drm/panthor: Don't add write fences to the shared BOs

[drm/drm-misc.git] / arch / x86 / kernel / uprobes.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * User-space Probes (UProbes) for x86
 *
 * Copyright (C) IBM Corporation, 2008-2011
 * Authors:
 *      Srikar Dronamraju
 *      Jim Keniston
 */
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/ptrace.h>
#include <linux/uprobes.h>
#include <linux/uaccess.h>
#include <linux/syscalls.h>

#include <linux/kdebug.h>
#include <asm/processor.h>
#include <asm/insn.h>
#include <asm/mmu_context.h>

/* Post-execution fixups. */

/* Adjust IP back to vicinity of actual insn */
#define UPROBE_FIX_IP           0x01

/* Adjust the return address of a call insn */
#define UPROBE_FIX_CALL         0x02

/* Instruction will modify TF, don't change it */
#define UPROBE_FIX_SETF         0x04

#define UPROBE_FIX_RIP_SI       0x08
#define UPROBE_FIX_RIP_DI       0x10
#define UPROBE_FIX_RIP_BX       0x20
#define UPROBE_FIX_RIP_MASK     \
        (UPROBE_FIX_RIP_SI | UPROBE_FIX_RIP_DI | UPROBE_FIX_RIP_BX)

#define UPROBE_TRAP_NR          UINT_MAX

/* Adaptations for mhiramat x86 decoder v14. */
#define OPCODE1(insn)           ((insn)->opcode.bytes[0])
#define OPCODE2(insn)           ((insn)->opcode.bytes[1])
#define OPCODE3(insn)           ((insn)->opcode.bytes[2])
#define MODRM_REG(insn)         X86_MODRM_REG((insn)->modrm.value)

#define W(row, b0, b1, b2, b3, b4, b5, b6, b7, b8, b9, ba, bb, bc, bd, be, bf)\
        (((b0##UL << 0x0)|(b1##UL << 0x1)|(b2##UL << 0x2)|(b3##UL << 0x3) |   \
          (b4##UL << 0x4)|(b5##UL << 0x5)|(b6##UL << 0x6)|(b7##UL << 0x7) |   \
          (b8##UL << 0x8)|(b9##UL << 0x9)|(ba##UL << 0xa)|(bb##UL << 0xb) |   \
          (bc##UL << 0xc)|(bd##UL << 0xd)|(be##UL << 0xe)|(bf##UL << 0xf))    \
         << (row % 32))

/*
 * Good-instruction tables for 32-bit apps.  This is non-const and volatile
 * to keep gcc from statically optimizing it out, as variable_test_bit makes
 * some versions of gcc to think only *(unsigned long*) is used.
 *
 * Opcodes we'll probably never support:
 * 6c-6f - ins,outs. SEGVs if used in userspace
 * e4-e7 - in,out imm. SEGVs if used in userspace
 * ec-ef - in,out acc. SEGVs if used in userspace
 * cc - int3. SIGTRAP if used in userspace
 * ce - into. Not used in userspace - no kernel support to make it useful. SEGVs
 *      (why we support bound (62) then? it's similar, and similarly unused...)
 * f1 - int1. SIGTRAP if used in userspace
 * f4 - hlt. SEGVs if used in userspace
 * fa - cli. SEGVs if used in userspace
 * fb - sti. SEGVs if used in userspace
 *
 * Opcodes which need some work to be supported:
 * 07,17,1f - pop es/ss/ds
 *      Normally not used in userspace, but would execute if used.
 *      Can cause GP or stack exception if tries to load wrong segment descriptor.
 *      We hesitate to run them under single step since kernel's handling
 *      of userspace single-stepping (TF flag) is fragile.
 *      We can easily refuse to support push es/cs/ss/ds (06/0e/16/1e)
 *      on the same grounds that they are never used.
 * cd - int N.
 *      Used by userspace for "int 80" syscall entry. (Other "int N"
 *      cause GP -> SEGV since their IDT gates don't allow calls from CPL 3).
 *      Not supported since kernel's handling of userspace single-stepping
 *      (TF flag) is fragile.
 * cf - iret. Normally not used in userspace. Doesn't SEGV unless arguments are bad
 */
#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
static volatile u32 good_insns_32[256 / 32] = {
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
        /*      ----------------------------------------------         */
        W(0x00, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1) | /* 00 */
        W(0x10, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 0) , /* 10 */
        W(0x20, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 20 */
        W(0x30, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 30 */
        W(0x40, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 40 */
        W(0x50, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 50 */
        W(0x60, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0) | /* 60 */
        W(0x70, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 70 */
        W(0x80, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 80 */
        W(0x90, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 90 */
        W(0xa0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* a0 */
        W(0xb0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* b0 */
        W(0xc0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0) | /* c0 */
        W(0xd0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* d0 */
        W(0xe0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0) | /* e0 */
        W(0xf0, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1)   /* f0 */
        /*      ----------------------------------------------         */
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
};
#else
#define good_insns_32   NULL
#endif

