linux-kernel - Re: [RFC PATCH] kbuild: pass objects instead of archives to linker

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Message-ID: <CAK7LNASjBm=WWpkSaL1+QuLokhnepUucvhYyP3CCeZYx6nOTHA@mail.gmail.com>
Date:   Wed, 2 Nov 2022 18:44:05 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, Jiri Slaby <jirislaby@...nel.org>,
        Michael Matz <matz@...e.de>, Kees Cook <keescook@...omium.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Fangrui Song <maskray@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [RFC PATCH] kbuild: pass objects instead of archives to linker

On Wed, Nov 2, 2022 at 6:21 PM Ard Biesheuvel <ardb@...nel.org> wrote:
>
> Hello Masahiro,
>
> On Wed, 2 Nov 2022 at 10:13, Masahiro Yamada <masahiroy@...nel.org> wrote:
> >
> > This is an experimental patch, driven by the feedback from Jiri Slaby
> > and Michael Matz. [1]
> >
> > Michael Matz says:
> >  "I know of no linker (outside LTO-like modes) that processes
> >   archives in a different order than first-to-last-member (under
> >   whole-archive), but that's not guaranteed anywhere. So relying on
> >   member-order within archives is always brittle."
> >
> > It is pretty easy to pass the list of objects instead of a thin archive
> > because the linker supports the '@...e' syntax, where command line
> > arguments are read from 'file'.
> >
>
> Can you explain which problem is solved by doing this?


Jiri Slaby reported that the (not-upstreamed) GCC-LTO tree got broken
due to 321648455061 ("kbuild: use obj-y instead extra-y for objects
placed at the head")

https://lore.kernel.org/linux-kbuild/ea468b86-abb7-bb2b-1e0a-4c8959d23f1c@kernel.org/


I am not pretty sure because I did not check the downstream code.


If I understood his report correctly, the reason for the breakage is
because I put all objects into the thin archive, expecting
the linker would preserve the object order in the archive.

By specifying the object order directly in the command line,
GCC-LTO should get back working again.






>
> If we can only produce a working kernel if each object is linked in
> the order it appears in the archive, I think we have bigger problems
> that need solving regardless. And for the .head.text objects that need
> to appear at the start of the binary image, I think the reported issue
> with __head annotated C functions on x86 needs to be addressed by
> getting rid of __head entirely (which seems to have been introduced
> without proper justification)


I agree that it is the correct approach.


I think my patch is unneeded (hence RFC), but I just wanted to know
if linkers (gnu ld and lld) see any difference.






--
Best Regards
Masahiro Yamada