//
// Copyright 2018 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
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "absl/debugging/internal/stack_consumption.h"
#ifdef ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
#include <signal.h>
#include <sys/mman.h>
#include <unistd.h>
#include <string.h>
#include "absl/base/attributes.h"
#include "absl/base/internal/raw_logging.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace debugging_internal {
namespace {
// This code requires that we know the direction in which the stack
// grows. It is commonly believed that this can be detected by putting
// a variable on the stack and then passing its address to a function
// that compares the address of this variable to the address of a
// variable on the function's own stack. However, this is unspecified
// behavior in C++: If two pointers p and q of the same type point to
// different objects that are not members of the same object or
// elements of the same array or to different functions, or if only
// one of them is null, the results of p<q, p>q, p<=q, and p>=q are
// unspecified. Therefore, instead we hardcode the direction of the
// stack on platforms we know about.
#if defined(__i386__) || defined(__x86_64__) || defined(__ppc__) || \
defined(__aarch64__)
constexpr bool kStackGrowsDown = true;
#else
#error Need to define kStackGrowsDown
#endif
// To measure the stack footprint of some code, we create a signal handler
// (for SIGUSR2 say) that exercises this code on an alternate stack. This
// alternate stack is initialized to some known pattern (0x55, 0x55, 0x55,
// ...). We then self-send this signal, and after the signal handler returns,
// look at the alternate stack buffer to see what portion has been touched.
//
// This trick gives us the the stack footprint of the signal handler. But the
// signal handler, even before the code for it is exercised, consumes some
// stack already. We however only want the stack usage of the code inside the
// signal handler. To measure this accurately, we install two signal handlers:
// one that does nothing and just returns, and the user-provided signal
// handler. The difference between the stack consumption of these two signals
// handlers should give us the stack foorprint of interest.
void EmptySignalHandler(int) {}
// This is arbitrary value, and could be increase further, at the cost of
// memset()ting it all to known sentinel value.
constexpr int kAlternateStackSize = 64 << 10; // 64KiB
constexpr int kSafetyMargin = 32;
constexpr char kAlternateStackFillValue = 0x55;
// These helper functions look at the alternate stack buffer, and figure
// out what portion of this buffer has been touched - this is the stack
// consumption of the signal handler running on this alternate stack.
// This function will return -1 if the alternate stack buffer has not been
// touched. It will abort the program if the buffer has overflowed or is about
// to overflow.
int GetStackConsumption(const void* const altstack) {
const char* begin;
int increment;
if (kStackGrowsDown) {
begin = reinterpret_cast<const char*>(altstack);
increment = 1;
} else {
begin = reinterpret_cast<const char*>(altstack) + kAlternateStackSize - 1;
increment = -1;
}
for (int usage_count = kAlternateStackSize; usage_count > 0; --usage_count) {
if (*begin != kAlternateStackFillValue) {
ABSL_RAW_CHECK(usage_count <= kAlternateStackSize - kSafetyMargin,
"Buffer has overflowed or is about to overflow");
return usage_count;
}
begin += increment;
}
ABSL_RAW_LOG(FATAL, "Unreachable code");
return -1;
}
} // namespace
int GetSignalHandlerStackConsumption(void (*signal_handler)(int)) {
// The alt-signal-stack cannot be heap allocated because there is a
// bug in glibc-2.2 where some signal handler setup code looks at the
// current stack pointer to figure out what thread is currently running.
// Therefore, the alternate stack must be allocated from the main stack
// itself.
void* altstack = mmap(nullptr, kAlternateStackSize, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
ABSL_RAW_CHECK(altstack != MAP_FAILED, "mmap() failed");
// Set up the alt-signal-stack (and save the older one).
stack_t sigstk;
memset(&sigstk, 0, sizeof(sigstk));
sigstk.ss_sp = altstack;
sigstk.ss_size = kAlternateStackSize;
sigstk.ss_flags = 0;
stack_t old_sigstk;
memset(&old_sigstk, 0, sizeof(old_sigstk));
ABSL_RAW_CHECK(sigaltstack(&sigstk, &old_sigstk) == 0,
"sigaltstack() failed");
// Set up SIGUSR1 and SIGUSR2 signal handlers (and save the older ones).
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
struct sigaction old_sa1, old_sa2;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_ONSTACK;
// SIGUSR1 maps to EmptySignalHandler.
sa.sa_handler = EmptySignalHandler;
ABSL_RAW_CHECK(sigaction(SIGUSR1, &sa, &old_sa1) == 0, "sigaction() failed");
// SIGUSR2 maps to signal_handler.
sa.sa_handler = signal_handler;
ABSL_RAW_CHECK(sigaction(SIGUSR2, &sa, &old_sa2) == 0, "sigaction() failed");
// Send SIGUSR1 signal and measure the stack consumption of the empty
// signal handler.
// The first signal might use more stack space. Run once and ignore the
// results to get that out of the way.
ABSL_RAW_CHECK(kill(getpid(), SIGUSR1) == 0, "kill() failed");
memset(altstack, kAlternateStackFillValue, kAlternateStackSize);
ABSL_RAW_CHECK(kill(getpid(), SIGUSR1) == 0, "kill() failed");
int base_stack_consumption = GetStackConsumption(altstack);
// Send SIGUSR2 signal and measure the stack consumption of signal_handler.
ABSL_RAW_CHECK(kill(getpid(), SIGUSR2) == 0, "kill() failed");
int signal_handler_stack_consumption = GetStackConsumption(altstack);
// Now restore the old alt-signal-stack and signal handlers.
if (old_sigstk.ss_sp == nullptr && old_sigstk.ss_size == 0 &&
(old_sigstk.ss_flags & SS_DISABLE)) {
// https://git.musl-libc.org/cgit/musl/commit/src/signal/sigaltstack.c?id=7829f42a2c8944555439380498ab8b924d0f2070
// The original stack has ss_size==0 and ss_flags==SS_DISABLE, but some
// versions of musl have a bug that rejects ss_size==0. Work around this by
// setting ss_size to MINSIGSTKSZ, which should be ignored by the kernel
// when SS_DISABLE is set.
old_sigstk.ss_size = MINSIGSTKSZ;
}
ABSL_RAW_CHECK(sigaltstack(&old_sigstk, nullptr) == 0,
"sigaltstack() failed");
ABSL_RAW_CHECK(sigaction(SIGUSR1, &old_sa1, nullptr) == 0,
"sigaction() failed");
ABSL_RAW_CHECK(sigaction(SIGUSR2, &old_sa2, nullptr) == 0,
"sigaction() failed");
ABSL_RAW_CHECK(munmap(altstack, kAlternateStackSize) == 0, "munmap() failed");
if (signal_handler_stack_consumption != -1 && base_stack_consumption != -1) {
return signal_handler_stack_consumption - base_stack_consumption;
}
return -1;
}
} // namespace debugging_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
