Linux 4.6-rc6

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

#define pr_fmt(fmt) "SMP alternatives: " fmt

#include <linux/module.h>
#include <linux/sched.h>
#include <linux/mutex.h>
#include <linux/list.h>
#include <linux/stringify.h>
#include <linux/mm.h>
#include <linux/vmalloc.h>
#include <linux/memory.h>
#include <linux/stop_machine.h>
#include <linux/slab.h>
#include <linux/kdebug.h>
#include <asm/alternative.h>
#include <asm/sections.h>
#include <asm/pgtable.h>
#include <asm/mce.h>
#include <asm/nmi.h>
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>
#include <asm/io.h>
#include <asm/fixmap.h>

int __read_mostly alternatives_patched;

EXPORT_SYMBOL_GPL(alternatives_patched);

#define MAX_PATCH_LEN (255-1)

static int __initdata_or_module debug_alternative;

static int __init debug_alt(char *str)
{
        debug_alternative = 1;
        return 1;
}
__setup("debug-alternative", debug_alt);

static int noreplace_smp;

static int __init setup_noreplace_smp(char *str)
{
        noreplace_smp = 1;
        return 1;
}
__setup("noreplace-smp", setup_noreplace_smp);

#ifdef CONFIG_PARAVIRT
static int __initdata_or_module noreplace_paravirt = 0;

static int __init setup_noreplace_paravirt(char *str)
{
        noreplace_paravirt = 1;
        return 1;
}
__setup("noreplace-paravirt", setup_noreplace_paravirt);
#endif

#define DPRINTK(fmt, args...)                                           \
do {                                                                    \
        if (debug_alternative)                                          \
                printk(KERN_DEBUG "%s: " fmt "\n", __func__, ##args);   \
} while (0)

#define DUMP_BYTES(buf, len, fmt, args...)                              \
do {                                                                    \
        if (unlikely(debug_alternative)) {                              \
                int j;                                                  \
                                                                        \
                if (!(len))                                             \
                        break;                                          \
                                                                        \
                printk(KERN_DEBUG fmt, ##args);                         \
                for (j = 0; j < (len) - 1; j++)                         \
                        printk(KERN_CONT "%02hhx ", buf[j]);            \
                printk(KERN_CONT "%02hhx\n", buf[j]);                   \
        }                                                               \
} while (0)

/*
 * Each GENERIC_NOPX is of X bytes, and defined as an array of bytes
 * that correspond to that nop. Getting from one nop to the next, we
 * add to the array the offset that is equal to the sum of all sizes of
 * nops preceding the one we are after.
 *
 * Note: The GENERIC_NOP5_ATOMIC is at the end, as it breaks the
 * nice symmetry of sizes of the previous nops.
 */
#if defined(GENERIC_NOP1) && !defined(CONFIG_X86_64)
static const unsigned char intelnops[] =
{
        GENERIC_NOP1,
        GENERIC_NOP2,
        GENERIC_NOP3,
        GENERIC_NOP4,
        GENERIC_NOP5,
        GENERIC_NOP6,
        GENERIC_NOP7,
        GENERIC_NOP8,
        GENERIC_NOP5_ATOMIC
};
static const unsigned char * const intel_nops[ASM_NOP_MAX+2] =
{
        NULL,
        intelnops,
        intelnops + 1,
        intelnops + 1 + 2,
        intelnops + 1 + 2 + 3,
        intelnops + 1 + 2 + 3 + 4,
        intelnops + 1 + 2 + 3 + 4 + 5,
        intelnops + 1 + 2 + 3 + 4 + 5 + 6,
        intelnops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
        intelnops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
};
#endif

#ifdef K8_NOP1
static const unsigned char k8nops[] =
{
        K8_NOP1,
        K8_NOP2,
        K8_NOP3,
        K8_NOP4,
        K8_NOP5,
        K8_NOP6,
        K8_NOP7,
        K8_NOP8,
        K8_NOP5_ATOMIC
};
static const unsigned char * const k8_nops[ASM_NOP_MAX+2] =
{
        NULL,
        k8nops,
        k8nops + 1,
        k8nops + 1 + 2,
        k8nops + 1 + 2 + 3,
        k8nops + 1 + 2 + 3 + 4,
        k8nops + 1 + 2 + 3 + 4 + 5,
        k8nops + 1 + 2 + 3 + 4 + 5 + 6,
        k8nops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
        k8nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
};
#endif

