mm: fix exec activate_mm vs TLB shootdown and lazy tlb switching race

[linux/fpc-iii.git] / arch / x86 / kernel / process.c

// SPDX-License-Identifier: GPL-2.0
#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/prctl.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/sched/idle.h>
#include <linux/sched/debug.h>
#include <linux/sched/task.h>
#include <linux/sched/task_stack.h>
#include <linux/init.h>
#include <linux/export.h>
#include <linux/pm.h>
#include <linux/tick.h>
#include <linux/random.h>
#include <linux/user-return-notifier.h>
#include <linux/dmi.h>
#include <linux/utsname.h>
#include <linux/stackprotector.h>
#include <linux/tick.h>
#include <linux/cpuidle.h>
#include <trace/events/power.h>
#include <linux/hw_breakpoint.h>
#include <asm/cpu.h>
#include <asm/apic.h>
#include <asm/syscalls.h>
#include <linux/uaccess.h>
#include <asm/mwait.h>
#include <asm/fpu/internal.h>
#include <asm/debugreg.h>
#include <asm/nmi.h>
#include <asm/tlbflush.h>
#include <asm/mce.h>
#include <asm/vm86.h>
#include <asm/switch_to.h>
#include <asm/desc.h>
#include <asm/prctl.h>
#include <asm/spec-ctrl.h>

#include "process.h"

/*
 * per-CPU TSS segments. Threads are completely 'soft' on Linux,
 * no more per-task TSS's. The TSS size is kept cacheline-aligned
 * so they are allowed to end up in the .data..cacheline_aligned
 * section. Since TSS's are completely CPU-local, we want them
 * on exact cacheline boundaries, to eliminate cacheline ping-pong.
 */
__visible DEFINE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw) = {
        .x86_tss = {
                /*
                 * .sp0 is only used when entering ring 0 from a lower
                 * privilege level.  Since the init task never runs anything
                 * but ring 0 code, there is no need for a valid value here.
                 * Poison it.
                 */
                .sp0 = (1UL << (BITS_PER_LONG-1)) + 1,

#ifdef CONFIG_X86_64
                /*
                 * .sp1 is cpu_current_top_of_stack.  The init task never
                 * runs user code, but cpu_current_top_of_stack should still
                 * be well defined before the first context switch.
                 */
                .sp1 = TOP_OF_INIT_STACK,
#endif

#ifdef CONFIG_X86_32
                .ss0 = __KERNEL_DS,
                .ss1 = __KERNEL_CS,
                .io_bitmap_base = INVALID_IO_BITMAP_OFFSET,
#endif
         },
#ifdef CONFIG_X86_32
         /*
          * Note that the .io_bitmap member must be extra-big. This is because
          * the CPU will access an additional byte beyond the end of the IO
          * permission bitmap. The extra byte must be all 1 bits, and must
          * be within the limit.
          */
        .io_bitmap              = { [0 ... IO_BITMAP_LONGS] = ~0 },
#endif
};
EXPORT_PER_CPU_SYMBOL(cpu_tss_rw);

DEFINE_PER_CPU(bool, __tss_limit_invalid);
EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid);

/*
 * this gets called so that we can store lazy state into memory and copy the
 * current task into the new thread.
 */
int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
{
        memcpy(dst, src, arch_task_struct_size);
#ifdef CONFIG_VM86
        dst->thread.vm86 = NULL;
#endif

        return fpu__copy(&dst->thread.fpu, &src->thread.fpu);
}

/*
 * Free current thread data structures etc..
 */
void exit_thread(struct task_struct *tsk)
{
        struct thread_struct *t = &tsk->thread;
        unsigned long *bp = t->io_bitmap_ptr;
        struct fpu *fpu = &t->fpu;

        if (bp) {
                struct tss_struct *tss = &per_cpu(cpu_tss_rw, get_cpu());

                t->io_bitmap_ptr = NULL;
                clear_thread_flag(TIF_IO_BITMAP);
                /*
                 * Careful, clear this in the TSS too:
                 */
                memset(tss->io_bitmap, 0xff, t->io_bitmap_max);
                t->io_bitmap_max = 0;
                put_cpu();
                kfree(bp);
        }

        free_vm86(t);

        fpu__drop(fpu);
}

void flush_thread(void)
{
        struct task_struct *tsk = current;

        flush_ptrace_hw_breakpoint(tsk);
        memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));

