[llvm] r247786 - llvm-mc-fuzzer: A fuzzing tool for the MC layer.
Kostya Serebryany via llvm-commits
llvm-commits at lists.llvm.org
Thu Sep 17 17:32:49 PDT 2015
On Thu, Sep 17, 2015 at 2:38 AM, Daniel Sanders <Daniel.Sanders at imgtec.com>
wrote:
> > I forgot to ask you to document the fuzzer at
> http://llvm.org/docs/LibFuzzer.html#fuzzing-components-of-llvm
>
>
>
> Will do
>
>
>
> > One problem: with the current structure of flags libFuzzer's -jobs=10
> does not work...
>
> > Thoughts?
>
>
>
> Hmm. I see why that happens, each spawned thread is calling system() to
> spawn a subprocess and that system() call is given a command built from the
> fuzzer config. The resulting command lacks any of the non-fuzzer args and
> so the child llvm-mc-fuzzer is trying to parse arguments meant for the
> underlying fuzzer. Why does it spawn a subprocess from the worker thread
> instead of doing the work directly inside the worker thread? Am I right in
> thinking that it's to stop a crash in one job from killing everything?
>
>
>
> I can think of four options:
>
> 1. fork() the new process instead of using system(). After the fork,
> the child should remove the effects of –job by setting it to 0 and reopen
> its stdout/stderr to achieve the same effect. This removes the need to
> reconstruct and reparse the command line since fork() will duplicate the
> result of the parse in the child process. Unfortunately, I don't think
> there's a direct Windows equivalent to this outside of Cygwin.
>
> 2. Separate fuzzer option parsing from the driver call. I'm
> thinking something along the lines of this quick sketch:
> FlagDescription *Config = FuzzerDriver::ParseFlags(FuzzerArgv);
> return FuzzerDriver::FuzzerDriver(argv, Config,
> DisassembleOneInput);
> That would allow argv to differ from the options the fuzzer understands
> which are in FuzzerArgv.
>
> 3. Make it possible to extend the fuzzer option parsing. The
> CommandLine library can do this nicely but you probably don't want the
> additional dependency in libFuzzer. Llvm-mc-fuzzer could always change to
> libFuzzer's approach to command line parsing.
>
> 4. Make it possible to modify the command before the system() call.
> The client of libFuzzer could install a callback that allows it to modify a
> std::vector containing the desired Argv.
>
I frankly like none of these, will need to think about it more...
It's probably not urgent for this particular fuzzer -- llvm-mc has pretty
small inputs and we can fuzz lots out of it in a single process.
But will need to figure out for future uses like this.
Maybe,
5. Add a libFuzzer option -target_options=-option1,param,-option2
and run llvm-mc-fuzzer like "./bin/llvm-mc-fuzzer
-target_options=-triple,x86_64-linux-gnu,-disassemble
BTW, I've found one llvm_unreachable with -triple x86_64-linux-gnu
already... will file a bug.
--kcc
>
>
> If all OS's had fork() then I'd favour #1 but Windows rules that out. Out
> of the rest #2 is seems the most flexible but #3/#4 are simpler. What's
> your opinion?
>
>
>
> *From:* Kostya Serebryany [mailto:kcc at google.com]
> *Sent:* 17 September 2015 05:38
> *To:* Daniel Sanders
> *Cc:* LLVM Commits
> *Subject:* Re: [llvm] r247786 - llvm-mc-fuzzer: A fuzzing tool for the MC
> layer.
>
>
>
> One problem: with the current structure of flags libFuzzer's -jobs=10 does
> not work...
>
> Thoughts?
>
>
>
> On Wed, Sep 16, 2015 at 9:25 PM, Kostya Serebryany <kcc at google.com> wrote:
>
> Cool! I'll add it to the bot when time permits.
>
> I forgot to ask you to document the fuzzer
>
> at http://llvm.org/docs/LibFuzzer.html#fuzzing-components-of-llvm
>
> Feel free to do it w/o prior review.
>
>
>
> On Wed, Sep 16, 2015 at 4:49 AM, Daniel Sanders via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
> Author: dsanders
> Date: Wed Sep 16 06:49:49 2015
> New Revision: 247786
>
> URL: http://llvm.org/viewvc/llvm-project?rev=247786&view=rev
> Log:
> llvm-mc-fuzzer: A fuzzing tool for the MC layer.
