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// Copyright 2015 Ted Mielczarek. See the COPYRIGHT
// file at the top-level directory of this distribution.
// Note since x86 and Amd64 have basically the same ABI, this implementation
// is written to largely erase the details of the two wherever possible,
// so that it can be copied between the two with minimal changes. It's not
// worth the effort to *actually* unify the implementations.
use super::impl_prelude::*;
use minidump::format::CONTEXT_X86;
use minidump::{MinidumpContext, MinidumpContextValidity, MinidumpModuleList, MinidumpRawContext};
use std::collections::HashSet;
use tracing::trace;
type Pointer = u32;
const POINTER_WIDTH: Pointer = 4;
const INSTRUCTION_REGISTER: &str = "eip";
const STACK_POINTER_REGISTER: &str = "esp";
const FRAME_POINTER_REGISTER: &str = "ebp";
const CALLEE_SAVED_REGS: &[&str] = &["ebp", "ebx", "edi", "esi"];
async fn get_caller_by_cfi<P>(
ctx: &CONTEXT_X86,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
trace!("trying cfi");
if let MinidumpContextValidity::Some(ref which) = args.valid() {
if !which.contains(STACK_POINTER_REGISTER) {
return None;
}
}
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_ip = stack_walker.caller_ctx.eip;
let caller_sp = stack_walker.caller_ctx.esp;
trace!(
"cfi evaluation was successful -- caller_ip: 0x{:08x}, caller_sp: 0x{:08x}",
caller_ip,
caller_sp,
);
// Do absolutely NO validation! Yep! As long as CFI evaluation succeeds
// (which does include ip 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?: breakpad is actually a little weary of the output of STACK WIN
// cfi, and does check that instruction_seems_valid() for eip. However,
// it doesn't immediately discard the results. It tentatively tries to
// scan, and then if that doesn't return anything compelling, it just goes
// forward with whatever STACK WIN came up with.
//
// The current layering of this code means that we don't actually know what
// kind of cfi was used here, and the code that *does* can't do scanning.
// For now let's just trust the results unconditionally. We can do something
// more hacky/robust if we find a compelling need to.
//
// It also has some weird scanning to try to adjust the computed bp?
trace!("cfi result seems valid");
let context = MinidumpContext {
raw: MinidumpRawContext::X86(stack_walker.caller_ctx),
valid: MinidumpContextValidity::Some(stack_walker.caller_validity),
};
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: &CONTEXT_X86,
args: &GetCallerFrameArgs<'_, P>,
) -> Option<StackFrame>
where
P: SymbolProvider + Sync,
{
trace!("trying frame pointer");
if let MinidumpContextValidity::Some(ref which) = args.valid() {
if !which.contains(FRAME_POINTER_REGISTER) {
return None;
}
}
let last_bp = ctx.ebp;
// Assume that the standard %bp-using x86 calling convention is in
// use.
//
// The typical x86 calling convention, when frame pointers are present,
// is for the calling procedure to use CALL, which pushes the return
// address onto the stack and sets the instruction pointer (%ip) to
// the entry point of the called routine. The called routine then
// PUSHes the calling routine's frame pointer (%bp) onto the stack
// before copying the stack pointer (%sp) to the frame pointer (%bp).
// Therefore, the calling procedure's frame pointer is always available
// by dereferencing the called procedure's frame pointer, and the return
// address is always available at the memory location immediately above
// the address pointed to by the called procedure's frame pointer. The
// calling procedure's stack pointer (%sp) is 2 pointers higher than the
// value of the called procedure's frame pointer at the time the calling
// procedure made the CALL: 1 pointer for the return address pushed by the
// CALL itself, and 1 pointer for the callee's PUSH of the caller's frame
// pointer.
//
// %ip_new = *(%bp_old + ptr)
// %bp_new = *(%bp_old)
// %sp_new = %bp_old + ptr*2
if last_bp >= u32::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_ip = args
.stack_memory
.get_memory_at_address(last_bp as u64 + POINTER_WIDTH as u64)?;
let caller_bp = args.stack_memory.get_memory_at_address(last_bp as u64)?;
let caller_sp = last_bp + POINTER_WIDTH * 2;
// NOTE: minor divergence from x64 impl here: doing extra validation on the
// value of `caller_sp` and `caller_bp` here encourages the stack scanner
// to kick in and start outputting extra frames for `/testdata/test.dmp`.