        fpu__clear(&tsk->thread.fpu);
}

void disable_TSC(void)
{
        preempt_disable();
        if (!test_and_set_thread_flag(TIF_NOTSC))
                /*
                 * Must flip the CPU state synchronously with
                 * TIF_NOTSC in the current running context.
                 */
                cr4_set_bits(X86_CR4_TSD);
        preempt_enable();
}

static void enable_TSC(void)
{
        preempt_disable();
        if (test_and_clear_thread_flag(TIF_NOTSC))
                /*
                 * Must flip the CPU state synchronously with
                 * TIF_NOTSC in the current running context.
                 */
                cr4_clear_bits(X86_CR4_TSD);
        preempt_enable();
}

int get_tsc_mode(unsigned long adr)
{
        unsigned int val;

        if (test_thread_flag(TIF_NOTSC))
                val = PR_TSC_SIGSEGV;
        else
                val = PR_TSC_ENABLE;

        return put_user(val, (unsigned int __user *)adr);
}

int set_tsc_mode(unsigned int val)
{
        if (val == PR_TSC_SIGSEGV)
                disable_TSC();
        else if (val == PR_TSC_ENABLE)
                enable_TSC();
        else
                return -EINVAL;

        return 0;
}

DEFINE_PER_CPU(u64, msr_misc_features_shadow);

static void set_cpuid_faulting(bool on)
{
        u64 msrval;

        msrval = this_cpu_read(msr_misc_features_shadow);
        msrval &= ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
        msrval |= (on << MSR_MISC_FEATURES_ENABLES_CPUID_FAULT_BIT);
        this_cpu_write(msr_misc_features_shadow, msrval);
        wrmsrl(MSR_MISC_FEATURES_ENABLES, msrval);
}

static void disable_cpuid(void)
{
        preempt_disable();
        if (!test_and_set_thread_flag(TIF_NOCPUID)) {
                /*
                 * Must flip the CPU state synchronously with
                 * TIF_NOCPUID in the current running context.
                 */
                set_cpuid_faulting(true);
        }
        preempt_enable();
}

static void enable_cpuid(void)
{
        preempt_disable();
        if (test_and_clear_thread_flag(TIF_NOCPUID)) {
                /*
                 * Must flip the CPU state synchronously with
                 * TIF_NOCPUID in the current running context.
                 */
                set_cpuid_faulting(false);
        }
        preempt_enable();
}

static int get_cpuid_mode(void)
{
        return !test_thread_flag(TIF_NOCPUID);
}

static int set_cpuid_mode(struct task_struct *task, unsigned long cpuid_enabled)
{
        if (!static_cpu_has(X86_FEATURE_CPUID_FAULT))
                return -ENODEV;

        if (cpuid_enabled)
                enable_cpuid();
        else
                disable_cpuid();

        return 0;
}

/*
 * Called immediately after a successful exec.
 */
void arch_setup_new_exec(void)
{
        /* If cpuid was previously disabled for this task, re-enable it. */
        if (test_thread_flag(TIF_NOCPUID))
                enable_cpuid();
}

static inline void switch_to_bitmap(struct thread_struct *prev,
                                    struct thread_struct *next,
                                    unsigned long tifp, unsigned long tifn)
{
        struct tss_struct *tss = this_cpu_ptr(&cpu_tss_rw);

        if (tifn & _TIF_IO_BITMAP) {
                /*
                 * Copy the relevant range of the IO bitmap.
                 * Normally this is 128 bytes or less:
                 */
                memcpy(tss->io_bitmap, next->io_bitmap_ptr,
                       max(prev->io_bitmap_max, next->io_bitmap_max));
                /*
                 * Make sure that the TSS limit is correct for the CPU
                 * to notice the IO bitmap.
                 */
                refresh_tss_limit();
        } else if (tifp & _TIF_IO_BITMAP) {
                /*
                 * Clear any possible leftover bits:
                 */
                memset(tss->io_bitmap, 0xff, prev->io_bitmap_max);
        }
}

#ifdef CONFIG_SMP

struct ssb_state {
        struct ssb_state        *shared_state;
        raw_spinlock_t          lock;
        unsigned int            disable_state;
        unsigned long           local_state;
};

#define LSTATE_SSB      0

static DEFINE_PER_CPU(struct ssb_state, ssb_state);

void speculative_store_bypass_ht_init(void)
{
        struct ssb_state *st = this_cpu_ptr(&ssb_state);
        unsigned int this_cpu = smp_processor_id();
        unsigned int cpu;

        st->local_state = 0;

