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// Copyright 2017 The Abseil Authors.
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Produce stack trace. I'm guessing (hoping!) the code is much like
// for x86. For apple machines, at least, it seems to be; see
// Linux has similar code:
#if defined(__linux__)
#include <asm/ptrace.h> // for PT_NIP.
#include <ucontext.h> // for ucontext_t
#include <unistd.h>
#include <cassert>
#include <cstdint>
#include <cstdio>
#include "absl/base/attributes.h"
#include "absl/base/optimization.h"
#include "absl/base/port.h"
#include "absl/debugging/stacktrace.h"
#include "absl/debugging/internal/address_is_readable.h"
#include "absl/debugging/internal/vdso_support.h" // a no-op on non-elf or non-glibc systems
// Given a stack pointer, return the saved link register value.
// Note that this is the link register for a callee.
static inline void *StacktracePowerPCGetLR(void **sp) {
// PowerPC has 3 main ABIs, which say where in the stack the
// Link Register is. For DARWIN and AIX (used by apple and
// linux ppc64), it's in sp[2]. For SYSV (used by linux ppc),
// it's in sp[1].
#if defined(_CALL_AIX) || defined(_CALL_DARWIN)
return *(sp+2);
#elif defined(_CALL_SYSV)
return *(sp+1);
#elif defined(__APPLE__) || defined(__FreeBSD__) || \
(defined(__linux__) && defined(__PPC64__))
// This check is in case the compiler doesn't define _CALL_AIX/etc.
return *(sp+2);
#elif defined(__linux)
// This check is in case the compiler doesn't define _CALL_SYSV.
return *(sp+1);
#error Need to specify the PPC ABI for your architecture.
// Given a pointer to a stack frame, locate and return the calling
// stackframe, or return null if no stackframe can be found. Perform sanity
// checks (the strictness of which is controlled by the boolean parameter
// "STRICT_UNWINDING") to reduce the chance that a bad pointer is returned.
ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS // May read random elements from stack.
ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY // May read random elements from stack.
static void **NextStackFrame(void **old_sp, const void *uc) {
void **new_sp = (void **) *old_sp;
enum { kStackAlignment = 16 };
// Check that the transition from frame pointer old_sp to frame
// pointer new_sp isn't clearly bogus
// With the stack growing downwards, older stack frame must be
// at a greater address that the current one.
if (new_sp <= old_sp) return nullptr;
// Assume stack frames larger than 100,000 bytes are bogus.
if ((uintptr_t)new_sp - (uintptr_t)old_sp > 100000) return nullptr;
} else {
// In the non-strict mode, allow discontiguous stack frames.
// (alternate-signal-stacks for example).
if (new_sp == old_sp) return nullptr;
// And allow frames upto about 1MB.
if ((new_sp > old_sp)
&& ((uintptr_t)new_sp - (uintptr_t)old_sp > 1000000)) return nullptr;
if ((uintptr_t)new_sp % kStackAlignment != 0) return nullptr;
#if defined(__linux__)
enum StackTraceKernelSymbolStatus {
kNotInitialized = 0, kAddressValid, kAddressInvalid };
if (IS_WITH_CONTEXT && uc != nullptr) {
static StackTraceKernelSymbolStatus kernel_symbol_status =
kNotInitialized; // Sentinel: not computed yet.
// Initialize with sentinel value: __kernel_rt_sigtramp_rt64 can not
// possibly be there.
static const unsigned char *kernel_sigtramp_rt64_address = nullptr;
if (kernel_symbol_status == kNotInitialized) {
absl::debugging_internal::VDSOSupport vdso;
if (vdso.IsPresent()) {
if (!vdso.LookupSymbol(
"__kernel_sigtramp_rt64", "LINUX_2.6.15",
&sigtramp_rt64_symbol_info) ||
sigtramp_rt64_symbol_info.address == nullptr) {
// Unexpected: VDSO is present, yet the expected symbol is missing
// or null.