>
> Summary:
> Only the disassembler is supported in this patch but it has already found
> a few
> issues in the Mips disassembler (mostly invalid instructions being
> successfully
> disassembled).
>
> Reviewers: kcc
>
> Subscribers: russell.gallop, silvas, kcc, llvm-commits
>
> Differential Revision: http://reviews.llvm.org/D12723
>
> Added:
> llvm/trunk/tools/llvm-mc-fuzzer/
> llvm/trunk/tools/llvm-mc-fuzzer/CMakeLists.txt
> llvm/trunk/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp
> Modified:
> llvm/trunk/docs/LibFuzzer.rst
>
> Modified: llvm/trunk/docs/LibFuzzer.rst
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/LibFuzzer.rst?rev=247786&r1=247785&r2=247786&view=diff
>
> ==============================================================================
> --- llvm/trunk/docs/LibFuzzer.rst (original)
> +++ llvm/trunk/docs/LibFuzzer.rst Wed Sep 16 06:49:49 2015
> @@ -453,7 +453,14 @@ Trophies
>
> * llvm-as: https://llvm.org/bugs/show_bug.cgi?id=24639
>
> -
> + * Disassembler:
> + * Mips: Discovered a number of untested instructions for the Mips
> target
> + (see valid-mips*.s in http://reviews.llvm.org/rL247405,
> + http://reviews.llvm.org/rL247414, http://reviews.llvm.org/rL247416,
> + http://reviews.llvm.org/rL247417, http://reviews.llvm.org/rL247420,
> + and http://reviews.llvm.org/rL247422) as well some instructions
> that
> + successfully disassembled on ISA's where they were not valid (see
> + invalid-xfail.s files in the same commits).
>
> .. _pcre2: http://www.pcre.org/
>
>
> Added: llvm/trunk/tools/llvm-mc-fuzzer/CMakeLists.txt
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/tools/llvm-mc-fuzzer/CMakeLists.txt?rev=247786&view=auto
>
> ==============================================================================
> --- llvm/trunk/tools/llvm-mc-fuzzer/CMakeLists.txt (added)
> +++ llvm/trunk/tools/llvm-mc-fuzzer/CMakeLists.txt Wed Sep 16 06:49:49 2015
> @@ -0,0 +1,18 @@
> +if( LLVM_USE_SANITIZE_COVERAGE )
> + include_directories(BEFORE
> + ${CMAKE_CURRENT_SOURCE_DIR}/../../lib/Fuzzer)
> +
> + set(LLVM_LINK_COMPONENTS
> + AllTargetsDescs
> + AllTargetsDisassemblers
> + AllTargetsInfos
> + MC
> + MCDisassembler
> + Support
> + )
> + add_llvm_tool(llvm-mc-fuzzer
> + llvm-mc-fuzzer.cpp)
> + target_link_libraries(llvm-mc-fuzzer
> + LLVMFuzzerNoMain
> + )
> +endif()
>
> Added: llvm/trunk/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp?rev=247786&view=auto
>
> ==============================================================================
> --- llvm/trunk/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp (added)
> +++ llvm/trunk/tools/llvm-mc-fuzzer/llvm-mc-fuzzer.cpp Wed Sep 16 06:49:49
> 2015
> @@ -0,0 +1,129 @@
> +//===--- llvm-mc-fuzzer.cpp - Fuzzer for the MC layer
> ---------------------===//
> +//
> +// The LLVM Compiler Infrastructure
> +//
> +// This file is distributed under the University of Illinois Open Source
> +// License. See LICENSE.TXT for details.
> +//
>
> +//===----------------------------------------------------------------------===//
> +//
>
> +//===----------------------------------------------------------------------===//
> +
> +#include "llvm-c/Disassembler.h"
> +#include "llvm-c/Target.h"
> +#include "llvm/ADT/ArrayRef.h"
> +#include "llvm/MC/SubtargetFeature.h"
> +#include "llvm/Support/CommandLine.h"
> +#include "llvm/Support/raw_ostream.h"
> +#include "FuzzerInterface.h"
> +
> +using namespace llvm;
> +
> +const unsigned AssemblyTextBufSize = 80;
> +
> +enum ActionType {
> + AC_Assemble,
> + AC_Disassemble
> +};
> +
> +static cl::opt<ActionType>
> +Action(cl::desc("Action to perform:"),
> + cl::init(AC_Assemble),
> + cl::values(clEnumValN(AC_Assemble, "assemble",
> + "Assemble a .s file (default)"),
> + clEnumValN(AC_Disassemble, "disassemble",
> + "Disassemble strings of hex bytes"),
> + clEnumValEnd));
> +
> +static cl::opt<std::string>
> + TripleName("triple", cl::desc("Target triple to assemble for, "
> + "see -version for available targets"));
> +
> +static cl::opt<std::string>
> + MCPU("mcpu",
> + cl::desc("Target a specific cpu type (-mcpu=help for details)"),
> + cl::value_desc("cpu-name"), cl::init(""));
> +
> +static cl::list<std::string>
> + MAttrs("mattr", cl::CommaSeparated,
> + cl::desc("Target specific attributes (-mattr=help for
> details)"),
> + cl::value_desc("a1,+a2,-a3,..."));
> +// The feature string derived from -mattr's values.
