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sdasgup3 / PLDI20-Artifact-Evaluation

Artifact Evaluation, PLDI'20

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llvm-irmcsemasymbolic-execution-enginesymbolic-summariesverification-queriesvirtualboxx86-64language-semanticsreverse-engineeringtranslation-validationvalidationcompiler-optimizationgraph-matchingformal-semanticsverification-conditionsdetecting-bugspldievaluationreproducing-bugs

PLDI 2020 Artifact Evaluation DOI

This gives the step-by-step instructions on how to evaluate the artifacts for [1].

[1] Sandeep Dasgupta, Sushant Dinesh, Deepan Venkatesh, Vikram S. Adve, and Christopher W. Fletcher 2020. Scalable Validation of Binary Lifters. In Proceedings of the 2020 ACM SIGPLAN Conference on Programming Language Design and Implementation. ACM. https://doi.org/10.1145/3385412.3385964

Artifact Submission

  • Accepted Paper pdf
  • VM Details
    • VM Player: VirtualBox 6.1 Or 5.1
    • Ubuntu Image: ova, size ~16G
      • md5 hash
      • login: sdasgup3
      • password: aecadmin123
    • Guest Machine recommended settings
      • RAM: Min-8GB, Max-16GB and Cores:Min-4, Max-8. However, it is recommended to configure the VM with more RAM and processors to allow parallel experiments during single instruction validation.
      • Allow 70GB of disk space as needed by the unpacked VM.
  • We have also included the current repository in the VM disk, so that one can access this README.md file at ~/Github/PLDI20-Artifact-Evaluation/README.md. This is just only to avoid the unfortunate scenario of the "bidirectional shared clipboard" not working for the VirtualBox.

Troubleshoot:

  1. For a Ubuntu host machine with Secure Boot enabled, the presented VirtualBox image may fail to be loaded. In that case, you can either disable the Secure Boot, or sign the VirtualBox module as described here.
  2. VM boot up with black login screen
  3. Virtualbox Error ID: BLKCACHE_IOERR: In the VB client, Storage » SATA Controller" Use the cache I/O host (all other values are those used by default VirtualBox))
  4. On startup of the VM, you will get the following warning, which we recommend to ignore.
On startup of the VM, I received this error message (although otherwise the VM functioned normally):
Error found when loading /home/sdasgup3/.profile:

tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
/home/sdasgup3/.bashrc: line 73: bind: warning: line editing not enabled
Could not open a connection to your authentication agent.
Could not open a connection to your authentication agent.

As a result the session will not be configured correctly.
You should fix the problem as soon as feasible

Getting Started

To check if the installations is working as expected, the reviewers should be able to run the following steps. The following steps (otherwise detailed in Step-by-step Instructions section) are responsible to run the program-level validation on function Queen::Doit

export NORM=CUSTOM
cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit/
make compd
make match

and provide the following output

$ export NORM=CUSTOM

$ cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit/

$ make compd
mkdir -p mcsema/
time /home/sdasgup3/Github//validating-binary-decompilation/source/build/bin//decompiler  --output mcsema/test.proposed.ll --path /home/sdasgup3/Github/compd_cache/ --function Doit --input ../binary/test.reloc --use-reloc-info 1>mcsema/compd.log 2>&1
Compd Pass:- /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit:Doit
opt -S  -inline   mcsema/test.proposed.ll -o mcsema/test.proposed.inline.ll

