linux-kernel - Re: [RFC PATCH 0/4] x86/build: Get rid of vmlinux postlink step

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Message-ID: <CAMj1kXEAw7qqRSPCnKj+sO5QEtWsFQ2P7gkxuNQhssWd3E6S=Q@mail.gmail.com>
Date: Tue, 11 Mar 2025 07:40:45 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	linux-kbuild@...r.kernel.org, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 0/4] x86/build: Get rid of vmlinux postlink step

On Tue, 11 Mar 2025 at 03:39, Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> On Sun, Mar 9, 2025 at 6:48 PM Ingo Molnar <mingo@...nel.org> wrote:
> >
> >
> > * Ard Biesheuvel <ardb@...nel.org> wrote:
> >
> > > On Sat, 8 Mar 2025 at 17:17, Masahiro Yamada <masahiroy@...nel.org> wrote:
> > > >
> > > ...
> > > > I do not think it is broken.
> > > > As I mentioned above, I regard vmlinux.relocs as a byproduct
> > > > of the atomic build rule of vmlinux. This works.
> >
> > Except when it doesn't work, such as when an intermediate linking step
> > fails, and intermediate build products are lost and cannot be recreated
> > easily (or at all without modifying the source)?
> >
> > And the thing is, there should be no such thing as an 'atomic build
> > rule of vmlinux' if it means lost information when the build is broken
> > at an intermediate step. What purpose does it have?
> >
> > > There is no make rule for vmlinux.relocs, and so
> > >
> > > - if it gets deleted, it cannot be rebuilt and even though the build
> > > does not break, the relocation data is missing from the compressed
> > > image, and this could potentially break the kaslr startup code,
> > > - it vmlinux.relocs is older than vmlinux for some reason, make will
> > > not notice and silently reuse the outdated version,
> > > - when creating vmlinux.relocs from vmlinux and an error occurs,
> > > vmlinux is deleted, making it difficult to diagnose the problem.
> > >
> > > I think this is badly broken, but if you think this is all working as
> > > it should, I am not going to debate this further, and you can consider
> > > the patch series withdrawn.
> >
> > That's very sad, as both the simplification is substantial:
> >
> >   19 files changed, 52 insertions(+), 87 deletions(-)
> >
> > and the increase in debuggability is substantial as well.
> >
> > Thanks,
> >
> >         Ingo
>
> When a byproduct is accidentally lost
> (for example, manually deleted), it is not automatically restored.
> Running 'rm vmlinux' or 'make clean' is needed.
>

Exactly. Make cannot detect this situation, and so the build breaks in some way.

> vmlinux.relocs is one such byproduct.
> Another is the map file when CONFIG_VMLINUX_MAP=y is enabled.
>

The linker map is not depended upon by other build targets, and is
typically for human debug consumption, so while not ideal, it is not
as broken as for the unstripped vmlinux.


> I am a bit concerned about having more and more
> intermediate vmlinux.* files, but maybe only me.
> I hope vmlinux.unstripped is the only/last one.
>

Maybe we should not strip vmlinux at all, but only remove any static
relocations when packaging? E.g., tar-pkg, etc

>
> OK, let's do this.
>
> Ard, please send v2 with .gitignore and 'clean' target updates.
>

Thanks, I'll prepare a v2, but in the meantime, can we think about not
removing the relocations in the first place?