[llvm-dev] Building LLVM's fuzzers
Kostya Serebryany via llvm-dev
llvm-dev at lists.llvm.org
Mon Sep 11 18:57:37 PDT 2017
On Mon, Sep 11, 2017 at 11:52 AM, Justin Bogner <mail at justinbogner.com>
wrote:
> Kostya Serebryany <kcc at google.com> writes:
> > Justin,
>
> > Calling appendToUsed has horrible complexity and if we call it in
> > every function clang consumes tons of memory (6Gb when compiling one
> > of the clang's source files). This killed my machine today :)
> >
> > The solution is to call appendToUsed once per module, instead of once
> > per function.
>
> Oh right, updating lists in metadata is O(n), so doing it per function
> is quadratic. This slipped my mind - sorry!
>
Yea. Very unexpected (although I've stumbled on it a few times, I still
don't remember about it)
>
> > Also, since this does not seem to be required for linux, I've put this
> > under if TargetTriple.isOSBinFormatMachO Submitted r312855, I'll see
> > if this breaks Mac (there seem to be no llvm tests for this, only
> > compiler-rt tests) but please also check if this looks ok.
>
> This looks fine, though I'd rather if we just did it on all platforms
> for consistency / clear semantic intent. Running appendToUsed once
> should be cheap, and we do it unguarded in our other instrumentation
> (like InstrProfiling.cpp).
>
It's unclear if it's going to hurt things on linux.
If I see it doesn't -- I'll remove the if TargetTriple.isOSBinFormatMachO
>
> > But this all still sounds bad on linux at least:
> > * with the old bfd linker and -ffunction-sections -Wl,-gc-sections
> these
> > arrays get removed (as discussed here)
>
> This is sad, and I don't think we have any particularly good ideas to
> fix this.
>
> > * with newer linkers the sanitizer coverage essentially disables
> > gc-sections
>
> I'm not sure I follow. We're making sure the global arrays /
> instrumentation data doesn't get dead stripped here, but dead stripping
> of functions should work as normal.
>
Ah, that's now my confusion.
inline-8bit-counters and trace-pc-guard seems to work just fine.
pc-table actually does break gc-sections as there is no a reference to
every function in the pc-table section.
+eugenis, who dealt with a similar issue in asan (although it's probably a
separate topic now).
--kcc
>
> > --kcc
> >
> >
> > On Thu, Aug 24, 2017 at 6:43 PM, Peter Collingbourne <peter at pcc.me.uk>
> > wrote:
> >
> >>
> >>
> >> On Thu, Aug 24, 2017 at 6:30 PM, Justin Bogner <mail at justinbogner.com>
> >> wrote:
> >>
> >>> Peter Collingbourne <peter at pcc.me.uk> writes:
> >>> > On Thu, Aug 24, 2017 at 3:38 PM, Kostya Serebryany <kcc at google.com>
> >>> wrote:
> >>> >
> >>> >>
> >>> >>
> >>> >> On Thu, Aug 24, 2017 at 3:35 PM, Peter Collingbourne <
> peter at pcc.me.uk>
> >>> >> wrote:
> >>> >>
> >>> >>> On Thu, Aug 24, 2017 at 3:21 PM, Kostya Serebryany via llvm-dev <
> >>> >>> llvm-dev at lists.llvm.org> wrote:
> >>> >>>
> >>> >>>>
> >>> >>>>
> >>> >>>> On Thu, Aug 24, 2017 at 3:20 PM, Justin Bogner <
> >>> mail at justinbogner.com>
> >>> >>>> wrote:
> >>> >>>>
> >>> >>>>> I think the simplest fix is something like this:
> >>> >>>>>
> >>> >>>>> diff --git a/lib/Transforms/Instrumentati
> on/SanitizerCoverage.cpp
> >>> >>>>> b/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
> >>> >>>>> index c6f0d17f8fe..e81957ab80a 100644
> >>> >>>>> --- a/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
> >>> >>>>> +++ b/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
> >>> >>>>> @@ -256,6 +256,7 @@ SanitizerCoverageModule::Creat
> >>> eSecStartEnd(Module
> >>> >>>>> &M, const char *Section,
> >>> >>>>> new GlobalVariable(M, Ty, false,
> >>> GlobalVariable::ExternalLinkag
> >>> >>>>> e,
> >>> >>>>> nullptr, getSectionEnd(Section));
> >>> >>>>> SecEnd->setVisibility(GlobalValue::HiddenVisibility);
> >>> >>>>> + appendToUsed(M, {SecStart, SecEnd});
> >>> >>>>>
> >>> >>>>> return std::make_pair(SecStart, SecEnd);
> >>> >>>>> }
> >>> >>>>>
> >>> >>>>> I'm trying it out now.