$ make match
opt -S -mem2reg -licm -gvn -early-cse -globalopt -simplifycfg -basicaa -aa -memdep -dse -deadargelim -libcalls-shrinkwrap -tailcallelim -simplifycfg -basicaa -aa -instcombine -simplifycfg -early-cse -gvn -basicaa -aa -memdep -dse -memcpyopt mcsema/test.proposed.inline.ll -o mcsema/test.proposed.opt.ll
opt -S -mem2reg -licm -gvn -early-cse -globalopt -simplifycfg -basicaa -aa -memdep -dse -deadargelim -libcalls-shrinkwrap -tailcallelim -simplifycfg -basicaa -aa -instcombine -simplifycfg -early-cse -gvn -basicaa -aa -memdep -dse -memcpyopt ../binary/test.mcsema.inline.ll -o mcsema/test.mcsema.opt.ll
( time /home/sdasgup3/Github//validating-binary-decompilation/source/build/bin//matcher --file1 mcsema/test.mcsema.opt.ll:Doit --file2 mcsema/test.proposed.opt.ll:Doit ) 1>mcsema/match_mcsema_proposed.log 2>&1
( time /home/sdasgup3/Github//validating-binary-decompilation/source/build/bin//matcher --file1 mcsema/test.proposed.opt.ll:Doit --file2 mcsema/test.mcsema.opt.ll:Doit ) 1>mcsema/match_proposed_mcsema.log 2>&1
Match Pass:both-exact-match:- /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit:Doit

Goals

The reviewer (or a user) should be able to

  1. reproduce the program-level validation (PLV) runs on single-source benchmark.
  2. check that PLV is effective in detecting bugs which are artificially injected.
  3. run single-instruction validation pipeline on individual instructions and generate the verification conditions to be dispatched to z3 solver.
  4. reproduce bugs and timeouts reported in the paper.

Step-by-step Instructions

Program-Level Validation (PLV)

Source Code

The two major components of PLV are (1) Compositional Lifter, with the core logic defined in library, and (2) matcher, with the core logic defined in library. Additionally, Compositional Lifter uses a Store of formally validated instructions to stitch the validated LLVM IR sequence of constituent instructions of a binary program.

Testing Arena for PLV

The test directory contains test-suites like toy-examples and single-source-benchmark. The single-source-benchmark contain folders for all the programs hosted by the test-suite. Each such program, for example the Queens program, has the following structure:

  • src/ (the source artifacts of the program)
    • test.ll or test.bc (Source LLVM code of Queens's program)
  • binary/
    • test: Binary compiled from src/test.bc
    • test.mcsema.bc: McSema lifted LLVM IR from binary/test
    • test.mcsema.calls_renamed.ll: A version of binary/test.mcsema.ll with function calls renamed so as to prevent inter-procedural optimization during normalization
    • test.mcsema.inline.ll: Inlined version of binary/test.mcsema.calls_renamed.ll
    • test.reloc: Binary compiled from src/test.bc with relocation information preserved
  • Makefile: With the following relevant targets
    • binary: To generate binary/test
    • reloc_binary: To generate binary/test.reloc
    • mcsema: To generate binary/test.mcsema.bc,binary/test.mcsema.inline.ll, and binary/test.mcsema.calls_renamed.ll using McSema tool (To be Skipped during evaluation for reason detailed below)
  • Doit/ (Similarly for Initrand/, Rand/, main/, Try/, or Queens/) (Artifacts related to individual functions of the Queens's program, extracted using external tool wllvm)
    • mcsema/test.proposed.ll: The LLVM IR, corresponding to function Doit, generated using Compositional Lifter.
    • mcsema/test.proposed.inlined.ll: Inlined version of mcsema/test.proposed.ll
    • Makefile: With the following relevant targets
      • compd: Invoke Compositional Lifter to generate mcsema/test.proposed.inline.ll
      • compd_opt: Invoke Normalizer to normalize mcsema/test.proposed.inline.ll using the LLVM opt pass list. Generates mcsema/test.proposed.opt.ll.
      • mcsema_opt: Invoke Normalizer to normalize ../binary/test.mcsema.inline.ll using the LLVM opt pass list. Generates mcsema/test.mcsema.opt.ll
      • match: Invoke Matcher to check for graph-isomorphism on the data-dependence graphs of the above normalized versions mcsema/test.proposed.opt.ll & mcsema/test.mcsema.opt.ll.

Note

  1. We have pre-populated the McSema lifted LLVM IR <program name>/binary/test.mcsema.ll because McSema needs a licensed disassembler IDA to generate this file, which is not provided because of some licensing issues. Hence, the Make target mcsema will not work.
  2. The Store has the validated IR sequences of individual binary instructions, generated using McSema. For similar reasons as above, we have packaged the entire Store in the VM, so that the reviewer (or user) do not have to invoke McSema.

