Migrate coverage to trace-pc-guard

[mikea]

Replace COV_FLAGS with -fsanitize-coverage=trace-pc-guard.

Easy test:

docker run -ti -e COV_FLAGS="-fsanitize-coverage=trace-pc-guard" ossfuzz/expat test

[mikea]

expat fuzzer crashes with:

==3296==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000a136f4 at pc 0x0000005cf95a bp 0x7fff09d70a70 sp 0x7fff09d70a68
WRITE of size 4 at 0x000000a136f4 thread T0
    #0 0x5cf959 in fuzzer::TracePC::HandleInit(unsigned int*, unsigned int*) /src/libfuzzer/FuzzerTracePC.cpp:49:8
    #1 0x5de349 in __sanitizer_cov_trace_pc_guard_init /src/libfuzzer/FuzzerTracePC.cpp:286:15
    #2 0x511a7b in sancov.module_ctor (/out/parse_fuzzer+0x511a7b)
    #3 0x772b4c in __libc_csu_init (/out/parse_fuzzer+0x772b4c)
    #4 0x7faf2bee77be in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x207be)
    #5 0x41c1a8 in _start (/out/parse_fuzzer+0x41c1a8)

0x000000a136f4 is located 44 bytes to the left of global variable '__sancov_guard.9' defined in '/src/parse_fuzzer.cc' (0xa13720) of size 12
0x000000a136f4 is located 0 bytes to the right of global variable '__sancov_guard' defined in '/src/parse_fuzzer.cc' (0xa136e0) of size 20
SUMMARY: AddressSanitizer: global-buffer-overflow /src/libfuzzer/FuzzerTracePC.cpp:49:8 in fuzzer::TracePC::HandleInit(unsigned int*, unsigned int*)
Shadow bytes around the buggy address:
  0x00008013a680: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008013a690: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008013a6a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008013a6b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008013a6c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x00008013a6d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00[04]f9
  0x00008013a6e0: f9 f9 f9 f9 00 04 f9 f9 f9 f9 f9 f9 04 f9 f9 f9
  0x00008013a6f0: f9 f9 f9 f9 00 00 00 00 00 00 00 00 00 00 00 00
  0x00008013a700: 04 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9 f9 f9 f9 f9
  0x00008013a710: 04 f9 f9 f9 f9 f9 f9 f9 00 00 00 00 00 00 00 00
  0x00008013a720: f9 f9 f9 f9 00 00 00 f9 f9 f9 f9 f9 00 f9 f9 f9
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==3296==ABORTING

[kcc]

how can I reproduce it? (w/o docker, please)

[mikea]

Sure. From the full output of the above (https://gist.github.com/mikea/90a119db04ab54aef09b26ec9ce5aa38) and expat build scrtipt (https://github.com/google/oss-fuzz/blob/master/targets/expat/build.sh):

# build libfuzzer
clang++ -g -stdlib=libc++ -std=c++11 -fsanitize=address -c /src/libfuzzer/FuzzerCrossOver.cpp /src/libfuzzer/FuzzerDriver.cpp /src/libfuzzer/FuzzerExtFunctionsDlsym.cpp /src/libfuzzer/FuzzerExtFunctionsWeak.cpp /src/libfuzzer/FuzzerIO.cpp /src/libfuzzer/FuzzerLoop.cpp /src/libfuzzer/FuzzerMain.cpp /src/libfuzzer/FuzzerMutate.cpp /src/libfuzzer/FuzzerSHA1.cpp /src/libfuzzer/FuzzerTracePC.cpp /src/libfuzzer/FuzzerTraceState.cpp /src/libfuzzer/FuzzerUtil.cpp /src/libfuzzer/FuzzerUtilDarwin.cpp /src/libfuzzer/FuzzerUtilLinux.cpp -I/src/libfuzzer

ar ruv /usr/lib/libfuzzer.a /work/libfuzzer/FuzzerCrossOver.o /work/libfuzzer/FuzzerDriver.o /work/libfuzzer/FuzzerExtFunctionsDlsym.o /work/libfuzzer/FuzzerExtFunctionsWeak.o /work/libfuzzer/FuzzerIO.o /work/libfuzzer/FuzzerLoop.o /work/libfuzzer/FuzzerMain.o /work/libfuzzer/FuzzerMutate.o /work/libfuzzer/FuzzerSHA1.o /work/libfuzzer/FuzzerTracePC.o /work/libfuzzer/FuzzerTraceState.o /work/libfuzzer/FuzzerUtil.o /work/libfuzzer/FuzzerUtilDarwin.o /work/libfuzzer/FuzzerUtilLinux.o

export 'CFLAGS=-g -fsanitize=address -fsanitize-coverage=trace-pc-guard'
export 'CXXFLAGS=-g -fsanitize=address -fsanitize-coverage=trace-pc-guard -stdlib=libc++'
export 'CC=clang'
export 'CXX=clang++'
export 'FUZZER_LDFLAGS=-Wl,-whole-archive /usr/local/lib/libc++.a /usr/local/lib/libc++abi.a -Wl,-no-whole-archive'

# build expat
cd /src/expat/expat

./buildconf.sh
./configure
make clean
make -j$(nproc) all

$CXX $CXXFLAGS -std=c++11 -Ilib/ \
    /src/parse_fuzzer.cc -o /out/parse_fuzzer \
    -lfuzzer .libs/libexpat.a $FUZZER_LDFLAGS

[kcc]

fixed in LLVM r287030.
The problem was that asan instrumented the coverage guards
emitted by -fsanitize-coverage=trace-pc-guard
which caused trouble if libFuzzer was asan-ified.
It caused inefficiency otherwise as well, as the guard section was longer. Good catch!

You'll need to update the compiler now...

[kcc]

also don't forget to add trace-cmp

[kcc]

Just to clarify: trace-pc-guard actually requires some work on the sanitizers' side and I want us first to switch to trace-pc-guard.

Then, separately (in a few days), I want to also enable trace-cmp.
I don't want to enable trace-cmp w/o trace-pc-guard because this configuration is not tested.