// Copyright 2015 Ted Mielczarek. See the COPYRIGHT
// file at the top-level directory of this distribution.
// NOTE: arm64_old.rs and arm64.rs should be identical except for the names of
// their context types.
use super::impl_prelude::*;
use minidump::{
CpuContext, MinidumpContext, MinidumpContextValidity, MinidumpModuleList, MinidumpRawContext,
Module,
};
use std::collections::HashSet;
use tracing::trace;
type ArmContext = minidump::format::CONTEXT_ARM64_OLD;
type Pointer = <ArmContext as CpuContext>::Register;
type Registers = minidump::format::Arm64RegisterNumbers;
const POINTER_WIDTH: Pointer = std::mem::size_of::<Pointer>() as Pointer;
const FRAME_POINTER: &str = Registers::FramePointer.name();
const LINK_REGISTER: &str = Registers::LinkRegister.name();
const STACK_POINTER: &str = "sp";
const PROGRAM_COUNTER: &str = "pc";
const CALLEE_SAVED_REGS: &[&str] = &[
"x19", "x20", "x21", "x22", "x23", "x24", "x25", "x26", "x27", "x28", "fp",
];
async fn get_caller_by_cfi<P>(
ctx: &ArmContext,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
trace!("trying cfi");
let _last_sp = ctx.get_register(STACK_POINTER, args.valid())?;
let mut stack_walker = CfiStackWalker::from_ctx_and_args(ctx, args, callee_forwarded_regs)?;
args.symbol_provider
.walk_frame(stack_walker.module, &mut stack_walker)
.await?;
let caller_pc = stack_walker.caller_ctx.get_register_always(PROGRAM_COUNTER);
let caller_sp = stack_walker.caller_ctx.get_register_always(STACK_POINTER);
let new_valid = MinidumpContextValidity::Some(stack_walker.caller_validity);
// Apply ptr auth stripping
let caller_pc = ptr_auth_strip(args.modules, caller_pc);
stack_walker
.caller_ctx
.set_register(PROGRAM_COUNTER, caller_pc);
// Nothing should really ever restore lr, but CFI is more magic so whatever sure
if let Some(lr) = stack_walker
.caller_ctx
.get_register(LINK_REGISTER, &new_valid)
{
stack_walker
.caller_ctx
.set_register(LINK_REGISTER, ptr_auth_strip(args.modules, lr));
}
if let Some(fp) = stack_walker
.caller_ctx
.get_register(FRAME_POINTER, &new_valid)
{
stack_walker
.caller_ctx
.set_register(FRAME_POINTER, ptr_auth_strip(args.modules, fp));
}
trace!(
"cfi evaluation was successful -- caller_pc: 0x{:016x}, caller_sp: 0x{:016x}",
caller_pc,
caller_sp,
);
// Do absolutely NO validation! Yep! As long as CFI evaluation succeeds
// (which does include pc and sp resolving), just blindly assume the
// values are correct. I Don't Like This, but it's what breakpad does and
// we should start with a baseline of parity.
// FIXME?: for whatever reason breakpad actually does block on the address
// being canonical *ONLY* for arm64, which actually rejects null pc early!
// Let's not do that to keep our code more uniform.
let context = MinidumpContext {
raw: MinidumpRawContext::OldArm64(stack_walker.caller_ctx),
valid: new_valid,
};
Some(StackFrame::from_context(context, FrameTrust::CallFrameInfo))
}
fn callee_forwarded_regs(valid: &MinidumpContextValidity) -> HashSet<&'static str> {
match valid {
MinidumpContextValidity::All => CALLEE_SAVED_REGS.iter().copied().collect(),
MinidumpContextValidity::Some(ref which) => CALLEE_SAVED_REGS
.iter()
.filter(|®| which.contains(reg))
.copied()
.collect(),
}
}
fn get_caller_by_frame_pointer<P>(
ctx: &ArmContext,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
trace!("trying frame pointer");
// Ok so there exists 3 kinds of stackframes in ARM64:
//
// * stackless leaves
// * stackful leaves
// * normal frames
//
//
// # Normal Frames
//
// Let's start with normal frames. In the standard calling convention, the following happens:
//
// lr := return_address (performed implicitly by ARM's function call instruction)
// PUSH fp, lr (save fp and lr to the stack -- ARM64 pushes in pairs)
// fp := sp (update the frame pointer to the current stack pointer)
//
// So to restore the caller's registers, we have:
//
// pc := *(fp + ptr) (this will get the return address, usual offset caveats apply)
// sp := fp + ptr*2
// fp := *fp
//
// Note that although we push lr, we don't restore lr. That's because lr is just our
// return address, and is therefore essentially a "saved" pc. lr is caller-saved *and*
// automatically overwritten by every CALL, so the callee (the frame we're unwinding right now)
// has no business ever knowing it, let alone restoring it. lr is generally just saved
// immediately and then used as a free general purpose register, and therefore will generally
// contain random garbage unrelated to unwinding.