/* Good-instruction tables for 64-bit apps.
 *
 * Genuinely invalid opcodes:
 * 06,07 - formerly push/pop es
 * 0e - formerly push cs
 * 16,17 - formerly push/pop ss
 * 1e,1f - formerly push/pop ds
 * 27,2f,37,3f - formerly daa/das/aaa/aas
 * 60,61 - formerly pusha/popa
 * 62 - formerly bound. EVEX prefix for AVX512 (not yet supported)
 * 82 - formerly redundant encoding of Group1
 * 9a - formerly call seg:ofs
 * ce - formerly into
 * d4,d5 - formerly aam/aad
 * d6 - formerly undocumented salc
 * ea - formerly jmp seg:ofs
 *
 * Opcodes we'll probably never support:
 * 6c-6f - ins,outs. SEGVs if used in userspace
 * e4-e7 - in,out imm. SEGVs if used in userspace
 * ec-ef - in,out acc. SEGVs if used in userspace
 * cc - int3. SIGTRAP if used in userspace
 * f1 - int1. SIGTRAP if used in userspace
 * f4 - hlt. SEGVs if used in userspace
 * fa - cli. SEGVs if used in userspace
 * fb - sti. SEGVs if used in userspace
 *
 * Opcodes which need some work to be supported:
 * cd - int N.
 *      Used by userspace for "int 80" syscall entry. (Other "int N"
 *      cause GP -> SEGV since their IDT gates don't allow calls from CPL 3).
 *      Not supported since kernel's handling of userspace single-stepping
 *      (TF flag) is fragile.
 * cf - iret. Normally not used in userspace. Doesn't SEGV unless arguments are bad
 */
#if defined(CONFIG_X86_64)
static volatile u32 good_insns_64[256 / 32] = {
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
        /*      ----------------------------------------------         */
        W(0x00, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 1) | /* 00 */
        W(0x10, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1, 0, 0) , /* 10 */
        W(0x20, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 0) | /* 20 */
        W(0x30, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 0) , /* 30 */
        W(0x40, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 40 */
        W(0x50, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 50 */
        W(0x60, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0) | /* 60 */
        W(0x70, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 70 */
        W(0x80, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 80 */
        W(0x90, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1) , /* 90 */
        W(0xa0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* a0 */
        W(0xb0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* b0 */
        W(0xc0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0) | /* c0 */
        W(0xd0, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* d0 */
        W(0xe0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0) | /* e0 */
        W(0xf0, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1)   /* f0 */
        /*      ----------------------------------------------         */
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
};
#else
#define good_insns_64   NULL
#endif

/* Using this for both 64-bit and 32-bit apps.
 * Opcodes we don't support:
 * 0f 00 - SLDT/STR/LLDT/LTR/VERR/VERW/-/- group. System insns
 * 0f 01 - SGDT/SIDT/LGDT/LIDT/SMSW/-/LMSW/INVLPG group.
 *      Also encodes tons of other system insns if mod=11.
 *      Some are in fact non-system: xend, xtest, rdtscp, maybe more
 * 0f 05 - syscall
 * 0f 06 - clts (CPL0 insn)
 * 0f 07 - sysret
 * 0f 08 - invd (CPL0 insn)
 * 0f 09 - wbinvd (CPL0 insn)
 * 0f 0b - ud2
 * 0f 30 - wrmsr (CPL0 insn) (then why rdmsr is allowed, it's also CPL0 insn?)
 * 0f 34 - sysenter
 * 0f 35 - sysexit
 * 0f 37 - getsec
 * 0f 78 - vmread (Intel VMX. CPL0 insn)
 * 0f 79 - vmwrite (Intel VMX. CPL0 insn)
 *      Note: with prefixes, these two opcodes are
 *      extrq/insertq/AVX512 convert vector ops.
 * 0f ae - group15: [f]xsave,[f]xrstor,[v]{ld,st}mxcsr,clflush[opt],
 *      {rd,wr}{fs,gs}base,{s,l,m}fence.
 *      Why? They are all user-executable.
 */
static volatile u32 good_2byte_insns[256 / 32] = {
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
        /*      ----------------------------------------------         */
        W(0x00, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 1, 1, 1) | /* 00 */
        W(0x10, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 10 */
        W(0x20, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 20 */
        W(0x30, 0, 1, 1, 1, 0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1) , /* 30 */
        W(0x40, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 40 */
        W(0x50, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 50 */
        W(0x60, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 60 */
        W(0x70, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 1) , /* 70 */
        W(0x80, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* 80 */
        W(0x90, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* 90 */
        W(0xa0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1) | /* a0 */
        W(0xb0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* b0 */
        W(0xc0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* c0 */
        W(0xd0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) , /* d0 */
        W(0xe0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1) | /* e0 */
        W(0xf0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1)   /* f0 */
        /*      ----------------------------------------------         */
        /*      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f         */
};
#undef W

/*
 * opcodes we may need to refine support for:
 *
 *  0f - 2-byte instructions: For many of these instructions, the validity
 *  depends on the prefix and/or the reg field.  On such instructions, we
 *  just consider the opcode combination valid if it corresponds to any
 *  valid instruction.
 *
 *  8f - Group 1 - only reg = 0 is OK
 *  c6-c7 - Group 11 - only reg = 0 is OK
 *  d9-df - fpu insns with some illegal encodings
 *  f2, f3 - repnz, repz prefixes.  These are also the first byte for
 *  certain floating-point instructions, such as addsd.
 *
 *  fe - Group 4 - only reg = 0 or 1 is OK
 *  ff - Group 5 - only reg = 0-6 is OK
 *
 * others -- Do we need to support these?
 *
 *  0f - (floating-point?) prefetch instructions
 *  07, 17, 1f - pop es, pop ss, pop ds
 *  26, 2e, 36, 3e - es:, cs:, ss:, ds: segment prefixes --
 *      but 64 and 65 (fs: and gs:) seem to be used, so we support them
 *  67 - addr16 prefix
 *  ce - into
 *  f0 - lock prefix
 */