#if defined(K7_NOP1) && !defined(CONFIG_X86_64)
static const unsigned char k7nops[] =
{
        K7_NOP1,
        K7_NOP2,
        K7_NOP3,
        K7_NOP4,
        K7_NOP5,
        K7_NOP6,
        K7_NOP7,
        K7_NOP8,
        K7_NOP5_ATOMIC
};
static const unsigned char * const k7_nops[ASM_NOP_MAX+2] =
{
        NULL,
        k7nops,
        k7nops + 1,
        k7nops + 1 + 2,
        k7nops + 1 + 2 + 3,
        k7nops + 1 + 2 + 3 + 4,
        k7nops + 1 + 2 + 3 + 4 + 5,
        k7nops + 1 + 2 + 3 + 4 + 5 + 6,
        k7nops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
        k7nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
};
#endif

#ifdef P6_NOP1
static const unsigned char p6nops[] =
{
        P6_NOP1,
        P6_NOP2,
        P6_NOP3,
        P6_NOP4,
        P6_NOP5,
        P6_NOP6,
        P6_NOP7,
        P6_NOP8,
        P6_NOP5_ATOMIC
};
static const unsigned char * const p6_nops[ASM_NOP_MAX+2] =
{
        NULL,
        p6nops,
        p6nops + 1,
        p6nops + 1 + 2,
        p6nops + 1 + 2 + 3,
        p6nops + 1 + 2 + 3 + 4,
        p6nops + 1 + 2 + 3 + 4 + 5,
        p6nops + 1 + 2 + 3 + 4 + 5 + 6,
        p6nops + 1 + 2 + 3 + 4 + 5 + 6 + 7,
        p6nops + 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8,
};
#endif

/* Initialize these to a safe default */
#ifdef CONFIG_X86_64
const unsigned char * const *ideal_nops = p6_nops;
#else
const unsigned char * const *ideal_nops = intel_nops;
#endif

void __init arch_init_ideal_nops(void)
{
        switch (boot_cpu_data.x86_vendor) {
        case X86_VENDOR_INTEL:
                /*
                 * Due to a decoder implementation quirk, some
                 * specific Intel CPUs actually perform better with
                 * the "k8_nops" than with the SDM-recommended NOPs.
                 */
                if (boot_cpu_data.x86 == 6 &&
                    boot_cpu_data.x86_model >= 0x0f &&
                    boot_cpu_data.x86_model != 0x1c &&
                    boot_cpu_data.x86_model != 0x26 &&
                    boot_cpu_data.x86_model != 0x27 &&
                    boot_cpu_data.x86_model < 0x30) {
                        ideal_nops = k8_nops;
                } else if (boot_cpu_has(X86_FEATURE_NOPL)) {
                           ideal_nops = p6_nops;
                } else {
#ifdef CONFIG_X86_64
                        ideal_nops = k8_nops;
#else
                        ideal_nops = intel_nops;
#endif
                }
                break;

        case X86_VENDOR_AMD:
                if (boot_cpu_data.x86 > 0xf) {
                        ideal_nops = p6_nops;
                        return;
                }

                /* fall through */

        default:
#ifdef CONFIG_X86_64
                ideal_nops = k8_nops;
#else
                if (boot_cpu_has(X86_FEATURE_K8))
                        ideal_nops = k8_nops;
                else if (boot_cpu_has(X86_FEATURE_K7))
                        ideal_nops = k7_nops;
                else
                        ideal_nops = intel_nops;
#endif
        }
}

/* Use this to add nops to a buffer, then text_poke the whole buffer. */
static void __init_or_module add_nops(void *insns, unsigned int len)
{
        while (len > 0) {
                unsigned int noplen = len;
                if (noplen > ASM_NOP_MAX)
                        noplen = ASM_NOP_MAX;
                memcpy(insns, ideal_nops[noplen], noplen);
                insns += noplen;
                len -= noplen;
        }
}

extern struct alt_instr __alt_instructions[], __alt_instructions_end[];
extern s32 __smp_locks[], __smp_locks_end[];
void *text_poke_early(void *addr, const void *opcode, size_t len);