Running the PLV pipeline

An Example Run

Here we will elaborate the process of running PLV on an isolated example function Queens/Doit/. We use shell variable NORM to specify which set of optimization passes to use for normalization. For example, the value CUSTOM enables using a set of 17 LLVM opt passes for normalization. As as aside, there is an option for NORM which enables AutoTuner for pass selection.

Running PLV on it involves the following steps

export NORM=CUSTOM
cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit/

## The step below (i.e., make mcsema) is responsible for running Mcsema to lift the binary "../binary/test"
## and creating  "../binary/test.mcsema.inline.ll" (already populated)
# make mcsema // Skip  this step as mentioned above

## Running Compositional lifter on the corresponding binary ../binary/test.reloc
## Creates mcsema/test.proposed.inline.ll
make compd

## Normalizing mcsema/test.proposed.inline.ll
## Creates mcsema/test.proposed.opt.ll
make compd_opt # expect "Compd Pass" upon execution

## Normalizing ../binary/test.mcsema.inline.ll (Already populated using make mcsema)
## Creates mcsema/test.mcsema.opt.ll
make mcsema_opt

## Matching mcsema/test.proposed.opt.ll & mcsema/test.mcsema.opt.ll
make match # expect "Match Pass" upon execution

Note The Make target match already includes the normalization actions corresponding to compd_opt & mcsema_opt, hence one can skip invoking such redundant targets. The reason we still have those targets are for legacy reasons and included in this presentation for better explanation of the steps. For example, instead of the above steps one can do

export NORM=CUSTOM
cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Doit/
make compd
make match

Batch run (Recommended)

To demonstrate a batch run, we have included a list of sample functions in a list ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/docs/AE_docs/samplePassList.txt. The list includes function himenobmtxpa/jacobi, the biggest function we tried lifting before submission, wherein the size of the extracted LLVM IR, using the Compositional Lifter, is 32105 LOC. Note that, the runtime of the matcher on himenobmtxpa/jacobi might take up-to 2 mins.

Running PLV in batch mode, over a sample list of functions, involves the following steps

cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/
../../scripts/run_batch_plv.sh docs/AE_docs/samplePassList.txt

# cat ../../scripts/run_batch_plv.sh
## LIST=$1
## 
## 
## TESTARENA=" ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/"
## 
## echo "Batch Run Begin"
## echo
## 
## echo "Setting NORMALIZATION to custom passes"
## export NORM=CUSTOM
## 
## echo
## echo "Running Compositional Lifter + Normalizer + Matcher"
## cat $LIST | parallel "echo ; echo {}; echo =======; cd {}; make  compd; make match; cd - "
## 
## echo "Batch Run Begin"

Note We have provided the list of 2189 passing cases ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/docs/AE_docs/matcherPassList.txt, which can be run using either batch mode or individually.

The reviewer (or user) is encouraged to pick a random count of entries form the file and invoke the batch run

export NORM=CUSTOM
cd ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/
sort -R docs/AE_docs/matcherPassList.txt | head -n 10 > /dev/stdout | ../../scripts/run_batch_plv.sh   /dev/stdin |& tee ~/Junk/log

PLV injected bug detection

We provided a list of test-cases, to demonstrate the effectiveness of PLV to catch potential bugs. The test-cases represent the four different category of injections as mentioned in paper (Section 6: Evaluation -> Program-level validation: Effectiveness at finding bugs) and generated by modifying the McSema-lifted-IR function Queens/Rand.

An example injection can be found at ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Inject.1/binary/test.mcsema.ll, where the bug is marked with comment tag INJECTED_BUG, along with the original line of code marked by tag ORIG. This bug is related to choosing a wrong instruction template. Running the PLV pipeline on this (and other injected bugs) will result in matcher bug, as follows:

cd  /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark
../../scripts/run_batch_injected_bug_plv.sh docs/AE_docs/sampleInjectedBugs.txt |& tee ~/Junk/log

However, commenting-in the line below the INJECTED_BUG tag, and un-commenting the line below ORIG tag, will make the matcher pass in all the cases.