        /*
         * Shared state setup happens once on the first bringup
         * of the CPU. It's not destroyed on CPU hotunplug.
         */
        if (st->shared_state)
                return;

        raw_spin_lock_init(&st->lock);

        /*
         * Go over HT siblings and check whether one of them has set up the
         * shared state pointer already.
         */
        for_each_cpu(cpu, topology_sibling_cpumask(this_cpu)) {
                if (cpu == this_cpu)
                        continue;

                if (!per_cpu(ssb_state, cpu).shared_state)
                        continue;

                /* Link it to the state of the sibling: */
                st->shared_state = per_cpu(ssb_state, cpu).shared_state;
                return;
        }

        /*
         * First HT sibling to come up on the core.  Link shared state of
         * the first HT sibling to itself. The siblings on the same core
         * which come up later will see the shared state pointer and link
         * themself to the state of this CPU.
         */
        st->shared_state = st;
}

/*
 * Logic is: First HT sibling enables SSBD for both siblings in the core
 * and last sibling to disable it, disables it for the whole core. This how
 * MSR_SPEC_CTRL works in "hardware":
 *
 *  CORE_SPEC_CTRL = THREAD0_SPEC_CTRL | THREAD1_SPEC_CTRL
 */
static __always_inline void amd_set_core_ssb_state(unsigned long tifn)
{
        struct ssb_state *st = this_cpu_ptr(&ssb_state);
        u64 msr = x86_amd_ls_cfg_base;

        if (!static_cpu_has(X86_FEATURE_ZEN)) {
                msr |= ssbd_tif_to_amd_ls_cfg(tifn);
                wrmsrl(MSR_AMD64_LS_CFG, msr);
                return;
        }

        if (tifn & _TIF_SSBD) {
                /*
                 * Since this can race with prctl(), block reentry on the
                 * same CPU.
                 */
                if (__test_and_set_bit(LSTATE_SSB, &st->local_state))
                        return;

                msr |= x86_amd_ls_cfg_ssbd_mask;

                raw_spin_lock(&st->shared_state->lock);
                /* First sibling enables SSBD: */
                if (!st->shared_state->disable_state)
                        wrmsrl(MSR_AMD64_LS_CFG, msr);
                st->shared_state->disable_state++;
                raw_spin_unlock(&st->shared_state->lock);
        } else {
                if (!__test_and_clear_bit(LSTATE_SSB, &st->local_state))
                        return;

                raw_spin_lock(&st->shared_state->lock);
                st->shared_state->disable_state--;
                if (!st->shared_state->disable_state)
                        wrmsrl(MSR_AMD64_LS_CFG, msr);
                raw_spin_unlock(&st->shared_state->lock);
        }
}
#else
static __always_inline void amd_set_core_ssb_state(unsigned long tifn)
{
        u64 msr = x86_amd_ls_cfg_base | ssbd_tif_to_amd_ls_cfg(tifn);

        wrmsrl(MSR_AMD64_LS_CFG, msr);
}
#endif

static __always_inline void amd_set_ssb_virt_state(unsigned long tifn)
{
        /*
         * SSBD has the same definition in SPEC_CTRL and VIRT_SPEC_CTRL,
         * so ssbd_tif_to_spec_ctrl() just works.
         */
        wrmsrl(MSR_AMD64_VIRT_SPEC_CTRL, ssbd_tif_to_spec_ctrl(tifn));
}

/*
 * Update the MSRs managing speculation control, during context switch.
 *
 * tifp: Previous task's thread flags
 * tifn: Next task's thread flags
 */
static __always_inline void __speculation_ctrl_update(unsigned long tifp,
                                                      unsigned long tifn)
{
        unsigned long tif_diff = tifp ^ tifn;
        u64 msr = x86_spec_ctrl_base;
        bool updmsr = false;

        /* Handle change of TIF_SSBD depending on the mitigation method. */
        if (static_cpu_has(X86_FEATURE_VIRT_SSBD)) {
                if (tif_diff & _TIF_SSBD)
                        amd_set_ssb_virt_state(tifn);
        } else if (static_cpu_has(X86_FEATURE_LS_CFG_SSBD)) {
                if (tif_diff & _TIF_SSBD)
                        amd_set_core_ssb_state(tifn);
        } else if (static_cpu_has(X86_FEATURE_SPEC_CTRL_SSBD) ||
                   static_cpu_has(X86_FEATURE_AMD_SSBD)) {
                updmsr |= !!(tif_diff & _TIF_SSBD);
                msr |= ssbd_tif_to_spec_ctrl(tifn);
        }