/*
 * Are we looking at a near JMP with a 1 or 4-byte displacement.
 */
static inline bool is_jmp(const u8 opcode)
{
        return opcode == 0xeb || opcode == 0xe9;
}

static void __init_or_module
recompute_jump(struct alt_instr *a, u8 *orig_insn, u8 *repl_insn, u8 *insnbuf)
{
        u8 *next_rip, *tgt_rip;
        s32 n_dspl, o_dspl;
        int repl_len;

        if (a->replacementlen != 5)
                return;

        o_dspl = *(s32 *)(insnbuf + 1);

        /* next_rip of the replacement JMP */
        next_rip = repl_insn + a->replacementlen;
        /* target rip of the replacement JMP */
        tgt_rip  = next_rip + o_dspl;
        n_dspl = tgt_rip - orig_insn;

        DPRINTK("target RIP: %p, new_displ: 0x%x", tgt_rip, n_dspl);

        if (tgt_rip - orig_insn >= 0) {
                if (n_dspl - 2 <= 127)
                        goto two_byte_jmp;
                else
                        goto five_byte_jmp;
        /* negative offset */
        } else {
                if (((n_dspl - 2) & 0xff) == (n_dspl - 2))
                        goto two_byte_jmp;
                else
                        goto five_byte_jmp;
        }

two_byte_jmp:
        n_dspl -= 2;

        insnbuf[0] = 0xeb;
        insnbuf[1] = (s8)n_dspl;
        add_nops(insnbuf + 2, 3);

        repl_len = 2;
        goto done;

five_byte_jmp:
        n_dspl -= 5;

        insnbuf[0] = 0xe9;
        *(s32 *)&insnbuf[1] = n_dspl;

        repl_len = 5;

done:

        DPRINTK("final displ: 0x%08x, JMP 0x%lx",
                n_dspl, (unsigned long)orig_insn + n_dspl + repl_len);
}

static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
{
        unsigned long flags;

        if (instr[0] != 0x90)
                return;

        local_irq_save(flags);
        add_nops(instr + (a->instrlen - a->padlen), a->padlen);
        sync_core();
        local_irq_restore(flags);

        DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
                   instr, a->instrlen - a->padlen, a->padlen);
}

/*
 * Replace instructions with better alternatives for this CPU type. This runs
 * before SMP is initialized to avoid SMP problems with self modifying code.
 * This implies that asymmetric systems where APs have less capabilities than
 * the boot processor are not handled. Tough. Make sure you disable such
 * features by hand.
 */
void __init_or_module apply_alternatives(struct alt_instr *start,
                                         struct alt_instr *end)
{
        struct alt_instr *a;
        u8 *instr, *replacement;
        u8 insnbuf[MAX_PATCH_LEN];

        DPRINTK("alt table %p -> %p", start, end);
        /*
         * The scan order should be from start to end. A later scanned
         * alternative code can overwrite previously scanned alternative code.
         * Some kernel functions (e.g. memcpy, memset, etc) use this order to
         * patch code.
         *
         * So be careful if you want to change the scan order to any other
         * order.
         */
        for (a = start; a < end; a++) {
                int insnbuf_sz = 0;

                instr = (u8 *)&a->instr_offset + a->instr_offset;
                replacement = (u8 *)&a->repl_offset + a->repl_offset;
                BUG_ON(a->instrlen > sizeof(insnbuf));
                BUG_ON(a->cpuid >= (NCAPINTS + NBUGINTS) * 32);
                if (!boot_cpu_has(a->cpuid)) {
                        if (a->padlen > 1)
                                optimize_nops(a, instr);

                        continue;
                }

                DPRINTK("feat: %d*32+%d, old: (%p, len: %d), repl: (%p, len: %d), pad: %d",
                        a->cpuid >> 5,
                        a->cpuid & 0x1f,
                        instr, a->instrlen,
                        replacement, a->replacementlen, a->padlen);

                DUMP_BYTES(instr, a->instrlen, "%p: old_insn: ", instr);
                DUMP_BYTES(replacement, a->replacementlen, "%p: rpl_insn: ", replacement);

                memcpy(insnbuf, replacement, a->replacementlen);
                insnbuf_sz = a->replacementlen;