> >>> >>>>>
> >>> >>>>
> >>> >>>> LGTM (if this works), thanks!
> >>> >>>>
> >>> >>>
> >>> >>> I wouldn't expect that to work because for ELF targets llvm.used
> has
> >>> no
> >>> >>> effect on the object file (only on the optimizer).
> >>> >>>
> >>> >>> Is there a simple way to reproduce the link failure?
> >>> >>>
> >>> >>
> >>> >>
> >>> >> ninja compiler-rt
> >>> >> echo 'extern "C" int LLVMFuzzerTestOneInput(const unsigned char *a,
> >>> >> unsigned long b){return 0; } ' > test.cc
> >>> >> clang -O3 test.cc -fsanitize=fuzzer # works
> >>> >> clang -O3 test.cc -Wl,-gc-sections -fsanitize=fuzzer # fails
> >>> >>
> >>> >
> >>> > It seems that the issue is that older versions of ld.bfd have a bug
> >>> which
> >>> > causes it not to define __start_ and __stop_ symbols if the only
> >>> reference
> >>> > to those symbols is from a constructor.
> >>>
> >>> It looks like this is a different problem from the one on macOS (and I
> >>> wasn't able to reproduce it with any bfd ld I had available, they were
> >>> all too new)
> >>>
> >>> I've gone ahead and fixed the issue on macOS in r311742.
> >>>
> >>> > If I add an artificial reference to the start symbol from libfuzzer's
> >>> main
> >>> > function, the program links correctly.
> >>> >
> >>> > diff --git a/compiler-rt/lib/fuzzer/FuzzerMain.cpp
> >>> > b/compiler-rt/lib/fuzzer/FuzzerMain.cpp
> >>> > index af8657200be2..c41e28e012db 100644
> >>> > --- a/compiler-rt/lib/fuzzer/FuzzerMain.cpp
> >>> > +++ b/compiler-rt/lib/fuzzer/FuzzerMain.cpp
> >>> > @@ -16,6 +16,10 @@ extern "C" {
> >>> > int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size);
> >>> > } // extern "C"
> >>> >
> >>> > +__attribute__((weak)) void nop(void *p) {}
> >>> > +extern void *__start___sancov_pcs;
> >>> > +
> >>> > int main(int argc, char **argv) {
> >>> > + nop(__start___sancov_pcs);
> >>> > return fuzzer::FuzzerDriver(&argc, &argv, LLVMFuzzerTestOneInput);
> >>> > }
> >>>
> >>> If we were to do this, we'd have to guard it appropriately - not all
> >>> platforms name the __start symbols like this.
> >>>
> >>
> >> Of course. There's also the issue of how to keep the symbols alive in
> DSOs.
> >>
> >> > The problem also goes away if I use "GNU ld (GNU Binutils)
> >>> > 2.28.51.20170105".
> >>>
> >>> 2.27 also doesn't have the issue. I don't know what our minimum version
> >>> of binutils is, and I'm under the impression most people use gold or
> lld
> >>> to link LLVM these days, so it isn't clear to me how big of a problem
> >>> this is.
> >>>
> >>
> >> For the record, the problem reproduces under 2.24, which is shipped by
> >> Ubuntu 14.04 LTS, which isn't that old. My view is that if we can find
> an
> >> unintrusive enough workaround, we should deploy it (with a comment to
> >> remove it after N years).
> >>
> >> Peter
> >>
> >>
> >>> > Peter
> >>> >
> >>> >
> >>> >
> >>> >>
> >>> >>
> >>> >>
> >>> >>
> >>> >>
> >>> >>>
> >>> >>> Peter
> >>> >>>
> >>> >>>
> >>> >>>>
> >>> >>>>>
> >>> >>>>> Kostya Serebryany <kcc at google.com> writes:
> >>> >>>>> > 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'
> >>> >>>>> >
> >>> >>>>> >
> >>> >>>>> >
> >>> >>>>> > 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=O
> >>> >>>>> n,
> >>> >>>>> >>> 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 <(408)%20916-6883>
> >>> >>>>> >>> >> 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?
> >>> >>>>> >>>
> >>> >>>>> >>
> >>> >>>>> >>
> >>> >>>>> >>
> >>> >>>>>
> >>> >>>>
> >>> >>>>
> >>> >>>> _______________________________________________
> >>> >>>> LLVM Developers mailing list
> >>> >>>> llvm-dev at lists.llvm.org
> >>> >>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>> >>>>
> >>> >>>>
> >>> >>>
> >>> >>>
> >>> >>> --
> >>> >>> --
> >>> >>> Peter
> >>> >>>
> >>> >>
> >>> >>
> >>> >
> >>> >
> >>> > --
> >>>
> >>
> >>
> >>
> >> --
> >> --
> >> Peter
> >>
>
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