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ref: Improve _sigtramp workaround and explanation #662

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Aug 17, 2022
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ref: Improve _sigtramp workaround and explanation #662

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65 changes: 61 additions & 4 deletions symbolic-cfi/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@
//! frame.
//!
//! Without CFI, the stackwalker needs to scan the stack memory for values that look like valid base
//! addresses. This fequently yields false-positives.
//! addresses. This frequently yields false-positives.
//!
//! [processor]: ../processor/index.html
//! [`CfiCache`]: struct.CfiCache.html
Expand Down Expand Up @@ -578,31 +578,88 @@ impl<W: Write> AsciiCfiWriter<W> {
let sym_name = symbols.lookup(start_addr).and_then(|sym| sym.name());

if sym_name == Some("_sigtramp") && cpu_family == CpuFamily::Amd64 {
// This specific function has some hand crafted dwarf expressions
// that we currently can't process. They encode how to restore the
// registers from a machine context accessible via `$rbx`
// This specific function has some hand crafted dwarf expressions.
// They encode how to restore the registers from a machine context accessible via `$rbx`
// See: https://github.com/apple/darwin-libplatform/blob/215b09856ab5765b7462a91be7076183076600df/src/setjmp/x86_64/_sigtramp.s#L198-L258

// We currently don't support DWARF expressions
// (`{Register,Cfa}Rule::{Val,}Expression`) at all.
// The register and CFA expressions are simple enough to implement
// in our DWARF to ASCII CFI translator. They look like:
//
// > DW_CFA_expression: RBP DW_OP_breg3 RBX+48, DW_OP_deref, DW_OP_plus_uconst 0x40
// > DW_CFA_def_cfa_expression: DW_OP_breg3 RBX+48, DW_OP_deref, DW_OP_plus_uconst 0x48, DW_OP_deref
//
// However the `.ra`/RIP expression is a lot more complex:
//
// > DW_CFA_val_expression: RIP DW_OP_breg3 RBX+48, DW_OP_deref, DW_OP_dup, DW_OP_plus_uconst 0x90,
// > DW_OP_deref, DW_OP_swap, DW_OP_plus_uconst 0x0, DW_OP_deref_size 0x4, DW_OP_dup, DW_OP_lit3,
// > DW_OP_ne, DW_OP_swap, DW_OP_lit4, DW_OP_ne, DW_OP_and, DW_OP_plus
// (the expression just subtracts 1 depending on the exception flag)
//
// The ASCII CFI syntax is not rich enough to express the above
// DWARF expression. DWARF expressions are "in theory" even turing-complete.
//
// As a workaround for this limitation, we generate some hard-coded
// ASCII CFI that is equivalent to the DWARF expressions.
// For simplicity, we only restore
// callee saves / call preserved / non-volatile registers:
// rbx, rsp, rbp, r12, r13, r14, and r15

let mc_offset = 48;
let rbx_offset = 24;
let rbp_offset = 64;
let rsp_offset = 72;
let r12_offset = 112;
let r13_offset = 120;
let r14_offset = 128;
let r15_offset = 136;
let rip_offset = 144;

write!(
self.inner,
"STACK CFI INIT {:x} {:x} ",
start_addr, entry.len
)?;

write!(
self.inner,
"$rbx: $rbx {} + ^ {} + ^ ",
mc_offset, rbx_offset
)?;
write!(
self.inner,
"$rbp: $rbx {} + ^ {} + ^ ",
mc_offset, rbp_offset
)?;
write!(
self.inner,
"$r12: $rbx {} + ^ {} + ^ ",
mc_offset, r12_offset
)?;
write!(
self.inner,
"$r13: $rbx {} + ^ {} + ^ ",
mc_offset, r13_offset
)?;
write!(
self.inner,
"$r14: $rbx {} + ^ {} + ^ ",
mc_offset, r14_offset
)?;
write!(
self.inner,
"$r15: $rbx {} + ^ {} + ^ ",
mc_offset, r15_offset
)?;

// rsp aka .cfa
write!(
self.inner,
".cfa: $rbx {} + ^ {} + ^ ",
mc_offset, rsp_offset,
)?;
// rip aka .ra
writeln!(
self.inner,
".ra: $rbx {} + ^ {} + ^",
Expand Down