                /* 0xe8 is a relative jump; fix the offset. */
                if (*insnbuf == 0xe8 && a->replacementlen == 5) {
                        *(s32 *)(insnbuf + 1) += replacement - instr;
                        DPRINTK("Fix CALL offset: 0x%x, CALL 0x%lx",
                                *(s32 *)(insnbuf + 1),
                                (unsigned long)instr + *(s32 *)(insnbuf + 1) + 5);
                }

                if (a->replacementlen && is_jmp(replacement[0]))
                        recompute_jump(a, instr, replacement, insnbuf);

                if (a->instrlen > a->replacementlen) {
                        add_nops(insnbuf + a->replacementlen,
                                 a->instrlen - a->replacementlen);
                        insnbuf_sz += a->instrlen - a->replacementlen;
                }
                DUMP_BYTES(insnbuf, insnbuf_sz, "%p: final_insn: ", instr);

                text_poke_early(instr, insnbuf, insnbuf_sz);
        }
}

#ifdef CONFIG_SMP
static void alternatives_smp_lock(const s32 *start, const s32 *end,
                                  u8 *text, u8 *text_end)
{
        const s32 *poff;

        mutex_lock(&text_mutex);
        for (poff = start; poff < end; poff++) {
                u8 *ptr = (u8 *)poff + *poff;

                if (!*poff || ptr < text || ptr >= text_end)
                        continue;
                /* turn DS segment override prefix into lock prefix */
                if (*ptr == 0x3e)
                        text_poke(ptr, ((unsigned char []){0xf0}), 1);
        }
        mutex_unlock(&text_mutex);
}

static void alternatives_smp_unlock(const s32 *start, const s32 *end,
                                    u8 *text, u8 *text_end)
{
        const s32 *poff;

        mutex_lock(&text_mutex);
        for (poff = start; poff < end; poff++) {
                u8 *ptr = (u8 *)poff + *poff;

                if (!*poff || ptr < text || ptr >= text_end)
                        continue;
                /* turn lock prefix into DS segment override prefix */
                if (*ptr == 0xf0)
                        text_poke(ptr, ((unsigned char []){0x3E}), 1);
        }
        mutex_unlock(&text_mutex);
}

struct smp_alt_module {
        /* what is this ??? */
        struct module   *mod;
        char            *name;

        /* ptrs to lock prefixes */
        const s32       *locks;
        const s32       *locks_end;

        /* .text segment, needed to avoid patching init code ;) */
        u8              *text;
        u8              *text_end;

        struct list_head next;
};
static LIST_HEAD(smp_alt_modules);
static DEFINE_MUTEX(smp_alt);
static bool uniproc_patched = false;    /* protected by smp_alt */

void __init_or_module alternatives_smp_module_add(struct module *mod,
                                                  char *name,
                                                  void *locks, void *locks_end,
                                                  void *text,  void *text_end)
{
        struct smp_alt_module *smp;

        mutex_lock(&smp_alt);
        if (!uniproc_patched)
                goto unlock;

        if (num_possible_cpus() == 1)
                /* Don't bother remembering, we'll never have to undo it. */
                goto smp_unlock;

        smp = kzalloc(sizeof(*smp), GFP_KERNEL);
        if (NULL == smp)
                /* we'll run the (safe but slow) SMP code then ... */
                goto unlock;

        smp->mod        = mod;
        smp->name       = name;
        smp->locks      = locks;
        smp->locks_end  = locks_end;
        smp->text       = text;
        smp->text_end   = text_end;
        DPRINTK("locks %p -> %p, text %p -> %p, name %s\n",
                smp->locks, smp->locks_end,
                smp->text, smp->text_end, smp->name);

        list_add_tail(&smp->next, &smp_alt_modules);
smp_unlock:
        alternatives_smp_unlock(locks, locks_end, text, text_end);
unlock:
        mutex_unlock(&smp_alt);
}

void __init_or_module alternatives_smp_module_del(struct module *mod)
{
        struct smp_alt_module *item;

        mutex_lock(&smp_alt);
        list_for_each_entry(item, &smp_alt_modules, next) {
                if (mod != item->mod)
                        continue;
                list_del(&item->next);
                kfree(item);
                break;
        }
        mutex_unlock(&smp_alt);
}

void alternatives_enable_smp(void)
{
        struct smp_alt_module *mod;

