool, 0644);

#ifdef CONFIG_KALLSYMS
struct blacklist_entry {
        struct list_head next;
        char *buf;
};

static __initdata_or_module LIST_HEAD(blacklisted_initcalls);

static int __init initcall_blacklist(char *str)
{
        char *str_entry;
        struct blacklist_entry *entry;

        /* str argument is a comma-separated list of functions */
        do {
                str_entry = strsep(&str, ",");
                if (str_entry) {
                        pr_debug("blacklisting initcall %s\n", str_entry);
                        entry = alloc_bootmem(sizeof(*entry));
                        entry->buf = alloc_bootmem(strlen(str_entry) + 1);
                        strcpy(entry->buf, str_entry);
                        list_add(&entry->next, &blacklisted_initcalls);
                }
        } while (str_entry);

        return 0;
}

static bool __init_or_module initcall_blacklisted(initcall_t fn)
{
        struct blacklist_entry *entry;
        char fn_name[KSYM_SYMBOL_LEN];
        unsigned long addr;

        if (list_empty(&blacklisted_initcalls))
                return false;

        addr = (unsigned long) dereference_function_descriptor(fn);
        sprint_symbol_no_offset(fn_name, addr);

        /*
         * fn will be "function_name [module_name]" where [module_name] is not
         * displayed for built-in init functions.  Strip off the [module_name].
         */
        strreplace(fn_name, ' ', '\0');

        list_for_each_entry(entry, &blacklisted_initcalls, next) {
                if (!strcmp(fn_name, entry->buf)) {
                        pr_debug("initcall %s blacklisted\n", fn_name);
                        return true;
                }
        }

        return false;
}
#else
static int __init initcall_blacklist(char *str)
{
        pr_warn("initcall_blacklist requires CONFIG_KALLSYMS\n");
        return 0;
}

static bool __init_or_module initcall_blacklisted(initcall_t fn)
{
        return false;
}
#endif
__setup("initcall_blacklist=", initcall_blacklist);

static int __init_or_module do_one_initcall_debug(initcall_t fn)
{
        ktime_t calltime, delta, rettime;
        unsigned long long duration;
        int ret;

        printk(KERN_DEBUG "calling  %pF @ %i\n", fn, task_pid_nr(current));
        calltime = ktime_get();
        ret = fn();
        rettime = ktime_get();
        delta = ktime_sub(rettime, calltime);
        duration = (unsigned long long) ktime_to_ns(delta) >> 10;
        printk(KERN_DEBUG "initcall %pF returned %d after %lld usecs\n",
                 fn, ret, duration);

        return ret;
}

int __init_or_module do_one_initcall(initcall_t fn)
{
        int count = preempt_count();
        int ret;
        char msgbuf[64];

        if (initcall_blacklisted(fn))
                return -EPERM;

        if (initcall_debug)
                ret = do_one_initcall_debug(fn);
        else
                ret = fn();

        msgbuf[0] = 0;

        if (preempt_count() != count) {
                sprintf(msgbuf, "preemption imbalance ");
                preempt_count_set(count);
        }
        if (irqs_disabled()) {
                strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));
                local_irq_enable();
        }
        WARN(msgbuf[0], "initcall %pF returned with %s\n", fn, msgbuf);

        add_latent_entropy();
        return ret;
}


extern initcall_t __initcall_start[];
extern initcall_t __initcall0_start[];
extern initcall_t __initcall1_start[];
extern initcall_t __initcall2_start[];
extern initcall_t __initcall3_start[];
extern initcall_t __initcall4_start[];
extern initcall_t __initcall5_start[];
extern initcall_t __initcall6_start[];
extern initcall_t __initcall7_start[];
extern initcall_t __initcall_end[];

static initcall_t *initcall_levels[] __initdata = {
        __initcall0_start,
        __initcall1_start,
        __initcall2_start,
        __initcall3_start,
        __initcall4_start,
        __initcall5_start,
        __initcall6_start,
        __initcall7_start,
        __initcall_end,
};

/* Keep these in sync with initcalls in include/linux/init.h */
static char *initcall_level_names[] __initdata = {
        "early",
        "core",
        "postcore",
        "arch",
        "subsys",
        "fs",
        "device",
        "late",
};

static void __init do_initcall_level(int level)
{
        initcall_t *fn;

        strcpy(initcall_command_line, saved_command_line);
        parse_args(initcall_level_names[level],
                   initcall_command_line, __start___param,
                   __stop___param - __start___param,
                   level, level,
                   NULL, &repair_env_string);

        for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
                do_one_initcall(*fn);
}

static void __init do_initcalls(void)
{
        int level;

        for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++)
                do_initcall_level(level);
}

