Re: RISC-V: Re-enable GCC+Rust builds

[Conor Dooley <conor@kernel.org>]

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On Tue, Sep 02, 2025 at 01:19:42AM +0800, Asuna wrote:
> > For example, there's a check in the riscv Kconfig menu to see if
> > stack-protector-guard=tls can be used via a cc-option check. If that
> > check passes with gcc as the compiler that option will be passed to the
> > rust side of the build, where llvm might not support it.
> If I understand correctly, the `-mstack-protector-guard` option is already
> always filtered out by `bindgen_skip_c_flags` in `rust/Makefile`, regardless
> of architecture. Therefore, we don't need to do anything more, right?

That particular one might be a problem not because of
-mstack-protector-guard itself, but rather three options get added at
once:
	$(eval KBUILD_CFLAGS += -mstack-protector-guard=tls		  \
				-mstack-protector-guard-reg=tp		  \
				-mstack-protector-guard-offset=$(shell	  \
			awk '{if ($$2 == "TSK_STACK_CANARY") print $$3;}' \
				$(objtree)/include/generated/asm-offsets.h))
and the other ones might be responsible for the error.
Similarly, something like -Wno-unterminated-string-initialization could
cause a problem if gcc supports it but not libclang.

I was doing some debugging today of another problem, and was able to
trigger both of those errors with llvm-21 and libclang-19, so they
definitely have the potential to be problems if there's a mismatch - I
just don't know how many of those issues affect a mixed build with rustc
and the gnu tools, mixing llvm and libclang versions already produces
a warning about it being a Bad IdeaTM (a warning that I think should be
an error).

> > Similarly, turning on an extension like Zacas via a cc-option check
> > could pass for gcc but not be usable when passed to the rust side,
> > causing errors.
> That makes sense. I might need to check the version of libclang for each
> extension that passes the cc-option check for GCC to ensure it supports
> them.
> 
> > These sorts of things should be prevented via Kconfig, not show up as
> > confusing build errors.
> I'm working on a patch, and intend to output an error message in
> `arch/riscv/Makefile` then exit 1 when detecting an incompatible
> gcc+libclang mix in use.

I think you're mostly better off catching that sort of thing in Kconfig,
where possible and just make incompatible mixes invalid. What's actually
incompatible is likely going to depend heavily on what options are
enabled.

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