/*
 * TODO:
 * - Where necessary, examine the modrm byte and allow only valid instructions
 * in the different Groups and fpu instructions.
 */

static bool is_prefix_bad(struct insn *insn)
{
        insn_byte_t p;
        int i;

        for_each_insn_prefix(insn, i, p) {
                insn_attr_t attr;

                attr = inat_get_opcode_attribute(p);
                switch (attr) {
                case INAT_MAKE_PREFIX(INAT_PFX_ES):
                case INAT_MAKE_PREFIX(INAT_PFX_CS):
                case INAT_MAKE_PREFIX(INAT_PFX_DS):
                case INAT_MAKE_PREFIX(INAT_PFX_SS):
                case INAT_MAKE_PREFIX(INAT_PFX_LOCK):
                        return true;
                }
        }
        return false;
}

static int uprobe_init_insn(struct arch_uprobe *auprobe, struct insn *insn, bool x86_64)
{
        enum insn_mode m = x86_64 ? INSN_MODE_64 : INSN_MODE_32;
        u32 volatile *good_insns;
        int ret;

        ret = insn_decode(insn, auprobe->insn, sizeof(auprobe->insn), m);
        if (ret < 0)
                return -ENOEXEC;

        if (is_prefix_bad(insn))
                return -ENOTSUPP;

        /* We should not singlestep on the exception masking instructions */
        if (insn_masking_exception(insn))
                return -ENOTSUPP;

        if (x86_64)
                good_insns = good_insns_64;
        else
                good_insns = good_insns_32;

        if (test_bit(OPCODE1(insn), (unsigned long *)good_insns))
                return 0;

        if (insn->opcode.nbytes == 2) {
                if (test_bit(OPCODE2(insn), (unsigned long *)good_2byte_insns))
                        return 0;
        }

        return -ENOTSUPP;
}

#ifdef CONFIG_X86_64

asm (
        ".pushsection .rodata\n"
        ".global uretprobe_trampoline_entry\n"
        "uretprobe_trampoline_entry:\n"
        "pushq %rax\n"
        "pushq %rcx\n"
        "pushq %r11\n"
        "movq $" __stringify(__NR_uretprobe) ", %rax\n"
        "syscall\n"
        ".global uretprobe_syscall_check\n"
        "uretprobe_syscall_check:\n"
        "popq %r11\n"
        "popq %rcx\n"

        /* The uretprobe syscall replaces stored %rax value with final
         * return address, so we don't restore %rax in here and just
         * call ret.
         */
        "retq\n"
        ".global uretprobe_trampoline_end\n"
        "uretprobe_trampoline_end:\n"
        ".popsection\n"
);

extern u8 uretprobe_trampoline_entry[];
extern u8 uretprobe_trampoline_end[];
extern u8 uretprobe_syscall_check[];

void *arch_uprobe_trampoline(unsigned long *psize)
{
        static uprobe_opcode_t insn = UPROBE_SWBP_INSN;
        struct pt_regs *regs = task_pt_regs(current);

        /*
         * At the moment the uretprobe syscall trampoline is supported
         * only for native 64-bit process, the compat process still uses
         * standard breakpoint.
         */
        if (user_64bit_mode(regs)) {
                *psize = uretprobe_trampoline_end - uretprobe_trampoline_entry;
                return uretprobe_trampoline_entry;
        }

        *psize = UPROBE_SWBP_INSN_SIZE;
        return &insn;
}

static unsigned long trampoline_check_ip(void)
{
        unsigned long tramp = uprobe_get_trampoline_vaddr();

        return tramp + (uretprobe_syscall_check - uretprobe_trampoline_entry);
}

SYSCALL_DEFINE0(uretprobe)
{
        struct pt_regs *regs = task_pt_regs(current);
        unsigned long err, ip, sp, r11_cx_ax[3];

        if (regs->ip != trampoline_check_ip())
                goto sigill;

        err = copy_from_user(r11_cx_ax, (void __user *)regs->sp, sizeof(r11_cx_ax));
        if (err)
                goto sigill;

        /* expose the "right" values of r11/cx/ax/sp to uprobe_consumer/s */
        regs->r11 = r11_cx_ax[0];
        regs->cx  = r11_cx_ax[1];
        regs->ax  = r11_cx_ax[2];
        regs->sp += sizeof(r11_cx_ax);
        regs->orig_ax = -1;

        ip = regs->ip;
        sp = regs->sp;

        uprobe_handle_trampoline(regs);

        /*
         * Some of the uprobe consumers has changed sp, we can do nothing,
         * just return via iret.
         * .. or shadow stack is enabled, in which case we need to skip
         * return through the user space stack address.
         */
        if (regs->sp != sp || shstk_is_enabled())
                return regs->ax;
        regs->sp -= sizeof(r11_cx_ax);

        /* for the case uprobe_consumer has changed r11/cx */
        r11_cx_ax[0] = regs->r11;
        r11_cx_ax[1] = regs->cx;

        /*
         * ax register is passed through as return value, so we can use
         * its space on stack for ip value and jump to it through the
         * trampoline's ret instruction
         */
        r11_cx_ax[2] = regs->ip;
        regs->ip = ip;

        err = copy_to_user((void __user *)regs->sp, r11_cx_ax, sizeof(r11_cx_ax));
        if (err)
                goto sigill;

