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use super::super::unwind_rule::UnwindRuleAarch64;
struct PrologueDetectorAarch64 {
sp_offset: i32,
}
#[derive(Clone, Debug, PartialEq, Eq)]
enum PrologueStepResult {
UnexpectedInstruction(UnexpectedInstructionType),
ValidPrologueInstruction,
}
#[derive(Clone, Debug, PartialEq, Eq)]
enum PrologueResult {
ProbablyAlreadyInBody(UnexpectedInstructionType),
FoundFunctionStart { sp_offset: i32 },
}
#[derive(Clone, Debug, PartialEq, Eq)]
enum PrologueInstructionType {
NotExpectedInPrologue,
CouldBePartOfPrologueIfThereIsAlsoAStackPointerSub,
VeryLikelyPartOfPrologue,
}
#[derive(Clone, Debug, PartialEq, Eq)]
enum UnexpectedInstructionType {
StoreOfWrongSize,
StoreReferenceRegisterNotSp,
AddSubNotOperatingOnSp,
NoNextInstruction,
NoStackPointerSubBeforeStore,
Unknown,
}
impl PrologueDetectorAarch64 {
pub fn new() -> Self {
Self { sp_offset: 0 }
}
pub fn analyze_slices(
&mut self,
slice_from_start: &[u8],
slice_to_end: &[u8],
) -> PrologueResult {
// There are at least two options of what we could do here:
// - We could walk forwards from the function start to the instruction pointer.
// - We could walk backwards from the instruction pointer to the function start.
// Walking backwards is fine on arm64 because instructions are fixed size.
// Walking forwards requires that we have a useful function start address.
//
// Unfortunately, we can't rely on having a useful function start address.
// We get the funcion start address from the __unwind_info, which often collapses
// consecutive functions with the same unwind rules into a single entry, discarding
// the original function start addresses.
// Concretely, this means that `slice_from_start` may start much earlier than the
// current function.
//
// So we walk backwards. We first check the next instruction, and then
// go backwards from the instruction pointer to the function start.
// If the instruction we're about to execute is one that we'd expect to find in a prologue,
// then we assume that we're in a prologue. Then we single-step backwards until we
// either run out of instructions (which means we've definitely hit the start of the
// function), or until we find an instruction that we would not expect in a prologue.
// At that point we guess that this instruction must be belonging to the previous
// function, and that we've succesfully found the start of the current function.
if slice_to_end.len() < 4 {
return PrologueResult::ProbablyAlreadyInBody(
UnexpectedInstructionType::NoNextInstruction,
);
}
let next_instruction = u32::from_le_bytes([
slice_to_end[0],
slice_to_end[1],
slice_to_end[2],
slice_to_end[3],
]);
let next_instruction_type = Self::analyze_prologue_instruction_type(next_instruction);
if next_instruction_type == PrologueInstructionType::NotExpectedInPrologue {
return PrologueResult::ProbablyAlreadyInBody(UnexpectedInstructionType::Unknown);
}
let instructions = slice_from_start
.chunks_exact(4)
.map(|c| u32::from_le_bytes([c[0], c[1], c[2], c[3]]))
.rev();
for instruction in instructions {
if let PrologueStepResult::UnexpectedInstruction(_) =
self.reverse_step_instruction(instruction)
{
break;
}
}
if next_instruction_type
== PrologueInstructionType::CouldBePartOfPrologueIfThereIsAlsoAStackPointerSub
&& self.sp_offset == 0
{
return PrologueResult::ProbablyAlreadyInBody(
UnexpectedInstructionType::NoStackPointerSubBeforeStore,
);
}
PrologueResult::FoundFunctionStart {
sp_offset: self.sp_offset,
}
}
/// Check if the instruction indicates that we're likely in a prologue.
pub fn analyze_prologue_instruction_type(word: u32) -> PrologueInstructionType {
// Detect pacibsp (verify stack pointer authentication) and `mov x29, sp`.
if word == 0xd503237f || word == 0x910003fd {
return PrologueInstructionType::VeryLikelyPartOfPrologue;
}
let bits_22_to_32 = word >> 22;
// Detect stores of register pairs to the stack.
if bits_22_to_32 & 0b1011111001 == 0b1010100000 {
// Section C3.3, Loads and stores.
// Only stores that are commonly seen in prologues (bits 22, 29 and 31 are set)
let writeback_bits = bits_22_to_32 & 0b110;
let reference_reg = ((word >> 5) & 0b11111) as u16;
if writeback_bits == 0b000 || reference_reg != 31 {
return PrologueInstructionType::NotExpectedInPrologue;
}
// We are storing a register pair to the stack. This is something that
// can happen in a prologue but it can also happen in the body of a
// function.
if writeback_bits == 0b100 {
// No writeback.
return PrologueInstructionType::CouldBePartOfPrologueIfThereIsAlsoAStackPointerSub;
}
return PrologueInstructionType::VeryLikelyPartOfPrologue;
}
// Detect sub instructions operating on the stack pointer.
