`-Zexport-executable-symbols` alone does not export symbols

[yshui]

I tried:

#[no_mangle]
pub fn exported() {}

building with:

rustc -Zexport-executable-symbols --crate-type bin

I expected to see this happen:

`exported` is in the list of dynamic symbols

Instead, this happened:

no symbol is exported

in addition, this test case here:

rust/src/test/run-make/export-executable-symbols/Makefile

Line 10 in 98f3001

nm $(TMPDIR)/main | $(CGREP) exported_symbol

is insufficient, as nm prints out every symbol, not only the exported ones.

Meta

Tested on:

• rustc 1.65.0-nightly (c07a8b4e0 2022-08-26)
• rustc 1.65.0-nightly (c2804e6ec 2022-09-07)

[yshui]

the test should have at least used nm -T, but i am not an expert on these tools.

[yshui]

seems like an additional -C link-arg=-rdynamic is needed.

[GoldsteinE]

An easy solution would be to just emit -rdynamic when export-executable-symbols is specified. I think this is a bit too aggressive though: it makes all the symbols exported, not only ones Rust considers used (so marked with #[no_mangle] and potentially #[export]).

A more precise solution would be to use --dynamic-list with a precise list of symbols rustc considers “exported”. I’m not yet sure whether it’s feasible to generate such a list.

[bjorn3]

We do already generate a version script for dynamic libraries to restrict the set of exported symbols. We just omit it for executables.

[GoldsteinE]

Thanks for the clarification.

Looking at the relevant code, it looks like version script does get generated for executables if -Zexport-executable-symbols is specified:

rust/compiler/rustc_codegen_ssa/src/back/linker.rs

Lines 722 to 730 in a91f7d7

	if crate_type == CrateType::Executable {
	let should_export_executable_symbols =
	self.sess.opts.unstable_opts.export_executable_symbols;
	if self.sess.target.override_export_symbols.is_none()
	&& !should_export_executable_symbols
	{
	return;
	}
	}

I’ll look into this code some more. I’m not sure version script is enough to get a symbol dynamically exported, I think --dynamic-list or --export-dynamic is also required.

[bjorn3]

Have you verified that the input object files export the symbols in the first place? I wouldn't be surprised if the mono collector skips them or internalizes them.

[GoldsteinE]

-rdynamic fixes the issue for me, so it looks like the problem is with the linker invocation.

Calling

rustc --emit=obj -Zexport-executable-symbols --crate-type=bin ttt.rs

gives me an object file with a global exported symbol:

$ objdump --syms ttt.o | rg exported
0000000000000000 l    d  .text.exported	0000000000000000 .text.exported
0000000000000000 g     F .text.exported	0000000000000001 exported

similar to a C function with external linkage:

$ cat t.c
void exported(void) {}
$ gcc -c t.c
$ objdump --syms t.o | rg exported
0000000000000000 g     F .text	0000000000000007 exported