        /* ensure sysret, see do_syscall_64() */
        regs->r11 = regs->flags;
        regs->cx  = regs->ip;

        return regs->ax;

sigill:
        force_sig(SIGILL);
        return -1;
}

/*
 * If arch_uprobe->insn doesn't use rip-relative addressing, return
 * immediately.  Otherwise, rewrite the instruction so that it accesses
 * its memory operand indirectly through a scratch register.  Set
 * defparam->fixups accordingly. (The contents of the scratch register
 * will be saved before we single-step the modified instruction,
 * and restored afterward).
 *
 * We do this because a rip-relative instruction can access only a
 * relatively small area (+/- 2 GB from the instruction), and the XOL
 * area typically lies beyond that area.  At least for instructions
 * that store to memory, we can't execute the original instruction
 * and "fix things up" later, because the misdirected store could be
 * disastrous.
 *
 * Some useful facts about rip-relative instructions:
 *
 *  - There's always a modrm byte with bit layout "00 reg 101".
 *  - There's never a SIB byte.
 *  - The displacement is always 4 bytes.
 *  - REX.B=1 bit in REX prefix, which normally extends r/m field,
 *    has no effect on rip-relative mode. It doesn't make modrm byte
 *    with r/m=101 refer to register 1101 = R13.
 */
static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
{
        u8 *cursor;
        u8 reg;
        u8 reg2;

        if (!insn_rip_relative(insn))
                return;

        /*
         * insn_rip_relative() would have decoded rex_prefix, vex_prefix, modrm.
         * Clear REX.b bit (extension of MODRM.rm field):
         * we want to encode low numbered reg, not r8+.
         */
        if (insn->rex_prefix.nbytes) {
                cursor = auprobe->insn + insn_offset_rex_prefix(insn);
                /* REX byte has 0100wrxb layout, clearing REX.b bit */
                *cursor &= 0xfe;
        }
        /*
         * Similar treatment for VEX3/EVEX prefix.
         * TODO: add XOP treatment when insn decoder supports them
         */
        if (insn->vex_prefix.nbytes >= 3) {
                /*
                 * vex2:     c5    rvvvvLpp   (has no b bit)
                 * vex3/xop: c4/8f rxbmmmmm wvvvvLpp
                 * evex:     62    rxbR00mm wvvvv1pp zllBVaaa
                 * Setting VEX3.b (setting because it has inverted meaning).
                 * Setting EVEX.x since (in non-SIB encoding) EVEX.x
                 * is the 4th bit of MODRM.rm, and needs the same treatment.
                 * For VEX3-encoded insns, VEX3.x value has no effect in
                 * non-SIB encoding, the change is superfluous but harmless.
                 */
                cursor = auprobe->insn + insn_offset_vex_prefix(insn) + 1;
                *cursor |= 0x60;
        }

        /*
         * Convert from rip-relative addressing to register-relative addressing
         * via a scratch register.
         *
         * This is tricky since there are insns with modrm byte
         * which also use registers not encoded in modrm byte:
         * [i]div/[i]mul: implicitly use dx:ax
         * shift ops: implicitly use cx
         * cmpxchg: implicitly uses ax
         * cmpxchg8/16b: implicitly uses dx:ax and bx:cx
         *   Encoding: 0f c7/1 modrm
         *   The code below thinks that reg=1 (cx), chooses si as scratch.
         * mulx: implicitly uses dx: mulx r/m,r1,r2 does r1:r2 = dx * r/m.
         *   First appeared in Haswell (BMI2 insn). It is vex-encoded.
         *   Example where none of bx,cx,dx can be used as scratch reg:
         *   c4 e2 63 f6 0d disp32   mulx disp32(%rip),%ebx,%ecx
         * [v]pcmpistri: implicitly uses cx, xmm0
         * [v]pcmpistrm: implicitly uses xmm0
         * [v]pcmpestri: implicitly uses ax, dx, cx, xmm0
         * [v]pcmpestrm: implicitly uses ax, dx, xmm0
         *   Evil SSE4.2 string comparison ops from hell.
         * maskmovq/[v]maskmovdqu: implicitly uses (ds:rdi) as destination.
         *   Encoding: 0f f7 modrm, 66 0f f7 modrm, vex-encoded: c5 f9 f7 modrm.
         *   Store op1, byte-masked by op2 msb's in each byte, to (ds:rdi).
         *   AMD says it has no 3-operand form (vex.vvvv must be 1111)
         *   and that it can have only register operands, not mem
         *   (its modrm byte must have mode=11).
         *   If these restrictions will ever be lifted,
         *   we'll need code to prevent selection of di as scratch reg!
         *
         * Summary: I don't know any insns with modrm byte which
         * use SI register implicitly. DI register is used only
         * by one insn (maskmovq) and BX register is used
         * only by one too (cmpxchg8b).
         * BP is stack-segment based (may be a problem?).
         * AX, DX, CX are off-limits (many implicit users).
         * SP is unusable (it's stack pointer - think about "pop mem";
         * also, rsp+disp32 needs sib encoding -> insn length change).
         */