// Detect `add fp, sp, #0xXX` instructions
if bits_22_to_32 & 0b1011111110 == 0b1001000100 {
// Section C3.4, Data processing - immediate
// unsigned add / sub imm, size class X (8 bytes)
let result_reg = (word & 0b11111) as u16;
let input_reg = ((word >> 5) & 0b11111) as u16;
let is_sub = ((word >> 30) & 0b1) == 0b1;
let expected_result_reg = if is_sub { 31 } else { 29 };
if input_reg != 31 || result_reg != expected_result_reg {
return PrologueInstructionType::NotExpectedInPrologue;
}
return PrologueInstructionType::VeryLikelyPartOfPrologue;
}
PrologueInstructionType::NotExpectedInPrologue
}
/// Step backwards over one (already executed) instruction.
pub fn reverse_step_instruction(&mut self, word: u32) -> PrologueStepResult {
// Detect pacibsp (verify stack pointer authentication)
if word == 0xd503237f {
return PrologueStepResult::ValidPrologueInstruction;
}
// Detect stores of register pairs to the stack.
if (word >> 22) & 0b1011111001 == 0b1010100000 {
// Section C3.3, Loads and stores.
// but only those that are commonly seen in prologues / prologues (bits 29 and 31 are set)
let writeback_bits = (word >> 23) & 0b11;
if writeback_bits == 0b00 {
// Not 64-bit load/store.
return PrologueStepResult::UnexpectedInstruction(
UnexpectedInstructionType::StoreOfWrongSize,
);
}
let reference_reg = ((word >> 5) & 0b11111) as u16;
if reference_reg != 31 {
return PrologueStepResult::UnexpectedInstruction(
UnexpectedInstructionType::StoreReferenceRegisterNotSp,
);
}
let is_preindexed_writeback = writeback_bits == 0b11;
let is_postindexed_writeback = writeback_bits == 0b01; // TODO: are there postindexed stores? What do they mean?
if is_preindexed_writeback || is_postindexed_writeback {
let imm7 = (((((word >> 15) & 0b1111111) as i16) << 9) >> 6) as i32;
self.sp_offset -= imm7; // - to undo the instruction
}
return PrologueStepResult::ValidPrologueInstruction;
}
// Detect sub instructions operating on the stack pointer.
if (word >> 23) & 0b111111111 == 0b110100010 {
// Section C3.4, Data processing - immediate
// unsigned sub imm, size class X (8 bytes)
let result_reg = (word & 0b11111) as u16;
let input_reg = ((word >> 5) & 0b11111) as u16;
if result_reg != 31 || input_reg != 31 {
return PrologueStepResult::UnexpectedInstruction(
UnexpectedInstructionType::AddSubNotOperatingOnSp,
);
}
let mut imm12 = ((word >> 10) & 0b111111111111) as i32;
let shift_immediate_by_12 = ((word >> 22) & 0b1) == 0b1;
if shift_immediate_by_12 {
imm12 <<= 12
}
self.sp_offset += imm12; // + to undo the sub instruction
return PrologueStepResult::ValidPrologueInstruction;
}
PrologueStepResult::UnexpectedInstruction(UnexpectedInstructionType::Unknown)
}
}
pub fn unwind_rule_from_detected_prologue(
slice_from_start: &[u8],
slice_to_end: &[u8],
) -> Option<UnwindRuleAarch64> {
let mut detector = PrologueDetectorAarch64::new();
match detector.analyze_slices(slice_from_start, slice_to_end) {
PrologueResult::ProbablyAlreadyInBody(_) => None,
PrologueResult::FoundFunctionStart { sp_offset } => {
let sp_offset_by_16 = u16::try_from(sp_offset / 16).ok()?;
let rule = if sp_offset_by_16 == 0 {
UnwindRuleAarch64::NoOp
} else {
UnwindRuleAarch64::OffsetSp { sp_offset_by_16 }
};
Some(rule)
}
}
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn test_prologue_1() {
// gimli::read::unit::parse_attribute
// 1000dfeb8 ff 43 01 d1 sub sp, sp, #0x50
// 1000dfebc f6 57 02 a9 stp x22, x21, [sp, #local_30]
// 1000dfec0 f4 4f 03 a9 stp x20, x19, [sp, #local_20]
// 1000dfec4 fd 7b 04 a9 stp x29, x30, [sp, #local_10]
// 1000dfec8 fd 03 01 91 add x29, sp, #0x40
// 1000dfecc f4 03 04 aa mov x20, x4
// 1000dfed0 f5 03 01 aa mov x21, x1
let bytes = &[
0xff, 0x43, 0x01, 0xd1, 0xf6, 0x57, 0x02, 0xa9, 0xf4, 0x4f, 0x03, 0xa9, 0xfd, 0x7b,
0x04, 0xa9, 0xfd, 0x03, 0x01, 0x91, 0xf4, 0x03, 0x04, 0xaa, 0xf5, 0x03, 0x01, 0xaa,
];
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..0], &bytes[0..]),
Some(UnwindRuleAarch64::NoOp)
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..4], &bytes[4..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 5 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..8], &bytes[8..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 5 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..12], &bytes[12..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 5 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..16], &bytes[16..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 5 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..20], &bytes[20..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..24], &bytes[24..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..28], &bytes[28..]),
None
);
}
#[test]
fn test_prologue_with_pacibsp() {
// 1801245c4 08 58 29 b8 str w8,[x0, w9, UXTW #0x2]
// 1801245c8 c0 03 5f d6 ret
// _malloc_zone_realloc
// 1801245cc 7f 23 03 d5 pacibsp
// 1801245d0 f8 5f bc a9 stp x24,x23,[sp, #local_40]!
