SanitizerCoverage

Introduction

LLVM has a simple code coverage instrumentation built in (SanitizerCoverage). It inserts calls to user-defined functions on function-, basic-block-, and edge- levels. Default implementations of those callbacks are provided and implement simple coverage reporting and visualization, however if you need just coverage visualization you may want to use SourceBasedCodeCoverage instead.

Tracing PCs with guards

With -fsanitize-coverage=trace-pc-guard the compiler will insert the following code on every edge:

__sanitizer_cov_trace_pc_guard(&guard_variable)

Every edge will have its own guard_variable (uint32_t).

The compler will also insert calls to a module constructor:

// The guards are [start, stop).
// This function will be called at least once per DSO and may be called
// more than once with the same values of start/stop.
__sanitizer_cov_trace_pc_guard_init(uint32_t *start, uint32_t *stop);

With an additional ...=trace-pc,indirect-calls flag __sanitizer_cov_trace_pc_indirect(void *callee) will be inserted on every indirect call.

The functions __sanitizer_cov_trace_pc_* should be defined by the user.

Example:

// trace-pc-guard-cb.cc
#include <stdint.h>
#include <stdio.h>
#include <sanitizer/coverage_interface.h>

// This callback is inserted by the compiler as a module constructor
// into every DSO. 'start' and 'stop' correspond to the
// beginning and end of the section with the guards for the entire
// binary (executable or DSO). The callback will be called at least
// once per DSO and may be called multiple times with the same parameters.
extern "C" void __sanitizer_cov_trace_pc_guard_init(uint32_t *start,
                                                    uint32_t *stop) {
  static uint64_t N;  // Counter for the guards.
  if (start == stop || *start) return;  // Initialize only once.
  printf("INIT: %p %p\n", start, stop);
  for (uint32_t *x = start; x < stop; x++)
    *x = ++N;  // Guards should start from 1.
}

// This callback is inserted by the compiler on every edge in the
// control flow (some optimizations apply).
// Typically, the compiler will emit the code like this:
//    if(*guard)
//      __sanitizer_cov_trace_pc_guard(guard);
// But for large functions it will emit a simple call:
//    __sanitizer_cov_trace_pc_guard(guard);
extern "C" void __sanitizer_cov_trace_pc_guard(uint32_t *guard) {
  if (!*guard) return;  // Duplicate the guard check.
  // If you set *guard to 0 this code will not be called again for this edge.
  // Now you can get the PC and do whatever you want:
  //   store it somewhere or symbolize it and print right away.
  // The values of `*guard` are as you set them in
  // __sanitizer_cov_trace_pc_guard_init and so you can make them consecutive
  // and use them to dereference an array or a bit vector.
  void *PC = __builtin_return_address(0);
  char PcDescr[1024];
  // This function is a part of the sanitizer run-time.
  // To use it, link with AddressSanitizer or other sanitizer.
  __sanitizer_symbolize_pc(PC, "%p %F %L", PcDescr, sizeof(PcDescr));
  printf("guard: %p %x PC %s\n", guard, *guard, PcDescr);
}
// trace-pc-guard-example.cc
void foo() { }
int main(int argc, char **argv) {
  if (argc > 1) foo();
}
clang++ -g  -fsanitize-coverage=trace-pc-guard trace-pc-guard-example.cc -c
clang++ trace-pc-guard-cb.cc trace-pc-guard-example.o -fsanitize=address
ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out
INIT: 0x71bcd0 0x71bce0
guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:2
guard: 0x71bcd8 3 PC 0x4ecd9e in main trace-pc-guard-example.cc:3:7
ASAN_OPTIONS=strip_path_prefix=`pwd`/ ./a.out with-foo
INIT: 0x71bcd0 0x71bce0
guard: 0x71bcd4 2 PC 0x4ecd5b in main trace-pc-guard-example.cc:3
guard: 0x71bcdc 4 PC 0x4ecdc7 in main trace-pc-guard-example.cc:4:17
guard: 0x71bcd0 1 PC 0x4ecd20 in foo() trace-pc-guard-example.cc:2:14

Tracing PCs

With -fsanitize-coverage=trace-pc the compiler will insert __sanitizer_cov_trace_pc() on every edge. With an additional ...=trace-pc,indirect-calls flag __sanitizer_cov_trace_pc_indirect(void *callee) will be inserted on every indirect call. These callbacks are not implemented in the Sanitizer run-time and should be defined by the user. This mechanism is used for fuzzing the Linux kernel (https://github.com/google/syzkaller).

Instrumentation points

Sanitizer Coverage offers different levels of instrumentation.

  • edge (default): edges are instrumented (see below).
  • bb: basic blocks are instrumented.
  • func: only the entry block of every function will be instrumented.

Use these flags together with trace-pc-guard or trace-pc, like this: -fsanitize-coverage=func,trace-pc-guard.

When edge or bb is used, some of the edges/blocks may still be left uninstrumented (pruned) if such instrumentation is considered redundant. Use no-prune (e.g. -fsanitize-coverage=bb,no-prune,trace-pc-guard) to disable pruning. This could be useful for better coverage visualization.