//
//
// # Leaf Functions
//
// Now leaf functions are a bit messier. These are functions which don't call other functions
// and therefore don't actually ever need to save lr or fp. As such, they can be entirely
// stackless, although they don't have to be. So calling a leaf function is just:
//
// lr := return_address
// <possibly some pushes, but maybe not>
//
// And to restore the caller's registers, we have:
//
// pc := lr
// sp := sp - <some arbitrary value>
// fp := fp
//
// Unfortunately, we're unaware of any way to "detect" that a function is a leaf or not
// without symbols/cfi just telling you that. Since we're in frame pointer unwinding,
// we probably don't have those available! And even if we did, we still wouldn't know if
// the frame was stackless or not, so we wouldn't know how to restore sp reliably and might
// get the stack in a weird state for subsequent (possibly CFI-based) frames.
// Also, if we incorrectly guess a frame is a leaf, we'll also use a probably-random-garbage
// lr as a pc and potentially halluncinate a bunch.
//
//
// # Conclusion
//
// At the moment we think it's safest/best to just always assume we're unwinding a normal
// frame. Statistically this is true (most frames are, even if they happen to be at the
// top of the stack when we crash), and if the frame *is* a leaf then our `fp` is likely
// to be the correct fp of the next frame. This will effectively result in us unwinding
// our caller instead of ourselves, causing the caller to be omitted from the backtrace
// but otherwise perfectly syncing up for the rest of the frames.
let last_fp = ctx.get_register(FRAME_POINTER, args.valid())?;
let last_sp = ctx.get_register(STACK_POINTER, args.valid())?;
if last_fp >= u64::MAX - POINTER_WIDTH * 2 {
// Although this code generally works fine if the pointer math overflows,
// debug builds will still panic, and this guard protects against it without
// drowning the rest of the code in checked_add.
return None;
}
let (caller_fp, caller_pc, caller_sp) = if last_fp == 0 {
// In this case we want unwinding to stop. One of the termination conditions in get_caller_frame
// is that caller_sp <= last_sp. Therefore we can force termination by setting caller_sp = last_sp.
(0, 0, last_sp)
} else {
(
args.stack_memory.get_memory_at_address(last_fp)?,
args.stack_memory
.get_memory_at_address(last_fp + POINTER_WIDTH)?,
last_fp + POINTER_WIDTH * 2,
)
};
let caller_fp = ptr_auth_strip(args.modules, caller_fp);
let caller_pc = ptr_auth_strip(args.modules, caller_pc);
// Don't accept obviously wrong instruction pointers.
if is_non_canonical(caller_pc) {
trace!("rejecting frame pointer result for unreasonable instruction pointer");
return None;
}
// Don't actually validate that the stack makes sense (duplicating breakpad behaviour).
trace!(
"frame pointer seems valid -- caller_pc: 0x{:016x}, caller_sp: 0x{:016x}",
caller_pc,
caller_sp,
);
let mut caller_ctx = ArmContext::default();
caller_ctx.set_register(PROGRAM_COUNTER, caller_pc);
caller_ctx.set_register(FRAME_POINTER, caller_fp);
caller_ctx.set_register(STACK_POINTER, caller_sp);
let mut valid = HashSet::new();
valid.insert(PROGRAM_COUNTER);
valid.insert(FRAME_POINTER);
valid.insert(STACK_POINTER);
let context = MinidumpContext {
raw: MinidumpRawContext::OldArm64(caller_ctx),
valid: MinidumpContextValidity::Some(valid),
};
Some(StackFrame::from_context(context, FrameTrust::FramePointer))
}
fn ptr_auth_strip(modules: &MinidumpModuleList, ptr: Pointer) -> Pointer {
// ARMv8.3 introduced a code hardening system called "Pointer Authentication"
// which is used on Apple platforms. It adds some extra high bits to the
// several pointers when they get pushed to memory, including the return
// address (lr) and frame pointer (fp), which both get pushed at the start
// of most non-leaf functions.