> +std::string FeaturesStr;
> +
> +static cl::list<std::string>
> + FuzzerArgv("fuzzer-args", cl::Positional,
> + cl::desc("Options to pass to the fuzzer"), cl::ZeroOrMore,
> + cl::PositionalEatsArgs);
> +
> +void DisassembleOneInput(const uint8_t *Data, size_t Size) {
> + char AssemblyText[AssemblyTextBufSize];
> +
> + std::vector<uint8_t> DataCopy(Data, Data + Size);
> +
> + LLVMDisasmContextRef Ctx = LLVMCreateDisasmCPUFeatures(
> + TripleName.c_str(), MCPU.c_str(), FeaturesStr.c_str(), nullptr, 0,
> + nullptr, nullptr);
> + assert(Ctx);
> + uint8_t *p = DataCopy.data();
> + unsigned Consumed;
> + do {
> + Consumed = LLVMDisasmInstruction(Ctx, p, Size, 0, AssemblyText,
> + AssemblyTextBufSize);
> + Size -= Consumed;
> + p += Consumed;
> + } while (Consumed != 0);
> + LLVMDisasmDispose(Ctx);
> +}
> +
> +int main(int argc, char **argv) {
> + // The command line is unusual compared to other fuzzers due to the
> need to
> + // specify the target. Options like -triple, -mcpu, and -mattr work like
> + // their counterparts in llvm-mc, while -fuzzer-args collects options
> for the
> + // fuzzer itself.
> + //
> + // Examples:
> + //
> + // Fuzz the big-endian MIPS32R6 disassembler using 100,000 inputs of up
> to
> + // 4-bytes each and use the contents of ./corpus as the test corpus:
> + // llvm-mc-fuzzer -triple mips-linux-gnu -mcpu=mips32r6 -disassemble \
> + // -fuzzer-args -max_len=4 -runs=100000 ./corpus
> + //
> + // Infinitely fuzz the little-endian MIPS64R2 disassembler with the MSA
> + // feature enabled using up to 64-byte inputs:
> + // llvm-mc-fuzzer -triple mipsel-linux-gnu -mcpu=mips64r2 -mattr=msa \
> + // -disassemble -fuzzer-args ./corpus
> + //
> + // If your aim is to find instructions that are not tested, then it is
> + // advisable to constrain the maximum input size to a single instruction
> + // using -max_len as in the first example. This results in a test
> corpus of
> + // individual instructions that test unique paths. Without this
> constraint,
> + // there will be considerable redundancy in the corpus.
> +
> + LLVMInitializeAllTargetInfos();
> + LLVMInitializeAllTargetMCs();
> + LLVMInitializeAllDisassemblers();
> +
> + cl::ParseCommandLineOptions(argc, argv);
> +
> + // Package up features to be passed to target/subtarget
> + // We have to pass it via a global since the callback doesn't
> + // permit any user data.
> + if (MAttrs.size()) {
> + SubtargetFeatures Features;
> + for (unsigned i = 0; i != MAttrs.size(); ++i)
> + Features.AddFeature(MAttrs[i]);
> + FeaturesStr = Features.getString();
> + }
> +
> + // Insert the program name into the FuzzerArgv.
> + FuzzerArgv.insert(FuzzerArgv.begin(), argv[0]);
> +
> + if (Action == AC_Assemble)
> + errs() << "error: -assemble is not implemented\n";
> + else if (Action == AC_Disassemble)
> + return fuzzer::FuzzerDriver(FuzzerArgv, DisassembleOneInput);
> +
> + llvm_unreachable("Unknown action");
> + return 1;
> +}
>
>
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