        /* Why bother if there are no other CPUs? */
        BUG_ON(num_possible_cpus() == 1);

        mutex_lock(&smp_alt);

        if (uniproc_patched) {
                pr_info("switching to SMP code\n");
                BUG_ON(num_online_cpus() != 1);
                clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
                clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
                list_for_each_entry(mod, &smp_alt_modules, next)
                        alternatives_smp_lock(mod->locks, mod->locks_end,
                                              mod->text, mod->text_end);
                uniproc_patched = false;
        }
        mutex_unlock(&smp_alt);
}

/* Return 1 if the address range is reserved for smp-alternatives */
int alternatives_text_reserved(void *start, void *end)
{
        struct smp_alt_module *mod;
        const s32 *poff;
        u8 *text_start = start;
        u8 *text_end = end;

        list_for_each_entry(mod, &smp_alt_modules, next) {
                if (mod->text > text_end || mod->text_end < text_start)
                        continue;
                for (poff = mod->locks; poff < mod->locks_end; poff++) {
                        const u8 *ptr = (const u8 *)poff + *poff;

                        if (text_start <= ptr && text_end > ptr)
                                return 1;
                }
        }

        return 0;
}
#endif /* CONFIG_SMP */

#ifdef CONFIG_PARAVIRT
void __init_or_module apply_paravirt(struct paravirt_patch_site *start,
                                     struct paravirt_patch_site *end)
{
        struct paravirt_patch_site *p;
        char insnbuf[MAX_PATCH_LEN];

        if (noreplace_paravirt)
                return;

        for (p = start; p < end; p++) {
                unsigned int used;

                BUG_ON(p->len > MAX_PATCH_LEN);
                /* prep the buffer with the original instructions */
                memcpy(insnbuf, p->instr, p->len);
                used = pv_init_ops.patch(p->instrtype, p->clobbers, insnbuf,
                                         (unsigned long)p->instr, p->len);

                BUG_ON(used > p->len);

                /* Pad the rest with nops */
                add_nops(insnbuf + used, p->len - used);
                text_poke_early(p->instr, insnbuf, p->len);
        }
}
extern struct paravirt_patch_site __start_parainstructions[],
        __stop_parainstructions[];
#endif  /* CONFIG_PARAVIRT */

void __init alternative_instructions(void)
{
        /* The patching is not fully atomic, so try to avoid local interruptions
           that might execute the to be patched code.
           Other CPUs are not running. */
        stop_nmi();

        /*
         * Don't stop machine check exceptions while patching.
         * MCEs only happen when something got corrupted and in this
         * case we must do something about the corruption.
         * Ignoring it is worse than a unlikely patching race.
         * Also machine checks tend to be broadcast and if one CPU
         * goes into machine check the others follow quickly, so we don't
         * expect a machine check to cause undue problems during to code
         * patching.
         */

        apply_alternatives(__alt_instructions, __alt_instructions_end);

#ifdef CONFIG_SMP
        /* Patch to UP if other cpus not imminent. */
        if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1)) {
                uniproc_patched = true;
                alternatives_smp_module_add(NULL, "core kernel",
                                            __smp_locks, __smp_locks_end,
                                            _text, _etext);
        }

        if (!uniproc_patched || num_possible_cpus() == 1)
                free_init_pages("SMP alternatives",
                                (unsigned long)__smp_locks,
                                (unsigned long)__smp_locks_end);
#endif

        apply_paravirt(__parainstructions, __parainstructions_end);

        restart_nmi();
        alternatives_patched = 1;
}

/**
 * text_poke_early - Update instructions on a live kernel at boot time
 * @addr: address to modify
 * @opcode: source of the copy
 * @len: length to copy
 *
 * When you use this code to patch more than one byte of an instruction
 * you need to make sure that other CPUs cannot execute this code in parallel.
 * Also no thread must be currently preempted in the middle of these
 * instructions. And on the local CPU you need to be protected again NMI or MCE
 * handlers seeing an inconsistent instruction while you patch.
 */
void *__init_or_module text_poke_early(void *addr, const void *opcode,
                                              size_t len)
{
        unsigned long flags;
        local_irq_save(flags);
        memcpy(addr, opcode, len);
        sync_core();
        local_irq_restore(flags);
        /* Could also do a CLFLUSH here to speed up CPU recovery; but
           that causes hangs on some VIA CPUs. */
        return addr;
}