Edge coverage

Consider this code:

void foo(int *a) {
  if (a)
    *a = 0;
}

It contains 3 basic blocks, let’s name them A, B, C:

A
|\
| \
|  B
| /
|/
C

If blocks A, B, and C are all covered we know for certain that the edges A=>B and B=>C were executed, but we still don’t know if the edge A=>C was executed. Such edges of control flow graph are called critical. The edge-level coverage simply splits all critical edges by introducing new dummy blocks and then instruments those blocks:

A
|\
| \
D  B
| /
|/
C

Tracing data flow

Support for data-flow-guided fuzzing. With -fsanitize-coverage=trace-cmp the compiler will insert extra instrumentation around comparison instructions and switch statements. Similarly, with -fsanitize-coverage=trace-div the compiler will instrument integer division instructions (to capture the right argument of division) and with -fsanitize-coverage=trace-gep – the LLVM GEP instructions (to capture array indices).

// Called before a comparison instruction.
// Arg1 and Arg2 are arguments of the comparison.
void __sanitizer_cov_trace_cmp1(uint8_t Arg1, uint8_t Arg2);
void __sanitizer_cov_trace_cmp2(uint16_t Arg1, uint16_t Arg2);
void __sanitizer_cov_trace_cmp4(uint32_t Arg1, uint32_t Arg2);
void __sanitizer_cov_trace_cmp8(uint64_t Arg1, uint64_t Arg2);

// Called before a comparison instruction if exactly one of the arguments is constant.
// Arg1 and Arg2 are arguments of the comparison, Arg1 is a compile-time constant.
// These callbacks are emitted by -fsanitize-coverage=trace-cmp since 2017-08-11
void __sanitizer_cov_trace_const_cmp1(uint8_t Arg1, uint8_t Arg2);
void __sanitizer_cov_trace_const_cmp2(uint16_t Arg1, uint16_t Arg2);
void __sanitizer_cov_trace_const_cmp4(uint32_t Arg1, uint32_t Arg2);
void __sanitizer_cov_trace_const_cmp8(uint64_t Arg1, uint64_t Arg2);

// Called before a switch statement.
// Val is the switch operand.
// Cases[0] is the number of case constants.
// Cases[1] is the size of Val in bits.
// Cases[2:] are the case constants.
void __sanitizer_cov_trace_switch(uint64_t Val, uint64_t *Cases);

// Called before a division statement.
// Val is the second argument of division.
void __sanitizer_cov_trace_div4(uint32_t Val);
void __sanitizer_cov_trace_div8(uint64_t Val);

// Called before a GetElemementPtr (GEP) instruction
// for every non-constant array index.
void __sanitizer_cov_trace_gep(uintptr_t Idx);

This interface is a subject to change.

Default implementation

The sanitizer run-time (AddressSanitizer, MemorySanitizer, etc) provide a default implementations of some of the coverage callbacks. You may use this implementation to dump the coverage on disk at the process exit.

Example:

% cat -n cov.cc
     1  #include <stdio.h>
     2  __attribute__((noinline))
     3  void foo() { printf("foo\n"); }
     4
     5  int main(int argc, char **argv) {
     6    if (argc == 2)
     7      foo();
     8    printf("main\n");
     9  }
% clang++ -g cov.cc -fsanitize=address -fsanitize-coverage=trace-pc-guard
% ASAN_OPTIONS=coverage=1 ./a.out; wc -c *.sancov
main
SanitizerCoverage: ./a.out.7312.sancov 2 PCs written
24 a.out.7312.sancov
% ASAN_OPTIONS=coverage=1 ./a.out foo ; wc -c *.sancov
foo
main
SanitizerCoverage: ./a.out.7316.sancov 3 PCs written
24 a.out.7312.sancov
32 a.out.7316.sancov

Every time you run an executable instrumented with SanitizerCoverage one *.sancov file is created during the process shutdown. If the executable is dynamically linked against instrumented DSOs, one *.sancov file will be also created for every DSO.

Sancov data format

The format of *.sancov files is very simple: the first 8 bytes is the magic, one of 0xC0BFFFFFFFFFFF64 and 0xC0BFFFFFFFFFFF32. The last byte of the magic defines the size of the following offsets. The rest of the data is the offsets in the corresponding binary/DSO that were executed during the run.

Sancov Tool

An simple sancov tool is provided to process coverage files. The tool is part of LLVM project and is currently supported only on Linux. It can handle symbolization tasks autonomously without any extra support from the environment. You need to pass .sancov files (named <module_name>.<pid>.sancov and paths to all corresponding binary elf files. Sancov matches these files using module names and binaries file names.

USAGE: sancov [options] <action> (<binary file>|<.sancov file>)...

Action (required)
  -print                    - Print coverage addresses
  -covered-functions        - Print all covered functions.
  -not-covered-functions    - Print all not covered functions.
  -symbolize                - Symbolizes the report.

Options
  -blacklist=<string>         - Blacklist file (sanitizer blacklist format).
  -demangle                   - Print demangled function name.
  -strip_path_prefix=<string> - Strip this prefix from file paths in reports

Coverage Reports

Experimental

.sancov files do not contain enough information to generate a source-level coverage report. The missing information is contained in debug info of the binary. Thus the .sancov has to be symbolized to produce a .symcov file first:

sancov -symbolize my_program.123.sancov my_program > my_program.123.symcov

The .symcov file can be browsed overlayed over the source code by running tools/sancov/coverage-report-server.py script that will start an HTTP server.

Output directory

By default, .sancov files are created in the current working directory. This can be changed with ASAN_OPTIONS=coverage_dir=/path:

% ASAN_OPTIONS="coverage=1:coverage_dir=/tmp/cov" ./a.out foo
% ls -l /tmp/cov/*sancov
-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
-rw-r----- 1 kcc eng 8 Nov 27 12:21 a.out.22679.sancov