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From:   Finn Behrens <me@kloenk.dev>
To:     Matthew Maurer <mmaurer@google.com>
Cc:     =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
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        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>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Nicolas Schier <nicolas@fjasle.eu>,
        rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-kbuild@vger.kernel.org
Subject: Re: [PATCH] rust: Respect HOSTCC when linking for host
Date:   Sun, 17 Sep 2023 00:39:46 +0200
Message-ID: <446E67B5-0200-4982-994B-B30C2DEDF098@kloenk.dev>
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On 16 Sep 2023, at 21:54, Matthew Maurer wrote:

> On Sat, Sep 16, 2023 at 11:07=E2=80=AFAM Finn Behrens <me@kloenk.dev> w=
rote:
>>
>>
>>
>> On 16 Sep 2023, at 19:53, Bj=C3=B6rn Roy Baron wrote:
>>
>>> On Saturday, September 16th, 2023 at 18:52, Finn Behrens <me@kloenk.d=
ev> wrote:
>>>
>>>>
>>>> On 15 Sep 2023, at 19:28, Matthew Maurer wrote:
>>>>
>>>>> Currently, rustc defaults to invoking `cc`, even if `HOSTCC` is def=
ined,
>>>>> resulting in build failures in hermetic environments where `cc` doe=
s not
>>>>> exist. This includes both hostprogs and proc-macros.
>>>>>
>>>>> Since we are setting the linker to `HOSTCC`, we set the linker flav=
or to
>>>>> `gcc` explicitly.
>>>> But as `HOSTCC` could also be clang, the linker flavor would then be=
 wrong, would that create a problem?
>>>
>>> Rustc uses the gcc linker flavor for clang too. There has been a prop=
osal to split it up, but I'm not sure of the status of that. In any case =
clang's cli is similar enough to gcc that it works fine to use the gcc li=
nker flavor.
>>>
>> In that case this looks very reasonable.
>>
>> Second thing I noticed is that `HOSTCC` could be the wrong variable, a=
nd `HOSTLD` would make more sense as we look for the linker and not the g=
eneral C compiler.
>>
> Yes, thanks Bjorn - "gcc" is the linker flavor used for "Use the C
> compiler as a linker".
>
> With regards to HOSTLD, I was trying to make the minimum possible
> change. Currently, it is using the command `cc` as a linker, so this
> would preserve existing behavior when HOSTCC is unset.
>
> If we would prefer `HOSTLD` instead we can do that, but we would need
> to additionally inspect `LLVM` to set the linker flavor accordingly
> (e.g. set ld vs ld.lld).
>
> Do folks have strong opinions between these? My primary concern is to
> avoid calling programs by foo when their HOSTFOO variable is set.
I don=E2=80=99t have an educated opinion at all, mainly nudged in the dir=
ection, so that the existence if `HOSTLD` does not get overlooked. If the=
re is a valid reason to not use it, this patch looks good to me.
>
> See https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-f=
lavor
> for details on linker flavor settings.
>>>>>
>>>>> Signed-off-by: Matthew Maurer <mmaurer@google.com>
>>>>> ---
>>>>>  rust/Makefile         | 1 +
>>>>>  scripts/Makefile.host | 1 +
>>>>>  2 files changed, 2 insertions(+)
>>>>>
>>>>> diff --git a/rust/Makefile b/rust/Makefile
>>>>> index 87958e864be0..2a2352638f11 100644
>>>>> --- a/rust/Makefile
>>>>> +++ b/rust/Makefile
>>>>> @@ -383,6 +383,7 @@ $(obj)/exports_kernel_generated.h: $(obj)/kerne=
l.o FORCE
>>>>>  quiet_cmd_rustc_procmacro =3D $(RUSTC_OR_CLIPPY_QUIET) P $@
>>>>>        cmd_rustc_procmacro =3D \
>>>>>     $(RUSTC_OR_CLIPPY) $(rust_common_flags) \
>>>>> +           -C linker-flavor=3Dgcc -C linker=3D$(HOSTCC) \
>>>>>             --emit=3Ddep-info=3D$(depfile) --emit=3Dlink=3D$@ --ext=
ern proc_macro \
>>>>>             --crate-type proc-macro \
>>>>>             --crate-name $(patsubst lib%.so,%,$(notdir $@)) $<
>>>>> diff --git a/scripts/Makefile.host b/scripts/Makefile.host
>>>>> index 8f7f842b54f9..0aa95a3af1c4 100644
>>>>> --- a/scripts/Makefile.host
>>>>> +++ b/scripts/Makefile.host
>>>>> @@ -91,6 +91,7 @@ hostcxx_flags  =3D -Wp,-MMD,$(depfile) \
>>>>>  # current working directory, which may be not accessible in the ou=
t-of-tree
>>>>>  # modules case.
>>>>>  hostrust_flags =3D --out-dir $(dir $@) --emit=3Ddep-info=3D$(depfi=
le) \
>>>>> +            -C linker-flavor=3Dgcc -C linker=3D$(HOSTCC) \
>>>>>                   $(KBUILD_HOSTRUSTFLAGS) $(HOST_EXTRARUSTFLAGS) \
>>>>>                   $(HOSTRUSTFLAGS_$(target-stem))
>>>>>
>>>>> --
>>>>> 2.42.0.459.ge4e396fd5e-goog