opcode.c

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#include <linux/types.h>

#include "opcode.h"

static bool opcode_is_prefix(uint8_t b)
{
    return
        /* Group 1 */
        b == 0xf0 || b == 0xf2 || b == 0xf3
        /* Group 2 */
        || b == 0x2e || b == 0x36 || b == 0x3e || b == 0x26
        || b == 0x64 || b == 0x65
        /* Group 3 */
        || b == 0x66
        /* Group 4 */
        || b == 0x67;
}

#ifdef CONFIG_X86_64
static bool opcode_is_rex_prefix(uint8_t b)
{
    return (b & 0xf0) == 0x40;
}
#else
static bool opcode_is_rex_prefix(uint8_t b)
{
    return false;
}
#endif

#define REX_W (1 << 3)

/*
 * This is a VERY crude opcode decoder. We only need to find the size of the
 * load/store that caused our #PF and this should work for all the opcodes
 * that we care about. Moreover, the ones who invented this instruction set
 * should be shot.
 */
void kmemcheck_opcode_decode(const uint8_t *op, unsigned int *size)
{
    /* Default operand size */
    int operand_size_override = 4;

    /* prefixes */
    for (; opcode_is_prefix(*op); ++op) {
        if (*op == 0x66)
            operand_size_override = 2;
    }

    /* REX prefix */
    if (opcode_is_rex_prefix(*op)) {
        uint8_t rex = *op;

        ++op;
        if (rex & REX_W) {
            switch (*op) {
            case 0x63:
                *size = 4;
                return;
            case 0x0f:
                ++op;

                switch (*op) {
                case 0xb6:
                case 0xbe:
                    *size = 1;
                    return;
                case 0xb7:
                case 0xbf:
                    *size = 2;
                    return;
                }

                break;
            }

            *size = 8;
            return;
        }
    }

    /* escape opcode */
    if (*op == 0x0f) {
        ++op;

        /*
         * This is move with zero-extend and sign-extend, respectively;
         * we don't have to think about 0xb6/0xbe, because this is
         * already handled in the conditional below.
         */
        if (*op == 0xb7 || *op == 0xbf)
            operand_size_override = 2;
    }

    *size = (*op & 1) ? operand_size_override : 1;
}

const uint8_t *kmemcheck_opcode_get_primary(const uint8_t *op)
{
    /* skip prefixes */
    while (opcode_is_prefix(*op))
        ++op;
    if (opcode_is_rex_prefix(*op))
        ++op;
    return op;
}