[llvm-dev] Building LLVM's fuzzers

[George Karpenkov via llvm-dev]

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?