// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_TRACE_EVENT_CFI_BACKTRACE_ANDROID_H_
#define BASE_TRACE_EVENT_CFI_BACKTRACE_ANDROID_H_
#include <stddef.h>
#include <stdint.h>
#include <memory>
#include "base/base_export.h"
#include "base/files/memory_mapped_file.h"
#include "base/gtest_prod_util.h"
#include "base/memory/raw_ptr.h"
namespace base {
namespace trace_event {
// This class is used to unwind stack frames in the current thread. The unwind
// information (dwarf debug info) is stripped from the chrome binary and we do
// not build with exception tables (ARM EHABI) in release builds. So, we use a
// custom unwind table which is generated and added to specific android builds,
// when add_unwind_tables_in_apk build option is specified. This unwind table
// contains information for unwinding stack frames when the functions calls are
// from lib[mono]chrome.so. The file is added as an asset to the apk and the
// table is used to unwind stack frames for profiling. This class implements
// methods to read and parse the unwind table and unwind stack frames using this
// data.
class BASE_EXPORT CFIBacktraceAndroid {
public:
// The CFI information that correspond to an instruction.
struct CFIRow {
bool operator==(const CFIBacktraceAndroid::CFIRow& o) const {
return cfa_offset == o.cfa_offset && ra_offset == o.ra_offset;
}
// The offset of the call frame address of previous function from the
// current stack pointer. Rule for unwinding SP: SP_prev = SP_cur +
// cfa_offset.
uint16_t cfa_offset = 0;
// The offset of location of return address from the previous call frame
// address. Rule for unwinding PC: PC_prev = * (SP_prev - ra_offset).
uint16_t ra_offset = 0;
};
// A simple cache that stores entries in table using prime modulo hashing.
// This cache with 500 entries already gives us 95% hit rate, and fits in a
// single system page (usually 4KiB). Using a thread local cache for each
// thread gives us 30% improvements on performance of heap profiling.
class CFICache {
public:
// Add new item to the cache. It replaces an existing item with same hash.
// Constant time operation.
void Add(uintptr_t address, CFIRow cfi);
// Finds the given address and fills |cfi| with the info for the address.
// returns true if found, otherwise false. Assumes |address| is never 0.
bool Find(uintptr_t address, CFIRow* cfi);
private:
FRIEND_TEST_ALL_PREFIXES(CFIBacktraceAndroidTest, TestCFICache);
// Size is the highest prime which fits the cache in a single system page,
// usually 4KiB. A prime is chosen to make sure addresses are hashed evenly.
static const int kLimit = 509;
struct AddrAndCFI {
uintptr_t address;
CFIRow cfi;
};
AddrAndCFI cache_[kLimit] = {};
};
static_assert(sizeof(CFIBacktraceAndroid::CFICache) < 4096,
"The cache does not fit in a single page.");
// Creates and initializes by memory mapping the unwind tables from apk assets
// on first call.
static CFIBacktraceAndroid* GetInitializedInstance();
// Returns true if the given program counter |pc| is mapped in chrome library.
static bool is_chrome_address(uintptr_t pc);
// Returns the start and end address of the current library.
static uintptr_t executable_start_addr();
static uintptr_t executable_end_addr();
// Returns true if stack unwinding is possible using CFI unwind tables in apk.
// There is no need to check this before each unwind call. Will always return
// the same value based on CFI tables being present in the binary.
bool can_unwind_stack_frames() const { return can_unwind_stack_frames_; }
// Returns the program counters by unwinding stack in the current thread in
// order of latest call frame first. Unwinding works only if
// can_unwind_stack_frames() returns true. For each stack frame, this method
// searches through the unwind table mapped in memory to find the unwind
// information for function and walks the stack to find all the return
// address. This only works until the last function call from the chrome.so.
// We do not have unwind information to unwind beyond any frame outside of
// chrome.so. Calls to Unwind() are thread safe and lock free.
size_t Unwind(const void** out_trace, size_t max_depth);
// Same as above function, but starts from a given program counter |pc|,
// stack pointer |sp| and link register |lr|. This can be from current thread
// or any other thread. But the caller must make sure that the thread's stack
// segment is not racy to read.
size_t Unwind(uintptr_t pc,
uintptr_t sp,
uintptr_t lr,
const void** out_trace,
size_t max_depth);
// Finds the CFI row for the given |func_addr| in terms of offset from
// the start of the current binary. Concurrent calls are thread safe.
bool FindCFIRowForPC(uintptr_t func_addr, CFIRow* out);
private:
FRIEND_TEST_ALL_PREFIXES(CFIBacktraceAndroidTest, TestCFICache);
FRIEND_TEST_ALL_PREFIXES(CFIBacktraceAndroidTest, TestFindCFIRow);
FRIEND_TEST_ALL_PREFIXES(CFIBacktraceAndroidTest, TestUnwinding);
// Initializes unwind tables using the CFI asset file in the apk if present.
// Also stores the limits of mapped region of the lib[mono]chrome.so binary,
// since the unwind is only feasible for addresses within the .so file. Once
// initialized, the memory map of the unwind table is never cleared since we
// cannot guarantee that all the threads are done using the memory map when
// heap profiling is turned off. But since we keep the memory map is clean,
// the system can choose to evict the unused pages when needed. This would
// still reduce the total amount of address space available in process.
CFIBacktraceAndroid();
~CFIBacktraceAndroid();
// Finds the UNW_INDEX and UNW_DATA tables in from the CFI file memory map.
void ParseCFITables();
// The start address of the memory mapped unwind table asset file. Unique ptr
// because it is replaced in tests.
std::unique_ptr<MemoryMappedFile> cfi_mmap_;
// The UNW_INDEX table: Start address of the function address column. The
// memory segment corresponding to this column is treated as an array of
// uintptr_t.
raw_ptr<const uintptr_t, AllowPtrArithmetic> unw_index_function_col_ =
nullptr;
// The UNW_INDEX table: Start address of the index column. The memory segment
// corresponding to this column is treated as an array of uint16_t.
raw_ptr<const uint16_t, AllowPtrArithmetic> unw_index_indices_col_ = nullptr;
// The number of rows in UNW_INDEX table.
size_t unw_index_row_count_ = 0;
// The start address of UNW_DATA table.
raw_ptr<const uint16_t, AllowPtrArithmetic> unw_data_start_addr_ = nullptr;
bool can_unwind_stack_frames_ = false;
};
} // namespace trace_event
} // namespace base
#endif // BASE_TRACE_EVENT_CFI_BACKTRACE_ANDROID_H_
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