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SanitizerCoverage

Introduction

NOTE: this document applies to the original Clang project, not the DirectX Compiler. It's made available for informational purposes only.

Sanitizer tools have a very simple code coverage tool built in. It allows to get function-level, basic-block-level, and edge-level coverage at a very low cost.

How to build and run

SanitizerCoverage can be used with :doc:`AddressSanitizer`, :doc:`LeakSanitizer`, :doc:`MemorySanitizer`, and UndefinedBehaviorSanitizer. In addition to -fsanitize=, pass one of the following compile-time flags:

  • -fsanitize-coverage=func for function-level coverage (very fast).
  • -fsanitize-coverage=bb for basic-block-level coverage (may add up to 30% extra slowdown).
  • -fsanitize-coverage=edge for edge-level coverage (up to 40% slowdown).

You may also specify -fsanitize-coverage=indirect-calls for additional caller-callee coverage.

At run time, pass coverage=1 in ASAN_OPTIONS, LSAN_OPTIONS, MSAN_OPTIONS or UBSAN_OPTIONS, as appropriate.

To get Coverage counters, add -fsanitize-coverage=8bit-counters to one of the above compile-time flags. At runtime, use *SAN_OPTIONS=coverage=1:coverage_counters=1.

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=func
% ASAN_OPTIONS=coverage=1 ./a.out; ls -l *sancov
main
-rw-r----- 1 kcc eng 4 Nov 27 12:21 a.out.22673.sancov
% ASAN_OPTIONS=coverage=1 ./a.out foo ; ls -l *sancov
foo
main
-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

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.

Postprocessing

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.

A simple script $LLVM/projects/compiler-rt/lib/sanitizer_common/scripts/sancov.py is provided to dump these offsets.

% sancov.py print a.out.22679.sancov a.out.22673.sancov
sancov.py: read 2 PCs from a.out.22679.sancov
sancov.py: read 1 PCs from a.out.22673.sancov
sancov.py: 2 files merged; 2 PCs total
0x465250
0x4652a0

You can then filter the output of sancov.py through addr2line --exe ObjectFile or llvm-symbolizer --obj ObjectFile to get file names and line numbers:

% sancov.py print a.out.22679.sancov a.out.22673.sancov 2> /dev/null | llvm-symbolizer --obj a.out
cov.cc:3
cov.cc:5

How good is the coverage?

It is possible to find out which PCs are not covered, by subtracting the covered set from the set of all instrumented PCs. The latter can be obtained by listing all callsites of __sanitizer_cov() in the binary. On Linux, sancov.py can do this for you. Just supply the path to binary and a list of covered PCs:

% sancov.py print a.out.12345.sancov > covered.txt
sancov.py: read 2 64-bit PCs from a.out.12345.sancov
sancov.py: 1 file merged; 2 PCs total
% sancov.py missing a.out < covered.txt
sancov.py: found 3 instrumented PCs in a.out
sancov.py: read 2 PCs from stdin
sancov.py: 1 PCs missing from coverage
0x4cc61c

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 (-fsanitize-coverage=edge) simply splits all critical edges by introducing new dummy blocks and then instruments those blocks:

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

Bitset

When coverage_bitset=1 run-time flag is given, the coverage will also be dumped as a bitset (text file with 1 for blocks that have been executed and 0 for blocks that were not).

% clang++ -fsanitize=address -fsanitize-coverage=edge cov.cc
% ASAN_OPTIONS="coverage=1:coverage_bitset=1" ./a.out
main
% ASAN_OPTIONS="coverage=1:coverage_bitset=1" ./a.out 1
foo
main
% head *bitset*
==> a.out.38214.bitset-sancov <==
01101
==> a.out.6128.bitset-sancov <==
11011%

For a given executable the length of the bitset is always the same (well, unless dlopen/dlclose come into play), so the bitset coverage can be easily used for bitset-based corpus distillation.

Caller-callee coverage

(Experimental!) Every indirect function call is instrumented with a run-time function call that captures caller and callee. At the shutdown time the process dumps a separate file called caller-callee.PID.sancov which contains caller/callee pairs as pairs of lines (odd lines are callers, even lines are callees)

a.out 0x4a2e0c
a.out 0x4a6510
a.out 0x4a2e0c
a.out 0x4a87f0

Current limitations:

  • Only the first 14 callees for every caller are recorded, the rest are silently ignored.
  • The output format is not very compact since caller and callee may reside in different modules and we need to spell out the module names.
  • The routine that dumps the output is not optimized for speed
  • Only Linux x86_64 is tested so far.
  • Sandboxes are not supported.

Coverage counters

This experimental feature is inspired by AFL's coverage instrumentation. With additional compile-time and run-time flags you can get more sensitive coverage information. In addition to boolean values assigned to every basic block (edge) the instrumentation will collect imprecise counters. On exit, every counter will be mapped to a 8-bit bitset representing counter ranges: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+ and those 8-bit bitsets will be dumped to disk.

