How rustc manage to link both panic-abort and panic-unwind on the same binary?

Embedded
1

This question is not really embedded, but is closer to embedded than general rust usage.

I created a trivial hello world application and built it with the following command line for this tier 3 target:

RUSTFLAGS="--emit=asm -g -C passes=loweratomic" cargo +nightly build -Zbuild-std=std,panic_abort -Zbuild-std-features=compiler-builtins-mem --target riscv32im-risc0-zkvm-elf --verbose

It all goes fine, but when I look into the assembly files emitted, I see that the symbol __rust_panic_cleanup is duplicated in both libpanic_abort and libpanic_unwind. At first I thought that was a quirk of cargo, and at most one was supplied to the final rustc invocation. But no, if I inspect the --verbose output, I can see that both are provided:

[...] --extern 'noprelude:panic_abort=[...]/target/riscv32im-risc0-zkvm-elf/debug/deps/libpanic_abort-be67a198f4b50054.rlib' --extern 'noprelude:panic_unwind=[...]/target/riscv32im-risc0-zkvm-elf/debug/deps/libpanic_unwind-edaa0a86e5842749.rlib' [...]

I am working on a context of a zkVM similar to risc0, and we do have to perform our own liking, so I would like to know how the repeated symbol is handled by rustc/lld, because our linker currently does not support repeated symbols and I will probably have to fix it.

2

Even though libstd declares a dependency on both the panic_abort and panic_unwind crates, rustc has special logic to only link one of the two depending on if code is compiled with -Cpanic=abort or -Cpanic=unwind.

3

I manually filtered out panic_unwind, but then I got this other, very similar, problem: when I tried to link with a crate defining the #[panic_handler], the symbol rust_begin_unwind is duplicated both in my crate and std. And I guess if our code didn't stop at the first error, we would also get an error due to #[global_allocator] being defined, rustc seems full of special logic when it comes to linking against std.

So, is there a not very complicated way to get what symbols from where are actually being delivered to linker, after all the special logic?

4

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