For example, follow the steps above steps to rectify ~/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Inject.1/binary/test.mcsema.ll and invoke:

cd  /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark
echo Inject.1/Rand/ > /dev/stdout | ../../scripts/run_batch_injected_bug_plv.sh  /dev/stdin

Note The script ../../scripts/run_batch_injected_bug_plv.sh differs from ../../scripts/run_batch_injected_bug_plv.sh in having an extra Make target mcsema. From the above discussion, this target is used to (1) Invoke IDA + McSema to generate binary/test.mcsema.ll, and (2) Sanitize the McSema generate file to binary/test.mcsema.inline.ll. For reasons about IDA licensing, we want to skip (1). However, (2) is required, as we want to sanitize the modified McSema lifted file binary/test.mcsema.ll. To accomplish this partial execution of Make target, we modified the corresponding Makefile;

Single-Instruction validation (SIV)

Source Code

An important component of the SIV is the tool spec-to-smt, which converts the symbolic summary (specified in K-AST) to SMTLIB queries, with the core functionality defined in library.

Testing Arena for SIV

For each binary instruction, for example the addq_r64_r64, has the following structure:

  • test.c: C file with an instruction instance of addq %rcx, %rbx wrapped in inline assembly.
  • test: Binary compiled from above C file.
  • test.ll: McSema lifted LLVM IR from binary test.
  • test.mod.ll: McSema lifts a lot af glue code other than the instructions in test. This is de-cluttered version of test.ll focusing on just the IR relevant to the lifting of addq instruction.
  • test-xspec.k: The K-specification file necessary to run a symbolic execution engine krpove (a tool that K-framework generates automatically from the semantics of X86-64 ISA) on binary instruction and to generate symbolic summary.
  • Output/test-xspec.out: Output of the above symbolic execution.
  • test-lspec.k: The K-specification file necessary to run another symbolic execution engine kprove (a tool that K-framework generates automatically from the semantics of LLVM IR) on test.mod.ll and to generate symbolic summary.
  • Output/test-lspec.out: Output of the above symbolic execution.
  • Output/test-z3.py: The python file encoding the verification queries in SMTLIB format. The queries are created using the tool spec-to-smt by processing the symbolic summaries in Output/test-xspec.out and Output/test-lspec.out.
  • Makefile: With the following relevant targets
    • binary: Generate test from test.c.
    • Target related to generating symbolic summary for binary instruction
      • collect: Prepare for building the symbolic execution engine using the semantics definitions of the binary instructions included in test.
      • kompile: Generate the symbolic execution engine for binary instructions included in test.
      • genxspec: Generate the test-xspec.k file.
      • xprove: Invoke symbolic execution using the spec file test-xspec.k. Generates Output/test-xspec.out containing the symbolic summary for the binary instruction addq.
    • Target related to generating symbolic summary for lifted LLVM IR
      • mcsema: Lift test to test.ll using McSema. (To Be Skipped because of IDA licensing)
      • declutter: Sanitize test.ll to test.mod.ll.
      • kli: Run concrete execution of test.mod.ll using the LLVM semantics. Output is stored in Output/test-lstate.out
      • genlspec: Generate the test-lspec.k file using many details from the concrete execution log Output/test-lstate.out.
      • lprove: Invoke symbolic execution using the spec file test-lspec.k. Generates Output/test-lspec.out containing the symbolic summary for the lifted LLVM IR test.mod.ll.
    • Target related to generating and proving verification conditions
      • genz3: Invoke spec-to-smt to create Output/test-z3.py containing verification queries.
      • provez3: Invoke z3 on Output/test-z3.py.

Note

  1. Similar files are available for other instructions and their variants (memory/immediate/register).
  2. We have pre-populated the McSema lifted LLVM IR <instruction opcode>/test.ll because McSema is not included in the VM distribution.

