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Decode VEX compact escapes more precisely

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The single opcode byte follows the VEX prefix immediately.
This commit is contained in:
Alexis Engelke
2019-02-24 10:07:18 +01:00
parent b2b29239b1
commit 9cdd35f82f
1 changed files with 23 additions and 30 deletions
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53
decode.c
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@@ -84,11 +84,6 @@ enum PrefixSet
PREFIX_REXX = 1 << 16, PREFIX_REXX = 1 << 16,
PREFIX_REXR = 1 << 17, PREFIX_REXR = 1 << 17,
PREFIX_REXW = 1 << 18, PREFIX_REXW = 1 << 18,
PREFIX_ESC_NONE = 0 << 19,
PREFIX_ESC_0F = 1 << 19,
PREFIX_ESC_0F38 = 2 << 19,
PREFIX_ESC_0F3A = 3 << 19,
PREFIX_ESC_MASK = 3 << 19,
PREFIX_VEX = 1 << 21, PREFIX_VEX = 1 << 21,
}; };
@@ -98,7 +93,8 @@ static
int int
decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode, decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode,
PrefixSet* out_prefixes, uint8_t* out_mandatory, PrefixSet* out_prefixes, uint8_t* out_mandatory,
uint8_t* out_segment, uint8_t* out_vex_operand) uint8_t* out_segment, uint8_t* out_vex_operand,
int* out_opcode_escape)
{ {
int off = 0; int off = 0;
PrefixSet prefixes = 0; PrefixSet prefixes = 0;
@@ -106,6 +102,7 @@ decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode,
uint8_t rep = 0; uint8_t rep = 0;
*out_mandatory = 0; *out_mandatory = 0;
*out_segment = FD_REG_NONE; *out_segment = FD_REG_NONE;
*out_opcode_escape = -1;
while (LIKELY(off < len)) while (LIKELY(off < len))
{ {
@@ -158,13 +155,7 @@ decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode,
prefixes |= byte & 0x40 ? 0 : PREFIX_REXX; prefixes |= byte & 0x40 ? 0 : PREFIX_REXX;
// SDM Vol 2A 2-15 (Dec. 2016): Ignored in 32-bit mode // SDM Vol 2A 2-15 (Dec. 2016): Ignored in 32-bit mode
prefixes |= mode == DECODE_64 || (byte & 0x20) ? 0 : PREFIX_REXB; prefixes |= mode == DECODE_64 || (byte & 0x20) ? 0 : PREFIX_REXB;
switch (byte & 0x1f) *out_opcode_escape = (byte & 0x1f);
{
case 0x01: prefixes |= PREFIX_ESC_0F; break;
case 0x02: prefixes |= PREFIX_ESC_0F38; break;
case 0x03: prefixes |= PREFIX_ESC_0F3A; break;
default: return -1;
}
// Load third byte of VEX prefix // Load third byte of VEX prefix
if (UNLIKELY(off + 2 >= len)) if (UNLIKELY(off + 2 >= len))
@@ -178,7 +169,7 @@ decode_prefixes(const uint8_t* buffer, int len, DecodeMode mode,
prefixes |= byte & 0x80 ? PREFIX_REXW : 0; prefixes |= byte & 0x80 ? PREFIX_REXW : 0;
} }
else // 2-byte VEX else // 2-byte VEX
prefixes |= PREFIX_ESC_0F; *out_opcode_escape = 1;
prefixes |= byte & 0x04 ? PREFIX_VEXL : 0; prefixes |= byte & 0x04 ? PREFIX_VEXL : 0;
*out_mandatory = byte & 0x03; *out_mandatory = byte & 0x03;
*out_vex_operand = ((byte & 0x78) >> 3) ^ 0xf; *out_vex_operand = ((byte & 0x78) >> 3) ^ 0xf;
@@ -343,10 +334,12 @@ fd_decode(const uint8_t* buffer, size_t len_sz, int mode_int, uintptr_t address,
int off = 0; int off = 0;
uint8_t vex_operand = 0; uint8_t vex_operand = 0;
uint8_t mandatory_prefix; uint8_t mandatory_prefix;
int opcode_escape;
PrefixSet prefixes = 0; PrefixSet prefixes = 0;
retval = decode_prefixes(buffer + off, len - off, mode, &prefixes, retval = decode_prefixes(buffer + off, len - off, mode, &prefixes,
&mandatory_prefix, &instr->segment, &vex_operand); &mandatory_prefix, &instr->segment, &vex_operand,
&opcode_escape);
if (UNLIKELY(retval < 0 || off + retval >= len)) if (UNLIKELY(retval < 0 || off + retval >= len))
{ {
return -1; return -1;
@@ -355,23 +348,23 @@ fd_decode(const uint8_t* buffer, size_t len_sz, int mode_int, uintptr_t address,
uint32_t kind = ENTRY_TABLE256; uint32_t kind = ENTRY_TABLE256;
if (UNLIKELY(prefixes & PREFIX_ESC_MASK)) // "Legacy" walk through table and escape opcodes
{ if (LIKELY(opcode_escape < 0))
uint32_t escape = prefixes & PREFIX_ESC_MASK;
table = (uint16_t*) &_decode_table[table[0x0F] & ~7];
if (escape == PREFIX_ESC_0F38)
{
table = (uint16_t*) &_decode_table[table[0x38] & ~7];
}
else if (escape == PREFIX_ESC_0F3A)
{
table = (uint16_t*) &_decode_table[table[0x3A] & ~7];
}
}
// First walk through full-byte opcodes. We do at most three iterations.
while (kind == ENTRY_TABLE256 && LIKELY(off < len)) while (kind == ENTRY_TABLE256 && LIKELY(off < len))
ENTRY_UNPACK(table, kind, _decode_table, table[buffer[off++]]); ENTRY_UNPACK(table, kind, _decode_table, table[buffer[off++]]);
// VEX/EVEX compact escapes; the prefix precedes the single opcode byte
else if (opcode_escape == 1 || opcode_escape == 2 || opcode_escape == 3)
{
ENTRY_UNPACK(table, kind, _decode_table, table[0x0F]);
if (opcode_escape == 2)
ENTRY_UNPACK(table, kind, _decode_table, table[0x38]);
if (opcode_escape == 3)
ENTRY_UNPACK(table, kind, _decode_table, table[0x3A]);
if (LIKELY(off < len))
ENTRY_UNPACK(table, kind, _decode_table, table[buffer[off++]]);
}
else
return -1;
// Then, walk through ModR/M-encoded opcode extensions. // Then, walk through ModR/M-encoded opcode extensions.
if ((kind == ENTRY_TABLE8 || kind == ENTRY_TABLE72) && LIKELY(off < len)) if ((kind == ENTRY_TABLE8 || kind == ENTRY_TABLE72) && LIKELY(off < len))