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{ Category Archives } Computer Science

API Fuzzing vs. File Fuzzing: A Cautionary Tale

Libraries that provide APIs should be rock solid, and so should file parsers. Although we can use fuzzing to ensure the solidity of both kinds of software, there are some big differences in how we do that. A file parser should be fully robust: it isn’t allowed to crash even if presented with a corrupted […]

Comments on a Formal Verification of PolarSSL

The C language has given the world many enduring gifts such as buffer overflows, uninitialized variables, and use-after-free errors. Since rewriting a code base in some other language is not easy, we’re often stuck trying to eliminate bugs in legacy C before they bite us, and of course bugs in network-facing code sometimes bite us […]

Secret Coders

Although I’m not sure that I’ve mentioned it here before, I’m a pretty big comic book nerd. So I was psyched when, late last year, Gene Luen Yang mailed me asking if I’d like a review copy of his upcoming graphic novel. I love Gene’s Avatar comics, which I had been reading with my kids, […]

A Few Synthesizing Superoptimizer Results

For this post, I crippled Souper by disabling its path conditions and limiting the depth of harvested expressions to two LLVM instructions. The first goal was to create a nice easy burn-in test for Souper’s instruction synthesizer, which uses a variant of this method; the second goal was to see if depth-limited, path-condition-free expressions would […]

Nibble Sort Denouement

Back in January my nibble sort contest resulted in entries that dramatically exceeded my expectations. Since then I’ve been trying to write up a post explaining the various strategies that people used and since you don’t care about my excuses I won’t tell you them, but I never got it written. However! I want to […]

Defending Against Compiler-Based Backdoors

Scotty Bauer (a Utah grad student), Pascal Cuoq, and I have an article in the latest PoC||GTFO about introducing a backdoor into sudo using a compiler bug. In other words, the C code implementing sudo does not contain a backdoor, but a backdoor appears when sudo is built using a particular compiler version (clang 3.3, […]

Generating a Random Program vs. Generating All Programs

Generating all possible inputs — up to some maximum length — to a software system is one way of creating test cases, and this technique even has a name: bounded exhaustive testing. Back when we were doing Csmith, my then-student Yang Chen spent a while on a bounded exhaustive C program generator which was in […]

What afl-fuzz Is Bad At

American fuzzy lop is a polished and effective fuzzing tool. It has found tons of bugs and there are any number of blog posts talking about that. Here we’re going to take a quick look at what it isn’t good at. For example, here’s a program that’s trivial to crash by hand, that afl-fuzz isn’t […]

UB Canaries

If you report an undefined behavior bug, a common reaction from software developers is “So what? Our code works just fine.” As a random example, here is a discussion I had with Rasmus Lerdorf about five years ago about some UBs in the PHP interpreter. One might point out that it wasn’t a very mature […]

Inexpensive CPU Monster

Rather than using the commercial cloud, my group tends to run day-to-day jobs on a tiny cluster of machines in my office and then to use Emulab when a serious amount of compute power is required. Recently I upgraded some nodes and thought I’d share the specs for the new machines on the off chance […]