Message-ID: <CAK7LNATH59wQxaMRur9S5=HCi4bo9pREWK5LQjnNwZ5uOt7g=A@mail.gmail.com>
Date:   Wed, 27 Sep 2023 01:51:51 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Matthew Maurer <mmaurer@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Finn Behrens <me@...enk.dev>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nicolas Schier <nicolas@...sle.eu>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] rust: Respect HOSTCC when linking for host

On Tue, Sep 19, 2023 at 3:55 AM Matthew Maurer <mmaurer@...gle.com> wrote:
>
> On Mon, Sep 18, 2023 at 11:43 AM Nick Desaulniers
> <ndesaulniers@...gle.com> wrote:
> >
> > On Mon, Sep 18, 2023 at 9:38 AM Matthew Maurer <mmaurer@...gle.com> wrote:
> > >
> > > On Mon, Sep 18, 2023 at 8:25 AM Nick Desaulniers
> > > <ndesaulniers@...gle.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.
> It does use the system linker (this is what -C linker is controlling),
> but the command line passed to the linker may change, extra object
> files may be added to the command line, etc.
> >
> > > > 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?
> That depends. I think it will usually look like "unrecognized flag:
> blah blah", but that's not guaranteed.
> >
> > command not found: asdfadfasdf
> This isn't about command-not-found, it's about "I set
> HOSTLD=foo/bar/weirdname, and it is an lld-like linker. rustc invoked
> it assuming it is an ld-like linker."
> >
> > Seems fine to me. If the user mis-specifies HOSTLD=, 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=lld`
> > > > for $KBUILD_HOSTCFLAGS, because its build environment doesn't contain
> > > > GNU binutils ("guilty, officer").
> > > Oddly, the Android kernel environment (Kleaf) is the one that I needed
> > > this patch to build in, but it seemed to be working without a manual
> > > KBUILD_HOSTCFLAGS forwarding.
> >
> > Surprising that worked.
> >
> > > Overall, it sounds like you'd prefer if I set this to use
> > > `KBUILD_HOSTLD` and `KBUILD_HOSTLDFLAGS`, and leave the linker flavor
> > > to autodetect?
> >
> > Yes for the first two.
> >
> > Dunno, what precisely is a linker flavor?  Is that like a flavor of ice cream?
> > Oh, right looking at your link:
> > https://doc.rust-lang.org/rustc/codegen-options/index.html#linker-flavor
> > Seems like if `LLVM=1` is set, or `LD=ld.lld`, or CONFIG_LD_IS_LLD, then
> > the flavor should be set to ld.lld, else ld.  Then the
> > KBUILD_HOSTLDFLAGS need to be passed, probably.
> >
> > But how are there "linker flavors" like ld or ld.lld if you just said
> > "Rust unfortunately currently doesn't support invoking the linker
> > directly: https://github.com/rust-lang/rust/issues/73632".  I'm having
> > trouble reconciling the two.
> Yes, what I meant by that is that *rustc* wants to invoke the linker,
> rather than having the surrounding build system invoke the linker. The
> exact command line passed to the linker in the final link, including
> potential synthetic objects, is considered an internal detail.
> >
> > Can we do something more like the below?
> >
> > ifdef CONFIG_LD_IS_LLD
> > hostrust_flags += -C linker-flavor=ld.lld
> > else
> > hostrust_flags += -C linker-flavor=ld
> > endif
> > hostrust_flags += -C linker=$(HOSTLD) <todo: figure out how to pass
> > KBUILD_HOSTLDFLAGS>
> Yes, I can make host linking use `$(HOSTLD)` and pass the flavor based
> on CONFIG_LD_IS_LLD. I'll send a variant that does that this
> afternoon.


CONFIG_LD_IS_LLD=y states that the linker for the
kernel space is LLD.

Host programs should not be affected.



-- 
Best Regards
Masahiro Yamada

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