// Since breakpad also doesn't output those frames, let's assume that's
// desirable.
trace!(
"frame pointer seems valid -- caller_ip: 0x{:08x}, caller_sp: 0x{:08x}",
caller_ip,
caller_sp,
);
let caller_ctx = CONTEXT_X86 {
eip: caller_ip,
esp: caller_sp,
ebp: caller_bp,
..CONTEXT_X86::default()
};
let mut valid = HashSet::new();
valid.insert(INSTRUCTION_REGISTER);
valid.insert(STACK_POINTER_REGISTER);
valid.insert(FRAME_POINTER_REGISTER);
let context = MinidumpContext {
raw: MinidumpRawContext::X86(caller_ctx),
valid: MinidumpContextValidity::Some(valid),
};
Some(StackFrame::from_context(context, FrameTrust::FramePointer))
}
async fn get_caller_by_scan<P>(
ctx: &CONTEXT_X86,
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 ip value that was pushed by the CALL instruction that created
// the current frame. The next frame is then assumed to end just before that
// ip value.
let last_bp = match args.valid() {
MinidumpContextValidity::All => Some(ctx.ebp),
MinidumpContextValidity::Some(ref which) => {
if !which.contains(STACK_POINTER_REGISTER) {
trace!("cannot scan without stack pointer");
return None;
}
if which.contains(FRAME_POINTER_REGISTER) {
Some(ctx.ebp)
} else {
None
}
}
};
let last_sp = ctx.esp;
// 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_ip = last_sp.checked_add(i * POINTER_WIDTH)?;
let caller_ip = args
.stack_memory
.get_memory_at_address(address_of_ip as u64)?;
if instruction_seems_valid(caller_ip, args.modules, args.symbol_provider).await {
// ip is pushed by CALL, so sp is just address_of_ip + ptr
let caller_sp = address_of_ip.checked_add(POINTER_WIDTH)?;
// Try to restore bp as well. This can be possible in two cases:
//
// 1. This function has the standard prologue that pushes bp and
// sets bp = sp. If this is the case, then the current bp should be
// immediately after (before in memory) address_of_ip.
//
// 2. This function does not use bp, and has just preserved it
// from the caller. If this is the case, bp should be before
// (after in memory) address_of_ip.
//
// We then try our best to eliminate bogus-looking bp's with some
// simple heuristics like "is a valid stack address".
let mut caller_bp = None;
// Max reasonable size for a single x86 frame is 128 KB. This value is used in
// a heuristic for recovering of the EBP chain after a scan for return address.
// This value is based on a stack frame size histogram built for a set of
// popular third party libraries which suggests that 99.5% of all frames are
// smaller than 128 KB.
const MAX_REASONABLE_GAP_BETWEEN_FRAMES: Pointer = 128 * 1024;
// If we're on the first iteration of the scan, there can't possibly be a frame pointer,
// because the entire stack frame is taken up by the return pointer. And if we're
// not on the first iteration, then the last iteration already loaded the location
// we expect the frame pointer to be in, so we can unconditionally load it here.
if i > 0 {
let address_of_bp = address_of_ip - POINTER_WIDTH;
let bp = args
.stack_memory
.get_memory_at_address(address_of_bp as u64)?;
if bp > address_of_ip && bp - address_of_bp <= MAX_REASONABLE_GAP_BETWEEN_FRAMES {
// Sanity check that resulting bp is still inside stack memory.
if args
.stack_memory
.get_memory_at_address::<Pointer>(bp as u64)
.is_some()
{
caller_bp = Some(bp);
}
} else if let Some(last_bp) = last_bp {
if last_bp >= caller_sp {
// Sanity check that resulting bp is still inside stack memory.
if args
.stack_memory
.get_memory_at_address::<Pointer>(last_bp as u64)
.is_some()
{
caller_bp = Some(last_bp);
}
}
}
}
trace!(
"scan seems valid -- caller_ip: 0x{:08x}, caller_sp: 0x{:08x}",
caller_ip,
caller_sp,
);
let caller_ctx = CONTEXT_X86 {
eip: caller_ip,
esp: caller_sp,
ebp: caller_bp.unwrap_or(0),
..CONTEXT_X86::default()
};
let mut valid = HashSet::new();
valid.insert(INSTRUCTION_REGISTER);
valid.insert(STACK_POINTER_REGISTER);
if caller_bp.is_some() {
valid.insert(FRAME_POINTER_REGISTER);
}
let context = MinidumpContext {
raw: MinidumpRawContext::X86(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 instruction == 0 {
return false;
}
super::instruction_seems_valid_by_symbols(instruction as u64, modules, symbol_provider).await
}
/*
// x86 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: &CONTEXT_X86,
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.
if frame.context.get_stack_pointer() <= ctx.esp as u64 {
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 one less than that, so it points within the
// CALL instruction. 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 - 1;
Some(frame)
}