[llvm-dev] Building LLVM's fuzzers

Thu Aug 24 15:20:06 PDT 2017

On Thu, Aug 24, 2017 at 3:16 PM, Kostya Serebryany <kcc at google.com> wrote:

> With -Wl,-gc-sections I get this:
> SimpleTest.cpp:(.text.sancov.module_ctor[sancov.module_ctor]+0x1b):
> undefined reference to `__start___sancov_pcs'
> SimpleTest.cpp:(.text.sancov.module_ctor[sancov.module_ctor]+0x20):
> undefined reference to `__stop___sancov_pcs'
>

This happens only with 'ld'.
lld and gold are fine.

>
>
>
> On Thu, Aug 24, 2017 at 3:07 PM, George Karpenkov <ekarpenkov at apple.com>
> wrote:
>
>>
>> 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>
>> wrote:
>>
>>> 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?
>>>
>>
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170824/602a9b07/attachment.html>