        /* Only evaluate TIF_SPEC_IB if conditional STIBP is enabled. */
        if (IS_ENABLED(CONFIG_SMP) &&
            static_branch_unlikely(&switch_to_cond_stibp)) {
                updmsr |= !!(tif_diff & _TIF_SPEC_IB);
                msr |= stibp_tif_to_spec_ctrl(tifn);
        }

        if (updmsr)
                wrmsrl(MSR_IA32_SPEC_CTRL, msr);
}

static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)
{
        if (test_and_clear_tsk_thread_flag(tsk, TIF_SPEC_FORCE_UPDATE)) {
                if (task_spec_ssb_disable(tsk))
                        set_tsk_thread_flag(tsk, TIF_SSBD);
                else
                        clear_tsk_thread_flag(tsk, TIF_SSBD);

                if (task_spec_ib_disable(tsk))
                        set_tsk_thread_flag(tsk, TIF_SPEC_IB);
                else
                        clear_tsk_thread_flag(tsk, TIF_SPEC_IB);
        }
        /* Return the updated threadinfo flags*/
        return task_thread_info(tsk)->flags;
}

void speculation_ctrl_update(unsigned long tif)
{
        unsigned long flags;

        /* Forced update. Make sure all relevant TIF flags are different */
        local_irq_save(flags);
        __speculation_ctrl_update(~tif, tif);
        local_irq_restore(flags);
}

/* Called from seccomp/prctl update */
void speculation_ctrl_update_current(void)
{
        preempt_disable();
        speculation_ctrl_update(speculation_ctrl_update_tif(current));
        preempt_enable();
}

void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p)
{
        struct thread_struct *prev, *next;
        unsigned long tifp, tifn;

        prev = &prev_p->thread;
        next = &next_p->thread;

        tifn = READ_ONCE(task_thread_info(next_p)->flags);
        tifp = READ_ONCE(task_thread_info(prev_p)->flags);
        switch_to_bitmap(prev, next, tifp, tifn);

        propagate_user_return_notify(prev_p, next_p);

        if ((tifp & _TIF_BLOCKSTEP || tifn & _TIF_BLOCKSTEP) &&
            arch_has_block_step()) {
                unsigned long debugctl, msk;

                rdmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
                debugctl &= ~DEBUGCTLMSR_BTF;
                msk = tifn & _TIF_BLOCKSTEP;
                debugctl |= (msk >> TIF_BLOCKSTEP) << DEBUGCTLMSR_BTF_SHIFT;
                wrmsrl(MSR_IA32_DEBUGCTLMSR, debugctl);
        }

        if ((tifp ^ tifn) & _TIF_NOTSC)
                cr4_toggle_bits(X86_CR4_TSD);

        if ((tifp ^ tifn) & _TIF_NOCPUID)
                set_cpuid_faulting(!!(tifn & _TIF_NOCPUID));

        if (likely(!((tifp | tifn) & _TIF_SPEC_FORCE_UPDATE))) {
                __speculation_ctrl_update(tifp, tifn);
        } else {
                speculation_ctrl_update_tif(prev_p);
                tifn = speculation_ctrl_update_tif(next_p);

                /* Enforce MSR update to ensure consistent state */
                __speculation_ctrl_update(~tifn, tifn);
        }
}

/*
 * Idle related variables and functions
 */
unsigned long boot_option_idle_override = IDLE_NO_OVERRIDE;
EXPORT_SYMBOL(boot_option_idle_override);

static void (*x86_idle)(void);

#ifndef CONFIG_SMP
static inline void play_dead(void)
{
        BUG();
}
#endif

void arch_cpu_idle_enter(void)
{
        tsc_verify_tsc_adjust(false);
        local_touch_nmi();
}

void arch_cpu_idle_dead(void)
{
        play_dead();
}

/*
 * Called from the generic idle code.
 */
void arch_cpu_idle(void)
{
        x86_idle();
}

/*
 * We use this if we don't have any better idle routine..
 */
void __cpuidle default_idle(void)
{
        trace_cpu_idle_rcuidle(1, smp_processor_id());
        safe_halt();
        trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
}
#ifdef CONFIG_APM_MODULE
EXPORT_SYMBOL(default_idle);
#endif