        reg = MODRM_REG(insn);  /* Fetch modrm.reg */
        reg2 = 0xff;            /* Fetch vex.vvvv */
        if (insn->vex_prefix.nbytes)
                reg2 = insn->vex_prefix.bytes[2];
        /*
         * TODO: add XOP vvvv reading.
         *
         * vex.vvvv field is in bits 6-3, bits are inverted.
         * But in 32-bit mode, high-order bit may be ignored.
         * Therefore, let's consider only 3 low-order bits.
         */
        reg2 = ((reg2 >> 3) & 0x7) ^ 0x7;
        /*
         * Register numbering is ax,cx,dx,bx, sp,bp,si,di, r8..r15.
         *
         * Choose scratch reg. Order is important: must not select bx
         * if we can use si (cmpxchg8b case!)
         */
        if (reg != 6 && reg2 != 6) {
                reg2 = 6;
                auprobe->defparam.fixups |= UPROBE_FIX_RIP_SI;
        } else if (reg != 7 && reg2 != 7) {
                reg2 = 7;
                auprobe->defparam.fixups |= UPROBE_FIX_RIP_DI;
                /* TODO (paranoia): force maskmovq to not use di */
        } else {
                reg2 = 3;
                auprobe->defparam.fixups |= UPROBE_FIX_RIP_BX;
        }
        /*
         * Point cursor at the modrm byte.  The next 4 bytes are the
         * displacement.  Beyond the displacement, for some instructions,
         * is the immediate operand.
         */
        cursor = auprobe->insn + insn_offset_modrm(insn);
        /*
         * Change modrm from "00 reg 101" to "10 reg reg2". Example:
         * 89 05 disp32  mov %eax,disp32(%rip) becomes
         * 89 86 disp32  mov %eax,disp32(%rsi)
         */
        *cursor = 0x80 | (reg << 3) | reg2;
}

static inline unsigned long *
scratch_reg(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        if (auprobe->defparam.fixups & UPROBE_FIX_RIP_SI)
                return &regs->si;
        if (auprobe->defparam.fixups & UPROBE_FIX_RIP_DI)
                return &regs->di;
        return &regs->bx;
}

/*
 * If we're emulating a rip-relative instruction, save the contents
 * of the scratch register and store the target address in that register.
 */
static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        if (auprobe->defparam.fixups & UPROBE_FIX_RIP_MASK) {
                struct uprobe_task *utask = current->utask;
                unsigned long *sr = scratch_reg(auprobe, regs);

                utask->autask.saved_scratch_register = *sr;
                *sr = utask->vaddr + auprobe->defparam.ilen;
        }
}

static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        if (auprobe->defparam.fixups & UPROBE_FIX_RIP_MASK) {
                struct uprobe_task *utask = current->utask;
                unsigned long *sr = scratch_reg(auprobe, regs);

                *sr = utask->autask.saved_scratch_register;
        }
}
#else /* 32-bit: */
/*
 * No RIP-relative addressing on 32-bit
 */
static void riprel_analyze(struct arch_uprobe *auprobe, struct insn *insn)
{
}
static void riprel_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
}
static void riprel_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
}
#endif /* CONFIG_X86_64 */

struct uprobe_xol_ops {
        bool    (*emulate)(struct arch_uprobe *, struct pt_regs *);
        int     (*pre_xol)(struct arch_uprobe *, struct pt_regs *);
        int     (*post_xol)(struct arch_uprobe *, struct pt_regs *);
        void    (*abort)(struct arch_uprobe *, struct pt_regs *);
};

static inline int sizeof_long(struct pt_regs *regs)
{
        /*
         * Check registers for mode as in_xxx_syscall() does not apply here.
         */
        return user_64bit_mode(regs) ? 8 : 4;
}

static int default_pre_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        riprel_pre_xol(auprobe, regs);
        return 0;
}

static int emulate_push_stack(struct pt_regs *regs, unsigned long val)
{
        unsigned long new_sp = regs->sp - sizeof_long(regs);

        if (copy_to_user((void __user *)new_sp, &val, sizeof_long(regs)))
                return -EFAULT;

        regs->sp = new_sp;
        return 0;
}

/*
 * We have to fix things up as follows:
 *
 * Typically, the new ip is relative to the copied instruction.  We need
 * to make it relative to the original instruction (FIX_IP).  Exceptions
 * are return instructions and absolute or indirect jump or call instructions.
 *
 * If the single-stepped instruction was a call, the return address that
 * is atop the stack is the address following the copied instruction.  We
 * need to make it the address following the original instruction (FIX_CALL).
 *
 * If the original instruction was a rip-relative instruction such as
 * "movl %edx,0xnnnn(%rip)", we have instead executed an equivalent
 * instruction using a scratch register -- e.g., "movl %edx,0xnnnn(%rsi)".
 * We need to restore the contents of the scratch register
 * (FIX_RIP_reg).
 */
static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        struct uprobe_task *utask = current->utask;

        riprel_post_xol(auprobe, regs);
        if (auprobe->defparam.fixups & UPROBE_FIX_IP) {
                long correction = utask->vaddr - utask->xol_vaddr;
                regs->ip += correction;
        } else if (auprobe->defparam.fixups & UPROBE_FIX_CALL) {
                regs->sp += sizeof_long(regs); /* Pop incorrect return address */
                if (emulate_push_stack(regs, utask->vaddr + auprobe->defparam.ilen))
                        return -ERESTART;
        }
        /* popf; tell the caller to not touch TF */
        if (auprobe->defparam.fixups & UPROBE_FIX_SETF)
                utask->autask.saved_tf = true;

        return 0;
}

static void default_abort_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        riprel_post_xol(auprobe, regs);
}

static const struct uprobe_xol_ops default_xol_ops = {
        .pre_xol  = default_pre_xol_op,
        .post_xol = default_post_xol_op,
        .abort    = default_abort_op,
};

static bool branch_is_call(struct arch_uprobe *auprobe)
{
        return auprobe->branch.opc1 == 0xe8;
}