//
// We lack some of the proper context to implement the "strip" primitive, because
// the amount of bits that are "real" pointer depends on various extensions like
// pointer tagging and how big page tables are. If we allocate too many bits to
// "real" then we can get ptr_auth bits in our pointers, and if we allocate too
// few we can end up truncating our pointers. Thankfully we'll usually have a bit
// of margin from pointers not having the highest real bits set.
//
// To help us guess, we have a few pieces of information:
//
// * Apple seems to default to a 17/47 split, so 47 bits for "real" is a good baseline
// * We know the address ranges of various loaded (and unloaded modules)
// * We know the address range of the stacks
// * We *can* know the address range of some sections of the heap (MemoryList)
// * We *can* know the page mappings (MemoryInfo)
//
// Right now we only incorporate the first two. Ideally we would process all those sources
// once at the start of stack walking and pass it down to the ARM stackwalker but that's
// a lot of annoying rewiring that won't necessarily improve results.
let apple_default_max_addr = (1 << 47) - 1;
let max_module_addr = modules
.by_addr()
.next_back()
.map(|last_module| {
last_module
.base_address()
.saturating_add(last_module.size())
})
.unwrap_or(0);
let max_addr = u64::max(apple_default_max_addr, max_module_addr);
// We can convert a "highest" address into a suitable mask by getting the next_power_of_two
// (a single bit >= the max) and subtracting one from it (producing all 1's <= that bit).
// There are two corner cases to this:
//
// * the next_power_of_two being 2^65, in which case our mask should be !0 (all ones)
// * the max addr being a power of two already means we will actually lose that one value
//
// The first case is handled by using checked_next_power_of_two. The second case isn't really
// handled by it very improbable. We do however make sure the apple max isn't a power of two.
let mask = max_addr
.checked_next_power_of_two()
.map(|high_bit| high_bit - 1)
.unwrap_or(!0);
// In principle, if we've done a good job of computing the mask, we can apply it regardless
// of if there's any ptr auth bits. Either it will clear the auth or be a noop. We don't
// check if this messes up, because there's too many subtleties like JITed code to reliably
// detect this going awry.
ptr & mask
}
async fn get_caller_by_scan<P>(
ctx: &ArmContext,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
trace!("trying scan");
// Stack scanning is just walking from the end of the frame until we encounter
// a value on the stack that looks like a pointer into some code (it's an address
// in a range covered by one of our modules). If we find such an instruction,
// we assume it's an pc value that was pushed by the CALL instruction that created
// the current frame. The next frame is then assumed to end just before that
// pc value.
let last_sp = ctx.get_register(STACK_POINTER, args.valid())?;
// Number of pointer-sized values to scan through in our search.
let default_scan_range = 40;
let extended_scan_range = default_scan_range * 4;
// Breakpad devs found that the first frame of an unwind can be really messed up,
// and therefore benefits from a longer scan. Let's do it too.
let scan_range = if let FrameTrust::Context = args.callee_frame.trust {
extended_scan_range
} else {
default_scan_range
};
for i in 0..scan_range {
let address_of_pc = last_sp.checked_add(i * POINTER_WIDTH)?;
let caller_pc = args.stack_memory.get_memory_at_address(address_of_pc)?;
if instruction_seems_valid(caller_pc, args.modules, args.symbol_provider).await {
// pc is pushed by CALL, so sp is just address_of_pc + ptr
let caller_sp = address_of_pc.checked_add(POINTER_WIDTH)?;
// Don't do any more validation, and don't try to restore fp
// (that's what breakpad does!)
trace!(
"scan seems valid -- caller_pc: 0x{:08x}, caller_sp: 0x{:08x}",
caller_pc,
caller_sp,
);
let mut caller_ctx = ArmContext::default();
caller_ctx.set_register(PROGRAM_COUNTER, caller_pc);
caller_ctx.set_register(STACK_POINTER, caller_sp);
let mut valid = HashSet::new();
valid.insert(PROGRAM_COUNTER);
valid.insert(STACK_POINTER);
let context = MinidumpContext {
raw: MinidumpRawContext::OldArm64(caller_ctx),
valid: MinidumpContextValidity::Some(valid),
};
return Some(StackFrame::from_context(context, FrameTrust::Scan));
}
}
None
}
/// The most strict validation we have for instruction pointers.