% clang++ -g cov.cc -fsanitize=address -fsanitize-coverage=edge,8bit-counters
% ASAN_OPTIONS="coverage=1:coverage_counters=1" ./a.out
% ls -l *counters-sancov
... a.out.17110.counters-sancov
% xxd *counters-sancov
0000000: 0001 0100 01

These counters may also be used for in-process coverage-guided fuzzers. See include/sanitizer/coverage_interface.h:

// The coverage instrumentation may optionally provide imprecise counters.
// Rather than exposing the counter values to the user we instead map
// the counters to a bitset.
// Every counter is associated with 8 bits in the bitset.
// We define 8 value ranges: 1, 2, 3, 4-7, 8-15, 16-31, 32-127, 128+
// The i-th bit is set to 1 if the counter value is in the i-th range.
// This counter-based coverage implementation is *not* thread-safe.

// Returns the number of registered coverage counters.
uintptr_t __sanitizer_get_number_of_counters();
// Updates the counter 'bitset', clears the counters and returns the number of
// new bits in 'bitset'.
// If 'bitset' is nullptr, only clears the counters.
// Otherwise 'bitset' should be at least
// __sanitizer_get_number_of_counters bytes long and 8-aligned.
uintptr_t
__sanitizer_update_counter_bitset_and_clear_counters(uint8_t *bitset);

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

Sudden death

Normally, coverage data is collected in memory and saved to disk when the program exits (with an atexit() handler), when a SIGSEGV is caught, or when __sanitizer_cov_dump() is called.

If the program ends with a signal that ASan does not handle (or can not handle at all, like SIGKILL), coverage data will be lost. This is a big problem on Android, where SIGKILL is a normal way of evicting applications from memory.

With ASAN_OPTIONS=coverage=1:coverage_direct=1 coverage data is written to a memory-mapped file as soon as it collected.

% ASAN_OPTIONS="coverage=1:coverage_direct=1" ./a.out
main
% ls
7036.sancov.map  7036.sancov.raw  a.out
% sancov.py rawunpack 7036.sancov.raw
sancov.py: reading map 7036.sancov.map
sancov.py: unpacking 7036.sancov.raw
writing 1 PCs to a.out.7036.sancov
% sancov.py print a.out.7036.sancov
sancov.py: read 1 PCs from a.out.7036.sancov
sancov.py: 1 files merged; 1 PCs total
0x4b2bae

Note that on 64-bit platforms, this method writes 2x more data than the default, because it stores full PC values instead of 32-bit offsets.

In-process fuzzing

Coverage data could be useful for fuzzers and sometimes it is preferable to run a fuzzer in the same process as the code being fuzzed (in-process fuzzer).

You can use __sanitizer_get_total_unique_coverage() from <sanitizer/coverage_interface.h> which returns the number of currently covered entities in the program. This will tell the fuzzer if the coverage has increased after testing every new input.

If a fuzzer finds a bug in the ASan run, you will need to save the reproducer before exiting the process. Use __asan_set_death_callback from <sanitizer/asan_interface.h> to do that.

An example of such fuzzer can be found in the LLVM tree.

Performance

This coverage implementation is fast. With function-level coverage (-fsanitize-coverage=func) the overhead is not measurable. With basic-block-level coverage (-fsanitize-coverage=bb) the overhead varies between 0 and 25%.

benchmark cov0 cov1 diff 0-1 cov2 diff 0-2 diff 1-2
400.perlbench 1296.00 1307.00 1.01 1465.00 1.13 1.12
401.bzip2 858.00 854.00 1.00 1010.00 1.18 1.18
403.gcc 613.00 617.00 1.01 683.00 1.11 1.11
429.mcf 605.00 582.00 0.96 610.00 1.01 1.05
445.gobmk 896.00 880.00 0.98 1050.00 1.17 1.19
456.hmmer 892.00 892.00 1.00 918.00 1.03 1.03
458.sjeng 995.00 1009.00 1.01 1217.00 1.22 1.21
462.libquantum 497.00 492.00 0.99 534.00 1.07 1.09
464.h264ref 1461.00 1467.00 1.00 1543.00 1.06 1.05
471.omnetpp 575.00 590.00 1.03 660.00 1.15 1.12
473.astar 658.00 652.00 0.99 715.00 1.09 1.10
483.xalancbmk 471.00 491.00 1.04 582.00 1.24 1.19
433.milc 616.00 627.00 1.02 627.00 1.02 1.00
444.namd 602.00 601.00 1.00 654.00 1.09 1.09
447.dealII 630.00 634.00 1.01 653.00 1.04 1.03
450.soplex 365.00 368.00 1.01 395.00 1.08 1.07
453.povray 427.00 434.00 1.02 495.00 1.16 1.14
470.lbm 357.00 375.00 1.05 370.00 1.04 0.99
482.sphinx3 927.00 928.00 1.00 1000.00 1.08 1.08

Why another coverage?

Why did we implement yet another code coverage?
  • We needed something that is lightning fast, plays well with AddressSanitizer, and does not significantly increase the binary size.
  • Traditional coverage implementations based in global counters suffer from contention on counters.