linkme crate doesn't link with wild

[mati865]

Clone https://github.com/dtolnay/linkme and run cargo t with wild as the linker and it will fail with:

Error: Failed copying from <epilogue> to output file

          Caused by:
              symbol `__start_linkme_SHENANIGANS` in section `linkme_SHENANIGANS` that we're not going to output Resolution { raw_value: 945344, dynamic_symbol_index: None, got_address: None, plt_address: None, resolution_flags: ResolutionFlags(DIRECT), value_flags: ValueFlags(ADDRESS) }

          Stack backtrace:
             0: anyhow::context::<impl anyhow::Context<T,core::convert::Infallible> for core::option::Option<T>>::with_context
             1: libwild::elf_writer::write_internal_symbols
             2: libwild::elf_writer::<impl libwild::layout::FileLayout>::write
             3: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
             4: rayon::iter::plumbing::bridge_producer_consumer::helper
             5: rayon_core::job::StackJob<L,F,R>::run_inline
             6: rayon_core::join::join_context::{{closure}}
             7: rayon_core::registry::in_worker
             8: rayon::iter::plumbing::bridge_producer_consumer::helper
             9: rayon_core::job::StackJob<L,F,R>::run_inline
            10: rayon_core::join::join_context::{{closure}}
            11: rayon_core::registry::in_worker
            12: rayon::iter::plumbing::bridge_producer_consumer::helper
            13: rayon_core::job::StackJob<L,F,R>::run_inline
            14: rayon_core::join::join_context::{{closure}}
            15: rayon_core::registry::in_worker
            16: rayon::iter::plumbing::bridge_producer_consumer::helper
            17: rayon_core::job::StackJob<L,F,R>::run_inline
            18: rayon_core::join::join_context::{{closure}}
            19: rayon_core::registry::in_worker
            20: rayon::iter::plumbing::bridge_producer_consumer::helper
            21: rayon_core::job::StackJob<L,F,R>::run_inline
            22: rayon_core::join::join_context::{{closure}}
            23: rayon_core::registry::in_worker
            24: rayon::iter::plumbing::bridge_producer_consumer::helper
            25: rayon_core::job::StackJob<L,F,R>::run_inline
            26: rayon_core::join::join_context::{{closure}}
            27: rayon_core::registry::in_worker
            28: rayon::iter::plumbing::bridge_producer_consumer::helper
            29: rayon_core::job::StackJob<L,F,R>::run_inline
            30: rayon_core::join::join_context::{{closure}}
            31: rayon_core::registry::in_worker
            32: rayon::iter::plumbing::bridge_producer_consumer::helper
            33: <rayon_core::job::StackJob<L,F,R> as rayon_core::job::Job>::execute
            34: rayon_core::registry::WorkerThread::wait_until_cold
            35: rayon_core::registry::ThreadBuilder::run
            36: std::sys::backtrace::__rust_begin_short_backtrace
            37: core::ops::function::FnOnce::call_once{{vtable.shim}}
            38: std::sys::pal::unix::thread::Thread::new::thread_start
            39: <unknown>
            40: <unknown>
          clang: error: linker command failed with exit code 255 (use -v to see invocation)
          WARNING: wild: -z relro is not yet supported

Given the nature of this crate it's not entirely surprising, but we should support it. Either by modifying wild or linkme code.
It also fails LLD, but probably for a different reason dtolnay/linkme#94

[mati865]

Also, release build fails differently, looks like the same issue as LLD:

Failed copying from <epilogue> to output file

          Caused by:
              symbol `__start_linkm2_NONCOPY` in section `linkm2_NONCOPY` that we're not going to output Resolution { raw_value: 222520, dynamic_symbol_index: None, got_address: None, plt_address: None, resolution_flags: ResolutionFlags(DIRECT), value_flags: ValueFlags(ADDRESS) }

          Stack backtrace:
             0: anyhow::context::<impl anyhow::Context<T,core::convert::Infallible> for core::option::Option<T>>::with_context
             1: libwild::elf_writer::write_internal_symbols
             2: libwild::elf_writer::<impl libwild::layout::FileLayout>::write
             3: core::ops::function::impls::<impl core::ops::function::FnOnce<A> for &mut F>::call_once
             4: rayon::iter::plumbing::bridge_producer_consumer::helper
             5: <rayon_core::job::StackJob<L,F,R> as rayon_core::job::Job>::execute
             6: rayon_core::registry::WorkerThread::wait_until_cold
             7: rayon_core::join::join_context::{{closure}}
             8: rayon_core::registry::in_worker
             9: rayon::iter::plumbing::bridge_producer_consumer::helper
            10: <rayon_core::job::StackJob<L,F,R> as rayon_core::job::Job>::execute
            11: rayon_core::registry::WorkerThread::wait_until_cold
            12: rayon_core::registry::ThreadBuilder::run
            13: std::sys::backtrace::__rust_begin_short_backtrace
            14: core::ops::function::FnOnce::call_once{{vtable.shim}}
            15: std::sys::pal::unix::thread::Thread::new::thread_start
            16: <unknown>
            17: <unknown>
          clang: error: linker command failed with exit code 255 (use -v to see invocation)
          WARNING: wild: -z relro is not yet supported

[davidlattimore]

It looks like the retain bit isn't set on the sections such as linkme_SHENANIGANS. Worse, it looks like rust doesn't provide any way to set the retain bit.

Wild and LLD both discard unreferenced sections unless they have the retain bit set.

GNU ld and Mold seems to treat a reference to __start_SECTION_NAME or __stop_SECTION_NAME as a signal to keep all instances of SECTION_NAME. That's probably not unreasonable behaviour. I'll have to have a think about how to do that without hurting performance.

[davidlattimore]

Correction: GNU ld does what I described above. It seems that Mold actually keeps all custom sections that don't start with '.'. It doesn't matter if there's a __start_X symbol or whether it's referenced. So that's an option too.

[davidlattimore]

It turned out to be relatively straightforward to implement this without affecting performance, so I did that. cargo test on the linkme crate now succeeds for me.

[mati865]

Thanks! Works great (both debug and release) and allows Zed editor to link when using wild!