assert(false && "VDSO is present, but doesn't have expected symbol");
kernel_symbol_status = kAddressInvalid;
} else {
kernel_sigtramp_rt64_address =
reinterpret_cast<const unsigned char *>(
kernel_symbol_status = kAddressValid;
} else {
kernel_symbol_status = kAddressInvalid;
if (new_sp != nullptr &&
kernel_symbol_status == kAddressValid &&
StacktracePowerPCGetLR(new_sp) == kernel_sigtramp_rt64_address) {
const ucontext_t* signal_context =
reinterpret_cast<const ucontext_t*>(uc);
void **const sp_before_signal =
#if defined(__PPC64__)
reinterpret_cast<void **>(signal_context->uc_mcontext.gp_regs[PT_R1]);
reinterpret_cast<void **>(
// Check that alleged sp before signal is nonnull and is reasonably
// aligned.
if (sp_before_signal != nullptr &&
((uintptr_t)sp_before_signal % kStackAlignment) == 0) {
// Check that alleged stack pointer is actually readable. This is to
// prevent a "double fault" in case we hit the first fault due to e.g.
// a stack corruption.
if (absl::debugging_internal::AddressIsReadable(sp_before_signal)) {
// Alleged stack pointer is readable, use it for further unwinding.
new_sp = sp_before_signal;
return new_sp;
// This ensures that absl::GetStackTrace sets up the Link Register properly.
ABSL_ATTRIBUTE_NOINLINE static void AbslStacktracePowerPCDummyFunction() {
template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS // May read random elements from stack.
ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY // May read random elements from stack.
static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
const void *ucp, int *min_dropped_frames) {
void **sp;
// Apple macOS uses an old version of gnu as -- both Darwin 7.9.0 (Panther)
// and Darwin 8.8.1 (Tiger) use as 1.38. This means we have to use a
// different asm syntax. I don't know quite the best way to discriminate
// systems using the old as from the new one; I've gone with __APPLE__.
#ifdef __APPLE__
__asm__ volatile ("mr %0,r1" : "=r" (sp));
__asm__ volatile ("mr %0,1" : "=r" (sp));
// On PowerPC, the "Link Register" or "Link Record" (LR), is a stack
// entry that holds the return address of the subroutine call (what
// instruction we run after our function finishes). This is the
// same as the stack-pointer of our parent routine, which is what we
// want here. While the compiler will always(?) set up LR for
// subroutine calls, it may not for leaf functions (such as this one).
// This routine forces the compiler (at least gcc) to push it anyway.
// The LR save area is used by the callee, so the top entry is bogus.
int n = 0;
// Unlike ABIs of X86 and ARM, PowerPC ABIs say that return address (in
// the link register) of a function call is stored in the caller's stack
// frame instead of the callee's. When we look for the return address
// associated with a stack frame, we need to make sure that there is a
// caller frame before it. So we call NextStackFrame before entering the
// loop below and check next_sp instead of sp for loop termination.
// The outermost frame is set up by runtimes and it does not have a
// caller frame, so it is skipped.
// The absl::GetStackFrames routine is called when we are in some
// informational context (the failure signal handler for example).
// Use the non-strict unwinding rules to produce a stack trace
// that is as complete as possible (even if it contains a few
// bogus entries in some rare cases).
void **next_sp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(sp, ucp);
while (next_sp && n < max_depth) {
if (skip_count > 0) {
} else {
result[n] = StacktracePowerPCGetLR(sp);
if (next_sp > sp) {
sizes[n] = (uintptr_t)next_sp - (uintptr_t)sp;
} else {
// A frame-size of 0 is used to indicate unknown frame size.
sizes[n] = 0;
sp = next_sp;
next_sp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(sp, ucp);
if (min_dropped_frames != nullptr) {
// Implementation detail: we clamp the max of frames we are willing to
// count, so as not to spend too much time in the loop below.
const int kMaxUnwind = 1000;
int num_dropped_frames = 0;
for (int j = 0; next_sp != nullptr && j < kMaxUnwind; j++) {
if (skip_count > 0) {
} else {
next_sp = NextStackFrame<!IS_STACK_FRAMES, IS_WITH_CONTEXT>(next_sp, ucp);
*min_dropped_frames = num_dropped_frames;
return n;
namespace absl {
namespace debugging_internal {
bool StackTraceWorksForTest() {
return true;
} // namespace debugging_internal
} // namespace absl