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use super::super::unwind_rule::UnwindRuleX86_64;
pub fn unwind_rule_from_detected_epilogue(
text_bytes: &[u8],
pc_offset: usize,
) -> Option<UnwindRuleX86_64> {
let (slice_from_start, slice_to_end) = text_bytes.split_at(pc_offset);
let mut sp_offset_by_8 = 0;
let mut bp_offset_by_8 = None;
let mut bytes = slice_to_end;
loop {
if bytes.is_empty() {
return None;
}
// Detect ret
if bytes[0] == 0xc3 {
break;
}
// Detect jmp
if bytes[0] == 0xeb || bytes[0] == 0xe9 || bytes[0] == 0xff {
// This could be a tail call, or just a regular jump inside the current function.
// Ideally, we would check whether the jump target is inside this function.
// But this would require having an accurate idea of where the current function
// starts and ends.
// For now, we instead use the following heuristic: Any jmp that directly follows
// a `pop` instruction is treated as a tail call.
if sp_offset_by_8 != 0 {
// We have detected a pop in the previous loop iteration.
break;
}
// This must be the first iteration. Look backwards.
if let Some(potential_pop_byte) = slice_from_start.last() {
// Get the previous byte. We have no idea how long the previous instruction
// is, so we might be looking at a random last byte of a wider instruction.
// Let's just pray that this is not the case.
if potential_pop_byte & 0xf8 == 0x58 {
// Assuming we haven't just misinterpreted the last byte of a wider
// instruction, this is a `pop rXX`.
break;
}
}
return None;
}
// Detect pop rbp
if bytes[0] == 0x5d {
bp_offset_by_8 = Some(sp_offset_by_8 as i16);
sp_offset_by_8 += 1;
bytes = &bytes[1..];
continue;
}
// Detect pop rXX
if (0x58..=0x5f).contains(&bytes[0]) {
sp_offset_by_8 += 1;
bytes = &bytes[1..];
continue;
}
// Detect pop rXX with prefix
if bytes.len() >= 2 && bytes[0] & 0xfe == 0x40 && bytes[1] & 0xf8 == 0x58 {
sp_offset_by_8 += 1;
bytes = &bytes[2..];
continue;
}
// Unexpected instruction.
// This probably means that we weren't in an epilogue after all.
return None;
}
// We've found the return or the tail call.
let rule = if sp_offset_by_8 == 0 {
UnwindRuleX86_64::JustReturn
} else {
sp_offset_by_8 += 1; // Add one for popping the return address.
if let Some(bp_storage_offset_from_sp_by_8) = bp_offset_by_8 {
UnwindRuleX86_64::OffsetSpAndRestoreBp {
sp_offset_by_8,
bp_storage_offset_from_sp_by_8,
}
} else {
UnwindRuleX86_64::OffsetSp { sp_offset_by_8 }
}
};
Some(rule)
}