[llvm-dev] Building LLVM's fuzzers

Thu Aug 24 15:07:34 PDT 2017

> On Aug 24, 2017, at 2:55 PM, Kostya Serebryany <kcc at google.com> wrote:
> 
> Interesting. 
> This is a relatively new addition (fsanitize-coverage=pc-tables, which is now a part of -fsanitize=fuzzer). 
> The tests worked (did they? On Mac?) so I thought everything is ok. 

For tests we never compile the tested target with -O3 (and that wouldn’t be sufficient),
and for testing fuzzers I was always building them in debug 

> Yea, we need to make sure the pc-tables are not stripped (this is a separate section with globals). 
> (I still haven't documented pc-tables, will do soon)

> Do you know what's the analog of Wl,-dead_strip on Linux?

Apparently -Wl,—gc-sections.
For some reason LLVM does not do it for gold, even though it seems to support this flag as well.
(that could be another reason why you don’t see the failure on Linux)

 1 if(NOT LLVM_NO_DEAD_STRIP)
 2   if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
 3     # ld64's implementation of -dead_strip breaks tools that use plugins.
 4     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
 5                  LINK_FLAGS " -Wl,-dead_strip")
 6   elseif(${CMAKE_SYSTEM_NAME} MATCHES "SunOS")
 7     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
 8                  LINK_FLAGS " -Wl,-z -Wl,discard-unused=sections")
 9   elseif(NOT WIN32 AND NOT LLVM_LINKER_IS_GOLD)
10     # Object files are compiled with -ffunction-data-sections.
11     # Versions of bfd ld < 2.23.1 have a bug in --gc-sections that breaks
12     # tools that use plugins. Always pass --gc-sections once we require
13     # a newer linker.
14     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
15                  LINK_FLAGS " -Wl,--gc-sections")
16   endif()
17 endif()

> 
> --kcc 
>  
> 
> 
> On Thu, Aug 24, 2017 at 2:49 PM, Justin Bogner <mail at justinbogner.com <mailto:mail at justinbogner.com>> wrote:
> George Karpenkov <ekarpenkov at apple.com <mailto: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 <mailto: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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