Running the SIV pipeline

An example run

Running SIV on an isolated example instruction addq_r64_r64 involves the following step

cd ~/Github/validating-binary-decompilation/tests/single_instruction_translation_validation/mcsema/
echo register-variants/addq_r64_r64 > /tmp/sample.txt
# Or try one of the following
#     echo immediate-variants/addq_r64_imm32 > /tmp/sample.txt        # Try an immediate variant
#     echo memory-variants/addq_r64_m64 > /tmp/sample.txt             # Try a memory variant
#     sort -R docs/AE_docs/non-bugs.txt | head -n 1 > /tmp/sample.txt # Try any random instruction from a list
                                                                      # of passing cases
../../scripts/run_batch_siv.sh /tmp/sample.txt |& tee ~/Junk/log

# cat ../../scripts/run_batch_siv.sh
## LIST=$1
##
## ## Number of jobs to issue in parallel
## P=$2
## 
## if [ -z "$P" ]; then
##   P=1
## fi
## 
## TESTARENA="~/Github/validating-binary-decompilation/tests/single_instruction_translation_validation/mcsema/" 
## 
## echo
## echo "Cleaning Stale instr semantics definitions"
## cd ~/Github/X86-64-semantics/semantics
## rm -rf underTestInstructions/*
## cd -
## 
## echo
## echo "Collecting instructions from binary"
## cat $LIST | parallel "cd $TESTARENA/{}; make collect; cd -"
## 
## echo
## echo "Compiling the collected X86 semantics to create a sym-ex"
## cd ~/Github/X86-64-semantics/semantics
## ../scripts/kompile.pl --backend java
## cd -
## 
## echo
## echo "Batch Run Begin using $P jobs in parallel"
## 
## cat $LIST | parallel -j $P "echo ; echo {}; echo ======; cd ${TESTARENA}/{}; \
##       echo; echo \"Generating symbolic summary for binary instruction\" \
##       make genxspec; make xprove; \
##       echo; echo \"Generating symbolic summary for lifted LLVM IR\" \
##       make declutter; make kli; make genlspec; make lprove; \
##       echo; echo \"Generating verification conditions\" \
##       make genz3; \
##       echo; echo \"Prove verification conditions\" \
##       make provez3; \
##       cd -"
## 
## echo "Batch Run End"

Note

  1. The Make target xprove will exit with non-zero status and it is a known issue. These errors are because of missing KAST (internal K AST) to SMT translations in the sym-ex engine backend. We do not need the translations as we have a separate tool spec-to-smt to achieve the translation from the summary that the target xprove produces. However, please note that all these error messages can be safely ignored as it does NOT affect the soundness of the summary generation process.

  2. The entire execution will take up-to 4-5 mins.

Batch runs

The goal of the entire SIV pipeline (as mentioned above) is to reproduce the file verification-query file Output/test-z3.py from scratch. Here is an example test-z3.py file encoding the verification conditions, which is then dispatched (using Make target provez3) to z3 for verification.

The python file has the following format:

 s = z3.solver()
 For each E in {registers, flags, memory values}: s.push()
  s.push()
  lvar = symbolic summary corresponding to E, obtained by sym-execution of the LLVM IR (generated by McSema by lifting the binary instruction)
  xvar = symbolic summary corresponding to E, obtained by sym-execution the binary instruction.

  s.add(lvar != xvar)

  solve s for unsat/sat/timeout

  s.pop()

Upon dispatch of the verification queries, for each register/flag/memory values, the output Test-Pass is generated if all the queries results in unsat. Conversely, the output says Test-Fail if any of query results in sat or timeout.