#ifdef CONFIG_XEN
bool xen_set_default_idle(void)
{
        bool ret = !!x86_idle;

        x86_idle = default_idle;

        return ret;
}
#endif

void stop_this_cpu(void *dummy)
{
        local_irq_disable();
        /*
         * Remove this CPU:
         */
        set_cpu_online(smp_processor_id(), false);
        disable_local_APIC();
        mcheck_cpu_clear(this_cpu_ptr(&cpu_info));

        /*
         * Use wbinvd on processors that support SME. This provides support
         * for performing a successful kexec when going from SME inactive
         * to SME active (or vice-versa). The cache must be cleared so that
         * if there are entries with the same physical address, both with and
         * without the encryption bit, they don't race each other when flushed
         * and potentially end up with the wrong entry being committed to
         * memory.
         */
        if (boot_cpu_has(X86_FEATURE_SME))
                native_wbinvd();
        for (;;) {
                /*
                 * Use native_halt() so that memory contents don't change
                 * (stack usage and variables) after possibly issuing the
                 * native_wbinvd() above.
                 */
                native_halt();
        }
}

/*
 * AMD Erratum 400 aware idle routine. We handle it the same way as C3 power
 * states (local apic timer and TSC stop).
 */
static void amd_e400_idle(void)
{
        /*
         * We cannot use static_cpu_has_bug() here because X86_BUG_AMD_APIC_C1E
         * gets set after static_cpu_has() places have been converted via
         * alternatives.
         */
        if (!boot_cpu_has_bug(X86_BUG_AMD_APIC_C1E)) {
                default_idle();
                return;
        }

        tick_broadcast_enter();

        default_idle();

        /*
         * The switch back from broadcast mode needs to be called with
         * interrupts disabled.
         */
        local_irq_disable();
        tick_broadcast_exit();
        local_irq_enable();
}

/*
 * Intel Core2 and older machines prefer MWAIT over HALT for C1.
 * We can't rely on cpuidle installing MWAIT, because it will not load
 * on systems that support only C1 -- so the boot default must be MWAIT.
 *
 * Some AMD machines are the opposite, they depend on using HALT.
 *
 * So for default C1, which is used during boot until cpuidle loads,
 * use MWAIT-C1 on Intel HW that has it, else use HALT.
 */
static int prefer_mwait_c1_over_halt(const struct cpuinfo_x86 *c)
{
        if (c->x86_vendor != X86_VENDOR_INTEL)
                return 0;

        if (!cpu_has(c, X86_FEATURE_MWAIT) || static_cpu_has_bug(X86_BUG_MONITOR))
                return 0;

        return 1;
}

/*
 * MONITOR/MWAIT with no hints, used for default C1 state. This invokes MWAIT
 * with interrupts enabled and no flags, which is backwards compatible with the
 * original MWAIT implementation.
 */
static __cpuidle void mwait_idle(void)
{
        if (!current_set_polling_and_test()) {
                trace_cpu_idle_rcuidle(1, smp_processor_id());
                if (this_cpu_has(X86_BUG_CLFLUSH_MONITOR)) {
                        mb(); /* quirk */
                        clflush((void *)&current_thread_info()->flags);
                        mb(); /* quirk */
                }

                __monitor((void *)&current_thread_info()->flags, 0, 0);
                if (!need_resched())
                        __sti_mwait(0, 0);
                else
                        local_irq_enable();
                trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
        } else {
                local_irq_enable();
        }
        __current_clr_polling();
}

void select_idle_routine(const struct cpuinfo_x86 *c)
{
#ifdef CONFIG_SMP
        if (boot_option_idle_override == IDLE_POLL && smp_num_siblings > 1)
                pr_warn_once("WARNING: polling idle and HT enabled, performance may degrade\n");
#endif
        if (x86_idle || boot_option_idle_override == IDLE_POLL)
                return;

        if (boot_cpu_has_bug(X86_BUG_AMD_E400)) {
                pr_info("using AMD E400 aware idle routine\n");
                x86_idle = amd_e400_idle;
        } else if (prefer_mwait_c1_over_halt(c)) {
                pr_info("using mwait in idle threads\n");
                x86_idle = mwait_idle;
        } else
                x86_idle = default_idle;
}

void amd_e400_c1e_apic_setup(void)
{
        if (boot_cpu_has_bug(X86_BUG_AMD_APIC_C1E)) {
                pr_info("Switch to broadcast mode on CPU%d\n", smp_processor_id());
                local_irq_disable();
                tick_broadcast_force();
                local_irq_enable();
        }
}

void __init arch_post_acpi_subsys_init(void)
{
        u32 lo, hi;

        if (!boot_cpu_has_bug(X86_BUG_AMD_E400))
                return;

