⚠️ 🧪 This post is work-in-progress. Changes are expected.

Enable AddressSanitizer in Rust *-sys crate


For our tlspuffin fuzzer we use Rust to implement the testing harness. The harness is statically linked to OpenSSL via the openssl-sys and openssl-src crates, where the latter just provides a Rust Build Script. openssl-sys runs the script, then looks at the build artifacts and statically links against them.

This is a usual setup when working with C libraries in Rust. How do we enable the wildly known AddressSanitizer?

1. Use Clang

The first step is to make the build script for OpenSSL use Clang. While GCC also supports sanitizers, Clang has a greater selection of available sanitizers. For example the SanitizerCoverage sanitizer is only available in Clang.

This can be done by setting the CC environment variable:

1let mut configure = ...;
3configure.env("CC", "clang");

Note: configure is an instance of std::process::Command which runs the ./Configure script with various arguments.

2. Usage of -fsanitize=address

The next step is to append -fsanitize=address flag to the invocation of the compiler. For OpenSSL we can do this by adding the enable-asan option.

1if cfg!(feature = "asan") {
2    configure.arg("enable-asan");

An other way of doing this is by appending the sanitize option directly to the CC variable.

1let mut cc = "clang".to_owned();
3if cfg!(feature = "asan") {
4    cc.push_str(" -fsanitize=address");
7configure.env("CC", cc);

The AddressSanitizer requires a runtime. The previous step instruments the library and adds calls into this runtime. This means the linker is responsible for adding this runtime. Therefore, -fsanitize=address is not only accepted by the compiler but also the linker. While it is possible pass -lasan to the linker, it is not recommended. The AddressSanitizer, which is also known as libasan must be the first library in the initial library list. That means it must have the possibility to overwrite function calls in other dynamically loaded libraries. The reason for this is that it needs to have control over calls to malloc or free to detect memory issues like null pointer dereferences.

-static-libsan vs -shared-libsan

There are two ways to instrument the library and link libasan. The two options -static-libsan and -shared-libsan are mentioned in the command line reference.

Clang uses -static-libsan as default whereas GCC uses -shared-libsan as default. The advantages and disadvantage of either are documented here.

In our example there is actually only one possibility. We want to instrument the library, but do not want to instrument the executable which calls into the static library. If we want to sanitize the whole Rust executable, including OpenSSL we can simply use the unstable Rust feature Sanitizers Support to link against the libasan runtime.

If we use -static-libsan the compiled static library of OpenSSL will have many undefined references the functions of the runtime. These need to be resolved during the final linking which rustc does. Same goes for -shared-libsan in our case. There will be undefined references.

Therefore, for our usecase there is no practical difference beween these two options, as we have to use the linker provided by rustc.

Methods for Linking libasan

There are two ways. Either we tell rustc directly to link to libasan

1RUSTFLAGS="-C link-arg=-lasan"

The other way it to do this using the directive cargo:rustc-link-lib in the build script of OpenSSL:

1if cfg!(feature = "asan") {
2    configure.arg("enable-asan");
3    println!("cargo:rustc-link-lib=asan");

An unanswered question is how to control in which order rustc links against shared libraries. If libasan is not the first in the list then the executable will abort with:

1==465915==ASan runtime does not come first in initial library list; you should either link runtime to your application or manually preload it with LD_PRELOAD.

If using the RUSTFLAGS, then the linker produces the following:

 1ldd target/debug/bin
 2    linux-vdso.so.1 (0x00007ffec712f000)
 3    libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f609c6f2000)
 4    librt.so.1 => /usr/lib/librt.so.1 (0x00007f609c6e7000)
 5    libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f609c6c6000)
 6    libm.so.6 => /usr/lib/libm.so.6 (0x00007f609c582000)
 7    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f609c57b000)
 8    libc.so.6 => /usr/lib/libc.so.6 (0x00007f609c3af000)
 9    /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f609f992000)
10    libasan.so.6 => /usr/lib/libasan.so.6 (0x00007f609b9ce000)
11    libstdc++.so.6 => /usr/lib/../lib/libstdc++.so.6 (0x00007f609b7b8000)

Note, that the libasan is at the very bottom. When linking using the build script, rustc did link correctly:

 1ldd target/debug/bin
 2    linux-vdso.so.1 (0x00007fffe93e5000)
 3    libasan.so.6 => /usr/lib/libasan.so.6 (0x00007f633b05d000)
 4    libgcc_s.so.1 => /usr/lib/libgcc_s.so.1 (0x00007f633b042000)
 5    librt.so.1 => /usr/lib/librt.so.1 (0x00007f633b037000)
 6    libpthread.so.0 => /usr/lib/libpthread.so.0 (0x00007f633b016000)
 7    libm.so.6 => /usr/lib/libm.so.6 (0x00007f633aed2000)
 8    libdl.so.2 => /usr/lib/libdl.so.2 (0x00007f633aecb000)
 9    libc.so.6 => /usr/lib/libc.so.6 (0x00007f633acfd000)
10    libstdc++.so.6 => /usr/lib/../lib/libstdc++.so.6 (0x00007f633aae7000)
11    /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f633e484000)

Note, that the libasan is at the very top.

One way to control it is to use LD_PRELOAD:

1LD_PRELOAD=$(clang -print-file-name=libasan.so) target/debug/bin

Hint: Cargo will crash if executed with libasan as preloaded library. Therefore, this is not ideal.

Source Code

If you want to take a look at my experimental changes, you can visit my fork on Github.

Failed Attempts

I’m not sure why the following attempt failed. But I’m quite certain that this is intended.

  • Statically linking against libclang_rt.asan-x86_64.a. This can not work because we are instrumenting only a library and not the executable. The linker crashes with: TLS transition from R_X86_64_TLSLD to R_X86_64_TPOFF32 against _ZN6__asanL14fake_stack_tlsE' at 0x667 in section .text' failed.

More References:

Do you have questions? Send an email to max@maxammann.org