#define CASE_COND                                       \
        COND(70, 71, XF(OF))                            \
        COND(72, 73, XF(CF))                            \
        COND(74, 75, XF(ZF))                            \
        COND(78, 79, XF(SF))                            \
        COND(7a, 7b, XF(PF))                            \
        COND(76, 77, XF(CF) || XF(ZF))                  \
        COND(7c, 7d, XF(SF) != XF(OF))                  \
        COND(7e, 7f, XF(ZF) || XF(SF) != XF(OF))

#define COND(op_y, op_n, expr)                          \
        case 0x ## op_y: DO((expr) != 0)                \
        case 0x ## op_n: DO((expr) == 0)

#define XF(xf)  (!!(flags & X86_EFLAGS_ ## xf))

static bool is_cond_jmp_opcode(u8 opcode)
{
        switch (opcode) {
        #define DO(expr)        \
                return true;
        CASE_COND
        #undef  DO

        default:
                return false;
        }
}

static bool check_jmp_cond(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        unsigned long flags = regs->flags;

        switch (auprobe->branch.opc1) {
        #define DO(expr)        \
                return expr;
        CASE_COND
        #undef  DO

        default:        /* not a conditional jmp */
                return true;
        }
}

#undef  XF
#undef  COND
#undef  CASE_COND

static bool branch_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        unsigned long new_ip = regs->ip += auprobe->branch.ilen;
        unsigned long offs = (long)auprobe->branch.offs;

        if (branch_is_call(auprobe)) {
                /*
                 * If it fails we execute this (mangled, see the comment in
                 * branch_clear_offset) insn out-of-line. In the likely case
                 * this should trigger the trap, and the probed application
                 * should die or restart the same insn after it handles the
                 * signal, arch_uprobe_post_xol() won't be even called.
                 *
                 * But there is corner case, see the comment in ->post_xol().
                 */
                if (emulate_push_stack(regs, new_ip))
                        return false;
        } else if (!check_jmp_cond(auprobe, regs)) {
                offs = 0;
        }

        regs->ip = new_ip + offs;
        return true;
}

static bool push_emulate_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        unsigned long *src_ptr = (void *)regs + auprobe->push.reg_offset;

        if (emulate_push_stack(regs, *src_ptr))
                return false;
        regs->ip += auprobe->push.ilen;
        return true;
}

static int branch_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        BUG_ON(!branch_is_call(auprobe));
        /*
         * We can only get here if branch_emulate_op() failed to push the ret
         * address _and_ another thread expanded our stack before the (mangled)
         * "call" insn was executed out-of-line. Just restore ->sp and restart.
         * We could also restore ->ip and try to call branch_emulate_op() again.
         */
        regs->sp += sizeof_long(regs);
        return -ERESTART;
}

static void branch_clear_offset(struct arch_uprobe *auprobe, struct insn *insn)
{
        /*
         * Turn this insn into "call 1f; 1:", this is what we will execute
         * out-of-line if ->emulate() fails. We only need this to generate
         * a trap, so that the probed task receives the correct signal with
         * the properly filled siginfo.
         *
         * But see the comment in ->post_xol(), in the unlikely case it can
         * succeed. So we need to ensure that the new ->ip can not fall into
         * the non-canonical area and trigger #GP.
         *
         * We could turn it into (say) "pushf", but then we would need to
         * divorce ->insn[] and ->ixol[]. We need to preserve the 1st byte
         * of ->insn[] for set_orig_insn().
         */
        memset(auprobe->insn + insn_offset_immediate(insn),
                0, insn->immediate.nbytes);
}

static const struct uprobe_xol_ops branch_xol_ops = {
        .emulate  = branch_emulate_op,
        .post_xol = branch_post_xol_op,
};

static const struct uprobe_xol_ops push_xol_ops = {
        .emulate  = push_emulate_op,
};

/* Returns -ENOSYS if branch_xol_ops doesn't handle this insn */
static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
{
        u8 opc1 = OPCODE1(insn);
        insn_byte_t p;
        int i;

        switch (opc1) {
        case 0xeb:      /* jmp 8 */
        case 0xe9:      /* jmp 32 */
                break;
        case 0x90:      /* prefix* + nop; same as jmp with .offs = 0 */
                goto setup;

        case 0xe8:      /* call relative */
                branch_clear_offset(auprobe, insn);
                break;

        case 0x0f:
                if (insn->opcode.nbytes != 2)
                        return -ENOSYS;
                /*
                 * If it is a "near" conditional jmp, OPCODE2() - 0x10 matches
                 * OPCODE1() of the "short" jmp which checks the same condition.
                 */
                opc1 = OPCODE2(insn) - 0x10;
                fallthrough;
        default:
                if (!is_cond_jmp_opcode(opc1))
                        return -ENOSYS;
        }

