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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 18 Sep 2023 11:43:04 -0700
Message-ID: <CAKwvOdnbbStoEeykYnx0jbnR=TAmmcdnOXbbA4Fx0BneFW8Fsg@mail.gmail.com>
Subject: Re: [PATCH] rust: Respect HOSTCC when linking for host
To:     Matthew Maurer <mmaurer@google.com>
Cc:     Finn Behrens <me@kloenk.dev>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alice Ryhl <aliceryhl@google.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nicolas Schier <nicolas@fjasle.eu>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org
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On Mon, Sep 18, 2023 at 9:38=E2=80=AFAM Matthew Maurer <mmaurer@google.com>=
 wrote:
>
> On Mon, Sep 18, 2023 at 8:25=E2=80=AFAM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > What happens if you invoke the linker directly?
> Rust unfortunately currently doesn't support invoking the linker
> directly: https://github.com/rust-lang/rust/issues/73632

Wait; does Rust have its own linker? It doesn't use the system linker?
 Perhaps that's necessary for the rust module format? If so, TIL.

> > Generally, the kernel either invokes the compiler or the linker
> > directly. (For assembler, it is typically preprocessed, as are linker
> > scripts!)  So invoking the linker directly is a common pattern in
> > kbuild.  It also makes me slightly sad if the rust compiler ends up
> > invoking a c compiler, even if it's simply to drive the linker.
> As mentioned earlier, we could pass $HOSTLD, but if the linker isn't
> named something accurate (e.g. if the linker is not named lld, but is
> lld), we need to know how to pass a flavor:
> https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-flavor
> Would it be appropriate to just assume the linker is named correctly?

If it were not, what does failure look like?

command not found: asdfadfasdf

Seems fine to me. If the user mis-specifies HOSTLD=3D, then they will
get a similar error, which should be prescriptive enough for them to
figure out how exactly they're "holding it wrong."

> > For example, Android carries a downstream patch to set `-fuse-ld=3Dlld`
> > for $KBUILD_HOSTCFLAGS, because its build environment doesn't contain
> > GNU binutils ("guilty, officer").
> Oddly, the Android kernel environment (Kleaf) is the one that I needed
> this patch to build in, but it seemed to be working without a manual
> KBUILD_HOSTCFLAGS forwarding.

Surprising that worked.

> Overall, it sounds like you'd prefer if I set this to use
> `KBUILD_HOSTLD` and `KBUILD_HOSTLDFLAGS`, and leave the linker flavor
> to autodetect?

Yes for the first two.

Dunno, what precisely is a linker flavor?  Is that like a flavor of ice cre=
am?
Oh, right looking at your link:
https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-flavor
Seems like if `LLVM=3D1` is set, or `LD=3Dld.lld`, or CONFIG_LD_IS_LLD, the=
n
the flavor should be set to ld.lld, else ld.  Then the
KBUILD_HOSTLDFLAGS need to be passed, probably.

But how are there "linker flavors" like ld or ld.lld if you just said
"Rust unfortunately currently doesn't support invoking the linker
directly: https://github.com/rust-lang/rust/issues/73632".  I'm having
trouble reconciling the two.

Can we do something more like the below?

ifdef CONFIG_LD_IS_LLD
hostrust_flags +=3D -C linker-flavor=3Dld.lld
else
hostrust_flags +=3D -C linker-flavor=3Dld
endif
hostrust_flags +=3D -C linker=3D$(HOSTLD) <todo: figure out how to pass
KBUILD_HOSTLDFLAGS>
--=20
Thanks,
~Nick Desaulniers