/**
 * text_poke - Update instructions on a live kernel
 * @addr: address to modify
 * @opcode: source of the copy
 * @len: length to copy
 *
 * Only atomic text poke/set should be allowed when not doing early patching.
 * It means the size must be writable atomically and the address must be aligned
 * in a way that permits an atomic write. It also makes sure we fit on a single
 * page.
 *
 * Note: Must be called under text_mutex.
 */
void *text_poke(void *addr, const void *opcode, size_t len)
{
        unsigned long flags;
        char *vaddr;
        struct page *pages[2];
        int i;

        if (!core_kernel_text((unsigned long)addr)) {
                pages[0] = vmalloc_to_page(addr);
                pages[1] = vmalloc_to_page(addr + PAGE_SIZE);
        } else {
                pages[0] = virt_to_page(addr);
                WARN_ON(!PageReserved(pages[0]));
                pages[1] = virt_to_page(addr + PAGE_SIZE);
        }
        BUG_ON(!pages[0]);
        local_irq_save(flags);
        set_fixmap(FIX_TEXT_POKE0, page_to_phys(pages[0]));
        if (pages[1])
                set_fixmap(FIX_TEXT_POKE1, page_to_phys(pages[1]));
        vaddr = (char *)fix_to_virt(FIX_TEXT_POKE0);
        memcpy(&vaddr[(unsigned long)addr & ~PAGE_MASK], opcode, len);
        clear_fixmap(FIX_TEXT_POKE0);
        if (pages[1])
                clear_fixmap(FIX_TEXT_POKE1);
        local_flush_tlb();
        sync_core();
        /* Could also do a CLFLUSH here to speed up CPU recovery; but
           that causes hangs on some VIA CPUs. */
        for (i = 0; i < len; i++)
                BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
        local_irq_restore(flags);
        return addr;
}

static void do_sync_core(void *info)
{
        sync_core();
}

static bool bp_patching_in_progress;
static void *bp_int3_handler, *bp_int3_addr;

int poke_int3_handler(struct pt_regs *regs)
{
        /* bp_patching_in_progress */
        smp_rmb();

        if (likely(!bp_patching_in_progress))
                return 0;

        if (user_mode(regs) || regs->ip != (unsigned long)bp_int3_addr)
                return 0;

        /* set up the specified breakpoint handler */
        regs->ip = (unsigned long) bp_int3_handler;

        return 1;

}

/**
 * text_poke_bp() -- update instructions on live kernel on SMP
 * @addr:       address to patch
 * @opcode:     opcode of new instruction
 * @len:        length to copy
 * @handler:    address to jump to when the temporary breakpoint is hit
 *
 * Modify multi-byte instruction by using int3 breakpoint on SMP.
 * We completely avoid stop_machine() here, and achieve the
 * synchronization using int3 breakpoint.
 *
 * The way it is done:
 *      - add a int3 trap to the address that will be patched
 *      - sync cores
 *      - update all but the first byte of the patched range
 *      - sync cores
 *      - replace the first byte (int3) by the first byte of
 *        replacing opcode
 *      - sync cores
 *
 * Note: must be called under text_mutex.
 */
void *text_poke_bp(void *addr, const void *opcode, size_t len, void *handler)
{
        unsigned char int3 = 0xcc;

        bp_int3_handler = handler;
        bp_int3_addr = (u8 *)addr + sizeof(int3);
        bp_patching_in_progress = true;
        /*
         * Corresponding read barrier in int3 notifier for
         * making sure the in_progress flags is correctly ordered wrt.
         * patching
         */
        smp_wmb();

        text_poke(addr, &int3, sizeof(int3));

        on_each_cpu(do_sync_core, NULL, 1);

        if (len - sizeof(int3) > 0) {
                /* patch all but the first byte */
                text_poke((char *)addr + sizeof(int3),
                          (const char *) opcode + sizeof(int3),
                          len - sizeof(int3));
                /*
                 * According to Intel, this core syncing is very likely
                 * not necessary and we'd be safe even without it. But
                 * better safe than sorry (plus there's not only Intel).
                 */
                on_each_cpu(do_sync_core, NULL, 1);
        }

        /* patch the first byte */
        text_poke(addr, opcode, sizeof(int3));

        on_each_cpu(do_sync_core, NULL, 1);

        bp_patching_in_progress = false;
        smp_wmb();

        return addr;
}