How to compile a static musl binary of a Rust project with native dependencies?

Question

I have a project with dependencies on Hyper and Diesel, and because of that, on native libraries OpenSSL and libpq. The project builds on nightly Rust because it uses compiler plugins.

My current attempt is to build on a Docker container. I have the MUSL libc and the libraries make'd and installed with prefix /usr/local/musl. I run cargo with the following command: (Not sure if some of the options are redundant, I'm not too well-versed with the compiler chain, and not even sure if they end up to the linker, but I have to try, right.)

LDFLAGS="-static -L/usr/local/musl/lib" \
LD_LIBRARY_PATH=/usr/local/musl/lib:$LD_LIBRARY_PATH \
CFLAGS="-I/usr/local/musl/include" \
PKG_CONFIG_PATH=/usr/local/musl/lib/pkgconfig \
cargo build --release --target=x86_64-unknown-linux-musl

When I ldd the resulting file, it reveals this:

$ ldd server
linux-vdso.so.1 (0x00007fffb878e000)
libpq.so.5 => /usr/local/musl/lib/libpq.so.5 (0x00007f4d730e7000)
libssl.so.1.0.0 => /usr/lib/x86_64-linux-gnu/libssl.so.1.0.0 (0x00007f4d72e82000)
libcrypto.so.1.0.0 => /usr/lib/x86_64-linux-gnu/libcrypto.so.1.0.0 (0x00007f4d72a85000)
libc.so => /usr/local/musl/lib/libc.so (0x00007f4d727f6000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f4d725f2000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f4d72246000)
/lib/ld64.so.1 => /lib64/ld-linux-x86-64.so.2 (0x000055e2124a2000)

There's all that dynamically linked stuff, and some even with the "x86_64-linux-gnu" chain! What went wrong?

I can make statically linked, simple pure-Rust projects without problems. ldd says that they are statically linked, and they run without problems, unlike the executable I have problems with.

When I used --verbose with Cargo, I got the following rustc command that actually builds the executable: http://pastebin.com/ywv0zNBK (Oops, that one had a custom outdir and -Z print-link-args, added by me) Adding the print-link-args flag, I got the following linker command: http://pastebin.com/Aw43qd7h

How do I get cargo or rustc to believe that I want a static binary?

Answer 1

The problem was that for each crate providing a native dependency – say OpenSSL – there is the build.rs build script that is in charge of communicating the build and linking options to Cargo and to rustc. (For example: they print out something like cargo:rustc-link-lib=static=ssl which Cargo then reads and acts accordingly.)

So just setting the "standard" GCC environmental variables is hardly going to have any effect. You must check each and every build.rs separately to know how to coerce that exact crate to convey cargo its options. For OpenSSL, its env vars like OPENSSL_DIR, OPENSSL_STATIC etc.

Another hurdle is that if you use compiler plugins, they might be compiled with the target triplet too (at least docker_codegen). On the other hand, they are linked dynamically during the compiling process. This mean that not only must static libraries be linked correctly, you must also have dynamic libraries of the target host variety, like Musl libc.so in place, and correctly set (LD_LIBRARY_PATH etc.).

I made a thoroughly commented Dockerfile that builds my project statically with some native dependencies. It might be of help for others too.

https://gitlab.com/rust_musl_docker/image