Issues linking __rust_probestack when cross compiling for UEFI on 0.1.23

[ALSchwalm]

Cross compiling a simple example crate (containing a function with sufficiently large stack allocation) with compiler_builtins v0.1.23 produces the following error:

error: linking with `rust-lld` failed: exit code: 1
  |
  = note: "rust-lld" "rust-lld" "-flavor" "link" "/NOLOGO" "/NXCOMPAT" "/nodefaultlib" "/entry:efi_main" "/subsystem:efi_application" "/LIBPATH:/home/adam/test/uefi-example/target/sysroot/lib/rustlib/x86_64-unknown-uefi/lib" "/home/adam/test/uefi-example/target/x86_64-unknown-uefi/debug/deps/uefi_example-33d6ce04242a6b68.1j5mfhubop672qu0.rcgu.o" "/OUT:/home/adam/test/uefi-example/target/x86_64-unknown-uefi/debug/deps/uefi_example-33d6ce04242a6b68.efi" "/OPT:REF,NOICF" "/DEBUG" "/LIBPATH:/home/adam/test/uefi-example/target/x86_64-unknown-uefi/debug/deps" "/LIBPATH:/home/adam/test/uefi-example/target/debug/deps" "/LIBPATH:/home/adam/test/uefi-example/target/sysroot/lib/rustlib/x86_64-unknown-uefi/lib" "/home/adam/test/uefi-example/target/sysroot/lib/rustlib/x86_64-unknown-uefi/lib/libcore-9d5e0b6ad79711be.rlib" "/home/adam/test/uefi-example/target/x86_64-unknown-uefi/debug/deps/libcompiler_builtins-e6a828d72c47e9cb.rlib"
  = note: rust-lld: error: undefined symbol: __rust_probestack
          >>> referenced by /home/adam/test/uefi-example/target/x86_64-unknown-uefi/debug/deps/uefi_example-33d6ce04242a6b68.1j5mfhubop672qu0.rcgu.o:(efi_main)

Produced using this crate with cargo +nightly xbuild --target x86_64-unknown-uefi on the current nightly.

Reverting to 0.1.21 resolves the issue (0.1.22 produces the expected issue with invalid assembly). Looking at the object files it seems the the symbol is present and defined but maybe not in a way the linker expects? I tried a bit to fix this but I don't know enough about COFF/UEFI linkage. I can only speculate it is not sufficient to just use .globl ___rust_probestack.

[IsaacWoods]

I wonder if this is because, for UEFI, __rust_probestack is defined using this:

// Same as above, but for Mach-O.
#[cfg(any(target_vendor = "apple", target_os = "uefi"))]
macro_rules! define_rust_probestack {
    ($body: expr) => {
        concat!(
            "
            .globl ___rust_probestack
        ___rust_probestack:
            ",
            $body
        )
    };
}

(notice that ___rust_probestack has three underscores, whereas the linker error refers to __rust_probestack (with two underscores))

I'm not sure if this is a typo, or if Mach-O requires a ___rust_probestack symbol but UEFI needs a __rust_probestack.

cc @tmandry who seemed to introduce the three-underscore version?

[ALSchwalm]

Nice catch! Changing it to two underscores seems to fix the issue on UEFI. I'll hold off on making a PR though as I'm not sure about Mach-O.

[tmandry]

Ah nice catch indeed. Yes, Mach-O does require an extra underscore, so we'll need to leave it as-is for apple targets. You can simply make a copy of this definition with two underscores for uefi targets. (And other non-ELF non-Mach-O targets that might get introduced in the future.)

It would also be good to add a comment somewhere pointing out the difference in underscores 😅