[llvm-dev] Building LLVM's fuzzers

[Kostya Serebryany via llvm-dev]

This is quite unexpected.
Do you have a minimized example of dead stripping eliminating the coverage
instrumentation?

--kcc

On Thu, Aug 24, 2017 at 2:37 PM, George Karpenkov <ekarpenkov at apple.com>
wrote:

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