        /*
         * AMD E400 detection needs to happen after ACPI has been enabled. If
         * the machine is affected K8_INTP_C1E_ACTIVE_MASK bits are set in
         * MSR_K8_INT_PENDING_MSG.
         */
        rdmsr(MSR_K8_INT_PENDING_MSG, lo, hi);
        if (!(lo & K8_INTP_C1E_ACTIVE_MASK))
                return;

        boot_cpu_set_bug(X86_BUG_AMD_APIC_C1E);

        if (!boot_cpu_has(X86_FEATURE_NONSTOP_TSC))
                mark_tsc_unstable("TSC halt in AMD C1E");
        pr_info("System has AMD C1E enabled\n");
}

static int __init idle_setup(char *str)
{
        if (!str)
                return -EINVAL;

        if (!strcmp(str, "poll")) {
                pr_info("using polling idle threads\n");
                boot_option_idle_override = IDLE_POLL;
                cpu_idle_poll_ctrl(true);
        } else if (!strcmp(str, "halt")) {
                /*
                 * When the boot option of idle=halt is added, halt is
                 * forced to be used for CPU idle. In such case CPU C2/C3
                 * won't be used again.
                 * To continue to load the CPU idle driver, don't touch
                 * the boot_option_idle_override.
                 */
                x86_idle = default_idle;
                boot_option_idle_override = IDLE_HALT;
        } else if (!strcmp(str, "nomwait")) {
                /*
                 * If the boot option of "idle=nomwait" is added,
                 * it means that mwait will be disabled for CPU C2/C3
                 * states. In such case it won't touch the variable
                 * of boot_option_idle_override.
                 */
                boot_option_idle_override = IDLE_NOMWAIT;
        } else
                return -1;

        return 0;
}
early_param("idle", idle_setup);

unsigned long arch_align_stack(unsigned long sp)
{
        if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space)
                sp -= get_random_int() % 8192;
        return sp & ~0xf;
}

unsigned long arch_randomize_brk(struct mm_struct *mm)
{
        return randomize_page(mm->brk, 0x02000000);
}

/*
 * Called from fs/proc with a reference on @p to find the function
 * which called into schedule(). This needs to be done carefully
 * because the task might wake up and we might look at a stack
 * changing under us.
 */
unsigned long get_wchan(struct task_struct *p)
{
        unsigned long start, bottom, top, sp, fp, ip, ret = 0;
        int count = 0;

        if (!p || p == current || p->state == TASK_RUNNING)
                return 0;

        if (!try_get_task_stack(p))
                return 0;

        start = (unsigned long)task_stack_page(p);
        if (!start)
                goto out;

        /*
         * Layout of the stack page:
         *
         * ----------- topmax = start + THREAD_SIZE - sizeof(unsigned long)
         * PADDING
         * ----------- top = topmax - TOP_OF_KERNEL_STACK_PADDING
         * stack
         * ----------- bottom = start
         *
         * The tasks stack pointer points at the location where the
         * framepointer is stored. The data on the stack is:
         * ... IP FP ... IP FP
         *
         * We need to read FP and IP, so we need to adjust the upper
         * bound by another unsigned long.
         */
        top = start + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
        top -= 2 * sizeof(unsigned long);
        bottom = start;

        sp = READ_ONCE(p->thread.sp);
        if (sp < bottom || sp > top)
                goto out;

        fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
        do {
                if (fp < bottom || fp > top)
                        goto out;
                ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
                if (!in_sched_functions(ip)) {
                        ret = ip;
                        goto out;
                }
                fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
        } while (count++ < 16 && p->state != TASK_RUNNING);

out:
        put_task_stack(p);
        return ret;
}

long do_arch_prctl_common(struct task_struct *task, int option,
                          unsigned long cpuid_enabled)
{
        switch (option) {
        case ARCH_GET_CPUID:
                return get_cpuid_mode();
        case ARCH_SET_CPUID:
                return set_cpuid_mode(task, cpuid_enabled);
        }

        return -EINVAL;
}