Error "cannot find -lunwind" when compiling static `mips-unknown-linux-musl`

[wcampbell0x2a]

I am using https://github.com/richfelker/musl-cross-make to generate a mips-linux-musl compiler to build mips-unknown-linux-musl, as it's tier 3 and needs -Zbuild-std.

First off, I needed to add a couple of -L to the config, in order to not have Could not find crt0 problems. I don't know if this is a rustc bug, or a cross-compiler bug.

However, the bug that I can't figure out, is that for static binaries(and/or panic=abort) it still needs the -lunwind library. Nevertheless, It seems like this should be something it finds from the ~/.rustup/toolchains/, but nothing I try helps rustc find this library?

I tried this code:

src/main.rs:

fn main() {
    println!("test");
}

Cargo.toml:

[profile.release]
strip = true
codegen-units = 1
lto = true
panic = "abort"
opt-level = "z"

.cargo/config:

[build]
rustflags = [
	"-C", "target-feature=+crt-static",
	"-L", "/home/wcampbell/projects/wcampbell/musl-cross-make/output/mips-linux-musl/lib",
	"-L", "/home/wcampbell/projects/wcampbell/musl-cross-make/output/lib/gcc/mips-linux-musl/9.4.0/",
]

[target.mips-unknown-linux-musl]
linker = "/home/wcampbell/projects/wcampbell/musl-cross-make/output/bin/mips-linux-musl-gcc"

[unstable]
build-std = ["core", "compiler_builtins", "alloc", "std", "panic_abort"]

I expected to see this happen: Statically compiled mips-musl binary with -Zbuild-std

Instead, this happened:

$ cargo build --bin mempeek --locked --release --target mips-unknown-linux-musl
error: linking with `/home/wcampbell/projects/wcampbell/musl-cross-make/output/bin/mips-linux-musl-gcc` failed: exit status: 1
  |
  = note: LC_ALL="C" PATH="/home/wcampbell/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin:/home/wcampbell/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/bin/self-contained:/home/wcampbell/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl" VSLANG="1033" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/bin/mips-linux-musl-gcc" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/mips-linux-musl/lib/crt1.o" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/mips-linux-musl/lib/crti.o" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/lib/gcc/mips-linux-musl/9.4.0/crtbegin.o" "/tmp/rustc2DONcX/symbols.o" "/home/wcampbell/projects/wcampbell/mempeek/target/mips-unknown-linux-musl/release/deps/mempeek-840512eed390a4d1.mempeek.eea5b45178c2269e-cgu.0.rcgu.o" "-Wl,--as-needed" "-L" "/home/wcampbell/projects/wcampbell/mempeek/target/mips-unknown-linux-musl/release/deps" "-L" "/home/wcampbell/projects/wcampbell/mempeek/target/release/deps" "-L" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/mips-linux-musl/lib" "-L" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/lib/gcc/mips-linux-musl/9.4.0/" "-L" "/home/wcampbell/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/mips-unknown-linux-musl/lib" "-Wl,-Bstatic" "-lunwind" "-lc" "/home/wcampbell/projects/wcampbell/mempeek/target/mips-unknown-linux-musl/release/deps/libcompiler_builtins-2264a038ec2e250c.rlib" "-Wl,-Bdynamic" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-nostartfiles" "-L" "/home/wcampbell/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/mips-unknown-linux-musl/lib" "-L" "/home/wcampbell/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/mips-unknown-linux-musl/lib/self-contained" "-o" "/home/wcampbell/projects/wcampbell/mempeek/target/mips-unknown-linux-musl/release/deps/mempeek-840512eed390a4d1" "-Wl,--gc-sections" "-static" "-no-pie" "-Wl,-z,relro,-z,now" "-Wl,--strip-all" "-nodefaultlibs" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/lib/gcc/mips-linux-musl/9.4.0/crtend.o" "/home/wcampbell/projects/wcampbell/musl-cross-make/output/mips-linux-musl/lib/crtn.o"
  = note: /home/wcampbell/projects/wcampbell/musl-cross-make/output/bin/../lib/gcc/mips-linux-musl/9.4.0/../../../../mips-linux-musl/bin/ld: cannot find -lunwind
          collect2: error: ld returned 1 exit status