        /*
         * 16-bit overrides such as CALLW (66 e8 nn nn) are not supported.
         * Intel and AMD behavior differ in 64-bit mode: Intel ignores 66 prefix.
         * No one uses these insns, reject any branch insns with such prefix.
         */
        for_each_insn_prefix(insn, i, p) {
                if (p == 0x66)
                        return -ENOTSUPP;
        }

setup:
        auprobe->branch.opc1 = opc1;
        auprobe->branch.ilen = insn->length;
        auprobe->branch.offs = insn->immediate.value;

        auprobe->ops = &branch_xol_ops;
        return 0;
}

/* Returns -ENOSYS if push_xol_ops doesn't handle this insn */
static int push_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
{
        u8 opc1 = OPCODE1(insn), reg_offset = 0;

        if (opc1 < 0x50 || opc1 > 0x57)
                return -ENOSYS;

        if (insn->length > 2)
                return -ENOSYS;
        if (insn->length == 2) {
                /* only support rex_prefix 0x41 (x64 only) */
#ifdef CONFIG_X86_64
                if (insn->rex_prefix.nbytes != 1 ||
                    insn->rex_prefix.bytes[0] != 0x41)
                        return -ENOSYS;

                switch (opc1) {
                case 0x50:
                        reg_offset = offsetof(struct pt_regs, r8);
                        break;
                case 0x51:
                        reg_offset = offsetof(struct pt_regs, r9);
                        break;
                case 0x52:
                        reg_offset = offsetof(struct pt_regs, r10);
                        break;
                case 0x53:
                        reg_offset = offsetof(struct pt_regs, r11);
                        break;
                case 0x54:
                        reg_offset = offsetof(struct pt_regs, r12);
                        break;
                case 0x55:
                        reg_offset = offsetof(struct pt_regs, r13);
                        break;
                case 0x56:
                        reg_offset = offsetof(struct pt_regs, r14);
                        break;
                case 0x57:
                        reg_offset = offsetof(struct pt_regs, r15);
                        break;
                }
#else
                return -ENOSYS;
#endif
        } else {
                switch (opc1) {
                case 0x50:
                        reg_offset = offsetof(struct pt_regs, ax);
                        break;
                case 0x51:
                        reg_offset = offsetof(struct pt_regs, cx);
                        break;
                case 0x52:
                        reg_offset = offsetof(struct pt_regs, dx);
                        break;
                case 0x53:
                        reg_offset = offsetof(struct pt_regs, bx);
                        break;
                case 0x54:
                        reg_offset = offsetof(struct pt_regs, sp);
                        break;
                case 0x55:
                        reg_offset = offsetof(struct pt_regs, bp);
                        break;
                case 0x56:
                        reg_offset = offsetof(struct pt_regs, si);
                        break;
                case 0x57:
                        reg_offset = offsetof(struct pt_regs, di);
                        break;
                }
        }

        auprobe->push.reg_offset = reg_offset;
        auprobe->push.ilen = insn->length;
        auprobe->ops = &push_xol_ops;
        return 0;
}

/**
 * arch_uprobe_analyze_insn - instruction analysis including validity and fixups.
 * @auprobe: the probepoint information.
 * @mm: the probed address space.
 * @addr: virtual address at which to install the probepoint
 * Return 0 on success or a -ve number on error.
 */
int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned long addr)
{
        struct insn insn;
        u8 fix_ip_or_call = UPROBE_FIX_IP;
        int ret;

        ret = uprobe_init_insn(auprobe, &insn, is_64bit_mm(mm));
        if (ret)
                return ret;

        ret = branch_setup_xol_ops(auprobe, &insn);
        if (ret != -ENOSYS)
                return ret;

        ret = push_setup_xol_ops(auprobe, &insn);
        if (ret != -ENOSYS)
                return ret;

        /*
         * Figure out which fixups default_post_xol_op() will need to perform,
         * and annotate defparam->fixups accordingly.
         */
        switch (OPCODE1(&insn)) {
        case 0x9d:              /* popf */
                auprobe->defparam.fixups |= UPROBE_FIX_SETF;
                break;
        case 0xc3:              /* ret or lret -- ip is correct */
        case 0xcb:
        case 0xc2:
        case 0xca:
        case 0xea:              /* jmp absolute -- ip is correct */
                fix_ip_or_call = 0;
                break;
        case 0x9a:              /* call absolute - Fix return addr, not ip */
                fix_ip_or_call = UPROBE_FIX_CALL;
                break;
        case 0xff:
                switch (MODRM_REG(&insn)) {
                case 2: case 3:                 /* call or lcall, indirect */
                        fix_ip_or_call = UPROBE_FIX_CALL;
                        break;
                case 4: case 5:                 /* jmp or ljmp, indirect */
                        fix_ip_or_call = 0;
                        break;
                }
                fallthrough;
        default:
                riprel_analyze(auprobe, &insn);
        }

        auprobe->defparam.ilen = insn.length;
        auprobe->defparam.fixups |= fix_ip_or_call;

        auprobe->ops = &default_xol_ops;
        return 0;
}

/*
 * arch_uprobe_pre_xol - prepare to execute out of line.
 * @auprobe: the probepoint information.
 * @regs: reflects the saved user state of current task.
 */
int arch_uprobe_pre_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        struct uprobe_task *utask = current->utask;

        if (auprobe->ops->pre_xol) {
                int err = auprobe->ops->pre_xol(auprobe, regs);
                if (err)
                        return err;
        }

        regs->ip = utask->xol_vaddr;
        utask->autask.saved_trap_nr = current->thread.trap_nr;
        current->thread.trap_nr = UPROBE_TRAP_NR;

