[llvm-dev] Building LLVM's fuzzers

[Justin Bogner via llvm-dev]

George Karpenkov <ekarpenkov at apple.com> writes:
> OK so with Kuba’s help I’ve found the error: with optimization, dead
> stripping of produced libraries is enabled,
> which removes coverage instrumentation.
>
> However, this has nothing to do with the move to compiler-rt, so I’m
> quite skeptical on whether it has worked
> beforehand.
>
> A trivial fix is to do:
>
> diff --git a/cmake/modules/HandleLLVMOptions.cmake b/cmake/modules/HandleLLVMOptions.cmake
> index 04596a6ff63..5465d8d95ba 100644
> --- a/cmake/modules/HandleLLVMOptions.cmake
> +++ b/cmake/modules/HandleLLVMOptions.cmake
> @@ -665,6 +665,9 @@ if(LLVM_USE_SANITIZER)
>    endif()
>    if (LLVM_USE_SANITIZE_COVERAGE)
>      append("-fsanitize=fuzzer-no-link" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
> +
> +    # Dead stripping messes up coverage instrumentation.
> +    set(LLVM_NO_DEAD_STRIP ON)
>    endif()
>  endif()
>
> Any arguments against that?

We shouldn't do this. We really only want to prevent dead stripping of
the counters themselves - disabling it completely isn't very nice.

> Apparently, a better way is to follow ASAN instrumentation pass,
> which uses some magic to protect against dead-stripping.

I thought this was already being done - how else did it work before?

>> On Aug 24, 2017, at 11:29 AM, Justin Bogner <mail at justinbogner.com> wrote:
>> 
>> (kcc, george: sorry for the re-send, the first was from a non-list email
>> address)
>> 
>> My configuration for building the fuzzers in the LLVM tree doesn't seem to
>> work any more (possibly as of moving libFuzzer to compiler-rt, but there
>> have been a few other changes in the last week or so that may be related).
>> 
>> I'm building with a fresh top-of-tree clang and setting
>> -DLLVM_USE_SANITIZER=Address and -DLLVM_USE_SANITIZE_COVERAGE=On, which
>> was working before:
>> 
>>  % cmake -GNinja \
>>          -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=On \
>>          -DLLVM_ENABLE_WERROR=On \
>>          -DLLVM_USE_SANITIZER=Address -DLLVM_USE_SANITIZE_COVERAGE=On \
>>          -DCMAKE_C_COMPILER=$HOME/llvm-lkgc/bin/clang \
>>          $HOME/code/llvm-src
>> 
>> But when I run any of the fuzzers, it looks like the sanitizer coverage
>> hasn't been set up correctly:
>> 
>>  % ./bin/llvm-as-fuzzer                                                                                   2017-08-24 11:14:33
>>  INFO: Seed: 4089166883
>>  INFO: Loaded 1 modules   (50607 guards): 50607 [0x10e14ef80, 0x10e18063c),
>>  INFO: Loaded 1 PC tables (0 PCs): 0 [0x10e2870a8,0x10e2870a8),
>>  ERROR: The size of coverage PC tables does not match the number of instrumented PCs. This might be a bug in the compiler, please contact the libFuzzer developers.
>> 
>> From the build logs, it looks like we're now building objects with these
>> sanitizer flags:
>> 
>>  -fsanitize=address
>>  -fsanitize-address-use-after-scope
>>  -fsanitize=fuzzer-no-link
>> 
>> We're then linking the fuzzer binaries with these:
>> 
>>  -fsanitize=address
>>  -fsanitize-address-use-after-scope
>>  -fsanitize=fuzzer-no-link
>>  -fsanitize=fuzzer
>> 
>> Any idea what's wrong or where to start looking?