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35 lines
1.3 KiB
Markdown
35 lines
1.3 KiB
Markdown
# InsTrim
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InsTrim: Lightweight Instrumentation for Coverage-guided Fuzzing
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## Introduction
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InsTrim uses CFG and markers to instrument just what is necessary in the
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binary in llvm_mode. It is about 10-15% faster without disadvantages.
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It requires at least llvm version 3.8.0.
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## Usage
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Set the environment variable `AFL_LLVM_INSTRUMENT=CFG` or `AFL_LLVM_INSTRIM=1`
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during compilation of the target.
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There is also an advanced mode which instruments loops in a way so that
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afl-fuzz can see which loop path has been selected but not being able to
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see how often the loop has been rerun.
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This again is a tradeoff for speed for less path information.
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To enable this mode set `AFL_LLVM_INSTRIM_LOOPHEAD=1`.
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There is an additional optimization option that skips single block
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functions. In 95% of the C targets and (guess) 50% of the C++ targets
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it is good to enable this, as otherwise pointless instrumentation occurs.
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The corner case where we want this instrumentation is when vtable/call table
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is used and the index to that vtable/call table is not set in specific
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basic blocks.
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To enable skipping these (most of the time) unnecessary instrumentations set
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`AFL_LLVM_INSTRIM_SKIPSINGLEBLOCK=1`
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## Background
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The paper: [InsTrim: Lightweight Instrumentation for Coverage-guided Fuzzing]
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(https://www.ndss-symposium.org/wp-content/uploads/2018/07/bar2018_14_Hsu_paper.pdf)
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