        utask->autask.saved_tf = !!(regs->flags & X86_EFLAGS_TF);
        regs->flags |= X86_EFLAGS_TF;
        if (test_tsk_thread_flag(current, TIF_BLOCKSTEP))
                set_task_blockstep(current, false);

        return 0;
}

/*
 * If xol insn itself traps and generates a signal(Say,
 * SIGILL/SIGSEGV/etc), then detect the case where a singlestepped
 * instruction jumps back to its own address. It is assumed that anything
 * like do_page_fault/do_trap/etc sets thread.trap_nr != -1.
 *
 * arch_uprobe_pre_xol/arch_uprobe_post_xol save/restore thread.trap_nr,
 * arch_uprobe_xol_was_trapped() simply checks that ->trap_nr is not equal to
 * UPROBE_TRAP_NR == -1 set by arch_uprobe_pre_xol().
 */
bool arch_uprobe_xol_was_trapped(struct task_struct *t)
{
        if (t->thread.trap_nr != UPROBE_TRAP_NR)
                return true;

        return false;
}

/*
 * Called after single-stepping. To avoid the SMP problems that can
 * occur when we temporarily put back the original opcode to
 * single-step, we single-stepped a copy of the instruction.
 *
 * This function prepares to resume execution after the single-step.
 */
int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        struct uprobe_task *utask = current->utask;
        bool send_sigtrap = utask->autask.saved_tf;
        int err = 0;

        WARN_ON_ONCE(current->thread.trap_nr != UPROBE_TRAP_NR);
        current->thread.trap_nr = utask->autask.saved_trap_nr;

        if (auprobe->ops->post_xol) {
                err = auprobe->ops->post_xol(auprobe, regs);
                if (err) {
                        /*
                         * Restore ->ip for restart or post mortem analysis.
                         * ->post_xol() must not return -ERESTART unless this
                         * is really possible.
                         */
                        regs->ip = utask->vaddr;
                        if (err == -ERESTART)
                                err = 0;
                        send_sigtrap = false;
                }
        }
        /*
         * arch_uprobe_pre_xol() doesn't save the state of TIF_BLOCKSTEP
         * so we can get an extra SIGTRAP if we do not clear TF. We need
         * to examine the opcode to make it right.
         */
        if (send_sigtrap)
                send_sig(SIGTRAP, current, 0);

        if (!utask->autask.saved_tf)
                regs->flags &= ~X86_EFLAGS_TF;

        return err;
}

/* callback routine for handling exceptions. */
int arch_uprobe_exception_notify(struct notifier_block *self, unsigned long val, void *data)
{
        struct die_args *args = data;
        struct pt_regs *regs = args->regs;
        int ret = NOTIFY_DONE;

        /* We are only interested in userspace traps */
        if (regs && !user_mode(regs))
                return NOTIFY_DONE;

        switch (val) {
        case DIE_INT3:
                if (uprobe_pre_sstep_notifier(regs))
                        ret = NOTIFY_STOP;

                break;

        case DIE_DEBUG:
                if (uprobe_post_sstep_notifier(regs))
                        ret = NOTIFY_STOP;

                break;

        default:
                break;
        }

        return ret;
}

/*
 * This function gets called when XOL instruction either gets trapped or
 * the thread has a fatal signal. Reset the instruction pointer to its
 * probed address for the potential restart or for post mortem analysis.
 */
void arch_uprobe_abort_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        struct uprobe_task *utask = current->utask;

        if (auprobe->ops->abort)
                auprobe->ops->abort(auprobe, regs);

        current->thread.trap_nr = utask->autask.saved_trap_nr;
        regs->ip = utask->vaddr;
        /* clear TF if it was set by us in arch_uprobe_pre_xol() */
        if (!utask->autask.saved_tf)
                regs->flags &= ~X86_EFLAGS_TF;
}

static bool __skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        if (auprobe->ops->emulate)
                return auprobe->ops->emulate(auprobe, regs);
        return false;
}

bool arch_uprobe_skip_sstep(struct arch_uprobe *auprobe, struct pt_regs *regs)
{
        bool ret = __skip_sstep(auprobe, regs);
        if (ret && (regs->flags & X86_EFLAGS_TF))
                send_sig(SIGTRAP, current, 0);
        return ret;
}

unsigned long
arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs *regs)
{
        int rasize = sizeof_long(regs), nleft;
        unsigned long orig_ret_vaddr = 0; /* clear high bits for 32-bit apps */

        if (copy_from_user(&orig_ret_vaddr, (void __user *)regs->sp, rasize))
                return -1;

        /* check whether address has been already hijacked */
        if (orig_ret_vaddr == trampoline_vaddr)
                return orig_ret_vaddr;

        nleft = copy_to_user((void __user *)regs->sp, &trampoline_vaddr, rasize);
        if (likely(!nleft)) {
                if (shstk_update_last_frame(trampoline_vaddr)) {
                        force_sig(SIGSEGV);
                        return -1;
                }
                return orig_ret_vaddr;
        }

        if (nleft != rasize) {
                pr_err("return address clobbered: pid=%d, %%sp=%#lx, %%ip=%#lx\n",
                       current->pid, regs->sp, regs->ip);

                force_sig(SIGSEGV);
        }

        return -1;
}

bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
                                struct pt_regs *regs)
{
        if (ctx == RP_CHECK_CALL) /* sp was just decremented by "call" insn */
                return regs->sp < ret->stack;
        else
                return regs->sp <= ret->stack;
}