In order to run this prover step on a random sample of non-buggy instructions, do the following:

cd ~/Github/validating-binary-decompilation/tests/single_instruction_translation_validation/mcsema/
sort -R docs/AE_docs/non-bugs.txt | head -n 50 | parallel "echo ; echo {}; echo ===; cd {}; make provez3; cd -" |& tee ~/Junk/log

Reproducing bugs

We provide the a list of bugs. In order to demonstrate the result of Z3-comparison on the corresponding verification queries, do the following:

cd ~/Github/validating-binary-decompilation/tests/single_instruction_translation_validation/mcsema/
cat docs/AE_docs/bugs.txt | parallel "echo ; echo {}; echo ===; cd {}; make provez3; cd -" |& tee ~/Junk/log

The above command will show, for each buggy instruction, the register that has a mismatching symbolic summary. We know, from our past experience, that having both the operands equal makes the semantics of some instructions pretty challenging. We leverage that experience by running SIV on instructions with its operands being the same registers and found couple of bugs mentioned in the above list as register-variants-samereg/*. Bug Reports to McSema can be found here, which are all acknowledged.

Reproducing timeouts

The timeouts (related to mulq) include solver constraints containing bit vector multiplication which the state-of-the-art SMT solvers are not very efficient at reasoning about. These cases timed-out (after 24h) reproducibly.

cd ~/Github/validating-binary-decompilation/tests/single_instruction_translation_validation/mcsema/register-variants/mulq_r64
make provez3 ## Timeout provides is 24 hrs

AutoTuner based normalization

In order to prove that two functions F & F ′ are semantically equivalent, they need to be reduced to isomorphic graphs via normalization. For normalization, we initially used a custom sequence of 17 LLVM optimization passes, discovered manually by pruning the LLVM -O3 search space. Later experimentation on normalizer revealed that (1) changing the order of passes improves the number of functions reducing to isomorphic graphs (the pass ordering problem) and thus reducing false negatives, and (2) not all of the 17 passes are needed for every pair of functions under equivalence check. Such observations intrigues us to frame the problem of selecting the best normalizing pass sequence as an application of program autotuning. We used the OpenTuner framework for the purpose. The OpenTuner framework requires the user to specify a space to search for, which in our case includes the passes from opt -O3 sequence. The framework then uses various machine learning techniques to find the best configuration which can minimize an objective function within a given resource budget.

In order to make the auto-tuner work, following is the requirement (which is already installed in VM)

sudo pip install opentuner # password: aecadmin123

An example auto-tuning run

Here we will show how the auto-tuner can help discover a pass sequence effective for normalization and subsequent matching.

cd /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/Queens/Queens
make tuner # Invoke the Opentuner and generate a file mcsema/normalizer_final_config.json
           # containing all the candidate pass sequences which meet the  objective function.
           # runtime: ~1 min.

unset NORM # Select the candidate pass sequences from  mcsema/normalizer_final_config.json file
make match # Invoke the matcher on each of the above candidate pass sequences till the matcher succeeds (declare Pass) or
           # all the candidate pass sequences are exhausted (declare Fail).

The file /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/docs/reported_stats/7.log shows the 65 false-negatives for which the matcher failed because of the the pass ordering problem. For all the functions in the above list, the matcher will Fail, when normalized using the fixed-length (17) pass sequence.

cd /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/
export NORM=CUSTOM
sort -R docs/reported_stats/7.log | head -n 5 > /dev/stdout |  cat /dev/stdin  | parallel "cd {}; make match ; cd -" |& tee ~/Junk/log

However, if the pass sequence is selected by the auto-tuner, the matcher will succeed in all the cases.

cd /home/sdasgup3/Github/validating-binary-decompilation/tests/program_translation_validation/single-source-benchmark/
unset NORM
sort -R docs/reported_stats/7.log | head -n 5 > /dev/stdout |  cat /dev/stdin  | parallel "cd {}; make match ; cd -" |& tee ~/Junk/log

Note that, for each function, the auto-tuner generated pass sequence is pre-populated at normalizer_final_config.json.

About

Artifact Evaluation, PLDI'20

llvm-irmcsemasymbolic-execution-enginesymbolic-summariesverification-queriesvirtualboxx86-64language-semanticsreverse-engineeringtranslation-validationvalidationcompiler-optimizationgraph-matchingformal-semanticsverification-conditionsdetecting-bugspldievaluationreproducing-bugs