// 1801245d4 f6 57 01 a9 stp x22,x21,[sp, #local_30]
// 1801245d8 f4 4f 02 a9 stp x20,x19,[sp, #local_20]
// 1801245dc fd 7b 03 a9 stp x29,x30,[sp, #local_10]
// 1801245e0 fd c3 00 91 add x29,sp,#0x30
// 1801245e4 f3 03 02 aa mov x19,x2
// 1801245e8 f4 03 01 aa mov x20,x1
let bytes = &[
0x08, 0x58, 0x29, 0xb8, 0xc0, 0x03, 0x5f, 0xd6, 0x7f, 0x23, 0x03, 0xd5, 0xf8, 0x5f,
0xbc, 0xa9, 0xf6, 0x57, 0x01, 0xa9, 0xf4, 0x4f, 0x02, 0xa9, 0xfd, 0x7b, 0x03, 0xa9,
0xfd, 0xc3, 0x00, 0x91, 0xf3, 0x03, 0x02, 0xaa, 0xf4, 0x03, 0x01, 0xaa,
];
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..0], &bytes[0..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..4], &bytes[4..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..8], &bytes[8..]),
Some(UnwindRuleAarch64::NoOp)
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..12], &bytes[12..]),
Some(UnwindRuleAarch64::NoOp)
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..16], &bytes[16..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 4 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..20], &bytes[20..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 4 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..24], &bytes[24..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 4 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..28], &bytes[28..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 4 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..32], &bytes[32..]),
None
);
}
#[test]
fn test_prologue_with_mov_fp_sp() {
// _tiny_free_list_add_ptr
// 180126e94 7f 23 03 d5 pacibsp
// 180126e98 fd 7b bf a9 stp x29,x30,[sp, #local_10]!
// 180126e9c fd 03 00 91 mov x29,sp
// 180126ea0 68 04 00 51 sub w8,w3,#0x1
let bytes = &[
0x7f, 0x23, 0x03, 0xd5, 0xfd, 0x7b, 0xbf, 0xa9, 0xfd, 0x03, 0x00, 0x91, 0x68, 0x04,
0x00, 0x51,
];
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..0], &bytes[0..]),
Some(UnwindRuleAarch64::NoOp)
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..4], &bytes[4..]),
Some(UnwindRuleAarch64::NoOp)
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..8], &bytes[8..]),
Some(UnwindRuleAarch64::OffsetSp { sp_offset_by_16: 1 })
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..12], &bytes[12..]),
None
);
}
#[test]
fn test_no_prologue_despite_stack_store() {
// We're in the middle of a function and are storing something to the stack.
// But this is not a prologue, so it shouldn't be detected as one.
//
// 1004073d0 e8 17 00 f9 str x8,[sp, #0x28]
// 1004073d4 03 00 00 14 b LAB_1004073e0
// 1004073d8 ff ff 01 a9 stp xzr,xzr,[sp, #0x18] ; <-- stores the pair xzr, xzr on the stack
// 1004073dc ff 17 00 f9 str xzr,[sp, #0x28]
// 1004073e0 e0 03 00 91 mov x0,sp
let bytes = &[
0xe8, 0x17, 0x00, 0xf9, 0x03, 0x00, 0x00, 0x14, 0xff, 0xff, 0x01, 0xa9, 0xff, 0x17,
0x00, 0xf9, 0xe0, 0x03, 0x00, 0x91,
];
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..0], &bytes[0..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..4], &bytes[4..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..8], &bytes[8..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..12], &bytes[12..]),
None
);
assert_eq!(
unwind_rule_from_detected_prologue(&bytes[..16], &bytes[16..]),
None
);
}
}