/*
 * Ok, the machine is now initialized. None of the devices
 * have been touched yet, but the CPU subsystem is up and
 * running, and memory and process management works.
 *
 * Now we can finally start doing some real work..
 */
static void __init do_basic_setup(void)
{
        cpuset_init_smp();
        shmem_init();
        driver_init();
        init_irq_proc();
        do_ctors();
        usermodehelper_enable();
        do_initcalls();
        random_int_secret_init();
}

static void __init do_pre_smp_initcalls(void)
{
        initcall_t *fn;

        for (fn = __initcall_start; fn < __initcall0_start; fn++)
                do_one_initcall(*fn);
}

/*
 * This function requests modules which should be loaded by default and is
 * called twice right after initrd is mounted and right before init is
 * exec'd.  If such modules are on either initrd or rootfs, they will be
 * loaded before control is passed to userland.
 */
void __init load_default_modules(void)
{
        load_default_elevator_module();
}

static int run_init_process(const char *init_filename)
{
        argv_init[0] = init_filename;
        return do_execve(getname_kernel(init_filename),
                (const char __user *const __user *)argv_init,
                (const char __user *const __user *)envp_init);
}

static int try_to_run_init_process(const char *init_filename)
{
        int ret;

        ret = run_init_process(init_filename);

        if (ret && ret != -ENOENT) {
                pr_err("Starting init: %s exists but couldn't execute it (error %d)\n",
                       init_filename, ret);
        }

        return ret;
}

static noinline void __init kernel_init_freeable(void);

#if defined(CONFIG_STRICT_KERNEL_RWX) || defined(CONFIG_STRICT_MODULE_RWX)
bool rodata_enabled __ro_after_init = true;
static int __init set_debug_rodata(char *str)
{
        return strtobool(str, &rodata_enabled);
}
__setup("rodata=", set_debug_rodata);
#endif

#ifdef CONFIG_STRICT_KERNEL_RWX
static void mark_readonly(void)
{
        if (rodata_enabled)
                mark_rodata_ro();
        else
                pr_info("Kernel memory protection disabled.\n");
}
#else
static inline void mark_readonly(void)
{
        pr_warn("This architecture does not have kernel memory protection.\n");
}
#endif

static int __ref kernel_init(void *unused)
{
        int ret;

        kernel_init_freeable();
        /* need to finish all async __init code before freeing the memory */
        async_synchronize_full();
        free_initmem();
        mark_readonly();
        system_state = SYSTEM_RUNNING;
        numa_default_policy();

        flush_delayed_fput();

        rcu_end_inkernel_boot();

        if (ramdisk_execute_command) {
                ret = run_init_process(ramdisk_execute_command);
                if (!ret)
                        return 0;
                pr_err("Failed to execute %s (error %d)\n",
                       ramdisk_execute_command, ret);
        }

        /*
         * We try each of these until one succeeds.
         *
         * The Bourne shell can be used instead of init if we are
         * trying to recover a really broken machine.
         */
        if (execute_command) {
                ret = run_init_process(execute_command);
                if (!ret)
                        return 0;
                panic("Requested init %s failed (error %d).",
                      execute_command, ret);
        }
        if (!try_to_run_init_process("/sbin/init") ||
            !try_to_run_init_process("/etc/init") ||
            !try_to_run_init_process("/bin/init") ||
            !try_to_run_init_process("/bin/sh"))
                return 0;

        panic("No working init found.  Try passing init= option to kernel. "
              "See Linux Documentation/admin-guide/init.rst for guidance.");
}

static noinline void __init kernel_init_freeable(void)
{
        /*
         * Wait until kthreadd is all set-up.
         */
        wait_for_completion(&kthreadd_done);

        /* Now the scheduler is fully set up and can do blocking allocations */
        gfp_allowed_mask = __GFP_BITS_MASK;

        /*
         * init can allocate pages on any node
         */
        set_mems_allowed(node_states[N_MEMORY]);
        /*
         * init can run on any cpu.
         */
        set_cpus_allowed_ptr(current, cpu_all_mask);

        cad_pid = task_pid(current);

        smp_prepare_cpus(setup_max_cpus);

        workqueue_init();

        do_pre_smp_initcalls();
        lockup_detector_init();

        smp_init();
        sched_init_smp();

        page_alloc_init_late();

        do_basic_setup();

        /* Open the /dev/console on the rootfs, this should never fail */
        if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
                pr_err("Warning: unable to open an initial console.\n");

        (void) sys_dup(0);
        (void) sys_dup(0);
        /*
         * check if there is an early userspace init.  If yes, let it do all
         * the work
         */

        if (!ramdisk_execute_command)
                ramdisk_execute_command = "/init";

        if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
                ramdisk_execute_command = NULL;
                prepare_namespace();
        }

        /*
         * Ok, we have completed the initial bootup, and
         * we're essentially up and running. Get rid of the
         * initmem segments and start the user-mode stuff..
         *
         * rootfs is available now, try loading the public keys
         * and default modules
         */

        integrity_load_keys();
        load_default_modules();
}