Meta

rustc --version --verbose:

rustc --version --verbose
rustc 1.77.0-nightly (5bd5d214e 2024-01-25)
binary: rustc
commit-hash: 5bd5d214effd494f4bafb29b3a7a2f6c2070ca5c
commit-date: 2024-01-25
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

[wcampbell0x2a]

@rustbot label -O-MIPS -O-musl -Zbuild-std

[rustbot]

Error: Label Zbuild-std can only be set by Rust team members

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[wcampbell0x2a]

@rustbot label +O-MIPS +O-musl

[saethlin]

libunwind is still required for aborting panics, because those still print backtraces.

The std feature panic_immediate_abort exists to disable all panic formatting and backtrace printing so that all the code that's used for it can be optimized away. It may or may not work here; it has no tests and I know it has users but I kind of maintain it on the side. I tried writing a run-make test for it and couldn't figure out how run-make tests work.

So I suppose there's a question of what you want to achieve here: Do you want a working panic=abort build, or do you want a build that doesn't depend on libunwind?

[wcampbell0x2a]

I guess I'm less concerned about panic_immediate_abort and panic=abort (although I use these for embedded projects).

I'm more concerned (if that's the right word) about how to resolve the library path errors when cross-compiling and what rustc expects of me when using -Zbuild-std.

I'm not sure about "do you want a build that doesn't depend on libunwind", although I guess with panic=abort the linker would remove the link invocation, as the symbol wouldnt be required (ideally).

[aroberts87]

This issue is timely for me because I'm doing the exact workflow as described above and getting the same results. I am also using https://github.com/richfelker/musl-cross-make to build a musl compiler, with the major difference for me that I'm targeting a different tier 3 platform, s390x-unknown-linux-musl.

The project I'm trying to build is sccache.

.cargo/config.toml:

[target.s390x-unknown-linux-musl]
rustflags = [
	"-C", "target-feature=+crt-static",
	"-C", "panic=abort",
	"-L/opt/s390x-linux-musl/s390x-linux-musl/lib",
	"-L/opt/s390x-linux-musl/lib/gcc/s390x-linux-musl/9.4.0/"
]
linker = "s390x-linux-musl-gcc"
ar = "s390x-linux-musl-gcc-ar"

Cargo.toml:

[profile.release]
codegen-units = 1
lto = true
strip = true
panic = "abort"

Build Command:

export PATH="/opt/s390x-linux-musl/bin/:$PATH"
cargo build -Zbuild-std=std,panic_abort -Zbuild-std-features=panic_immediate_abort --locked --bin sccache --features=openssl/vendored

Observed Error:

  = note: /opt/s390x-linux-musl/bin/../lib/gcc/s390x-linux-musl/9.4.0/../../../../s390x-linux-musl/bin/ld: cannot find -lunwind
          collect2: error: ld returned 1 exit status

rustc --version --verbose:

rustc --version --verbose 
rustc 1.77.0-nightly (595bc6f00 2024-01-05)
binary: rustc
commit-hash: 595bc6f00369475047538fdae1ff8cea692ac385
commit-date: 2024-01-05
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

I'm pretty new to the Rust ecosystem and most of my configuration in .cargo/config.toml is based on solutions to other posts that I found while attempting to find a solution to this problem, but when I found this issue and realized it was filed today I wanted to chime in as a second data point in case this is more than the user error that I thought I was having.

[wcampbell0x2a]

@aroberts87 fyi, ar is deprecated: https://doc.rust-lang.org/cargo/reference/config.html#targettriplear

[wcampbell0x2a]

Another question, for build-std and this issue, is libunwind expected to be in my tool chain, or is this something that rustc doesn't have packaged because it's a tier 3?

I guess with LTO and panic=abort the functions would be removed anyway( should have no call sites), so could it just be an "empty functions providing symbols" library?