///
/// This is only used for stack-scanning, because it's explicitly
/// trying to distinguish between total garbage and correct values.
/// cfi and frame_pointer approaches do not use this validation
/// because by default they're working with plausible/trustworthy
/// data.
///
/// Specifically, not using this validation allows cfi/fp methods
/// to unwind through frames we don't have mapped modules for (such as
/// OS APIs). This may seem confusing since we obviously don't have cfi
/// for unmapped modules!
///
/// The way this works is that we will use cfi to unwind some frame we
/// know about and *end up* in a function we know nothing about, but with
/// all the right register values. At this point, frame pointers will
/// often do the correct thing even though we don't know what code we're
/// in -- until we get back into code we do know about and cfi kicks back in.
/// At worst, this sets scanning up in a better position for success!
///
/// If we applied this more rigorous validation to cfi/fp methods, we
/// would just discard the correct register values from the known frame
/// and immediately start doing unreliable scans.
async fn instruction_seems_valid<P>(
instruction: Pointer,
modules: &MinidumpModuleList,
symbol_provider: &P,
) -> bool
where
P: SymbolProvider + Sync,
{
if is_non_canonical(instruction) || instruction == 0 {
return false;
}
super::instruction_seems_valid_by_symbols(instruction, modules, symbol_provider).await
}
fn is_non_canonical(instruction: Pointer) -> bool {
// Reject instructions in the first page or above the user-space threshold.
!(0x1000..=0x000fffffffffffff).contains(&instruction)
}
/*
// ARM64 is currently hyper-permissive, so we don't use this,
// but here it is in case we change our minds!
fn stack_seems_valid(
caller_sp: Pointer,
callee_sp: Pointer,
stack_memory: UnifiedMemory<'_, '_>,
) -> bool {
// The stack shouldn't *grow* when we unwind
if caller_sp < callee_sp {
return false;
}
// The stack pointer should be in the stack
stack_memory
.get_memory_at_address::<Pointer>(caller_sp as u64)
.is_some()
}
*/
pub async fn get_caller_frame<P>(
ctx: &ArmContext,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
// .await doesn't like closures, so don't use Option chaining
let mut frame = None;
if frame.is_none() {
frame = get_caller_by_cfi(ctx, args).await;
}
if frame.is_none() {
frame = get_caller_by_frame_pointer(ctx, args);
}
if frame.is_none() {
frame = get_caller_by_scan(ctx, args).await;
}
let mut frame = frame?;
// We now check the frame to see if it looks like unwinding is complete,
// based on the frame we computed having a nonsense value. Returning
// None signals to the unwinder to stop unwinding.
// if the instruction is within the first ~page of memory, it's basically
// null, and we can assume unwinding is complete.
if frame.context.get_instruction_pointer() < 4096 {
trace!("instruction pointer was nullish, assuming unwind complete");
return None;
}
// If the new stack pointer is at a lower address than the old,
// then that's clearly incorrect. Treat this as end-of-stack to
// enforce progress and avoid infinite loops.
let sp = frame.context.get_stack_pointer();
let last_sp = ctx.get_register_always("sp");
if sp <= last_sp {
// Arm leaf functions may not actually touch the stack (thanks
// to the link register allowing you to "push" the return address
// to a register), so we need to permit the stack pointer to not
// change for the first frame of the unwind. After that we need
// more strict validation to avoid infinite loops.
let is_leaf = args.callee_frame.trust == FrameTrust::Context && sp == last_sp;
if !is_leaf {
trace!("stack pointer went backwards, assuming unwind complete");
return None;
}
}
// Ok, the frame now seems well and truly valid, do final cleanup.
// A caller's ip is the return address, which is the instruction
// *after* the CALL that caused us to arrive at the callee. Set
// the value to 4 less than that, so it points to the CALL instruction
// (arm64 instructions are all 4 bytes wide). This is important because
// we use this value to lookup the CFI we need to unwind the next frame.
let ip = frame.context.get_instruction_pointer();
frame.instruction = ip - 4;
Some(frame)
}