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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Fri, 18 Feb 2022 20:08:43 +0100
Message-ID: <CAMj1kXHjAxf9gxcAXGpDwR2DUc5jYVdJFzWbwJxcMLg5-y=kxA@mail.gmail.com>
Subject: Re: [PATCH] arm64 module: remove (NOLOAD)
To:     Nathan Chancellor <nathan@kernel.org>
Cc:     Fangrui Song <maskray@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
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On Fri, 18 Feb 2022 at 17:34, Nathan Chancellor <nathan@kernel.org> wrote:
>
> On Fri, Feb 18, 2022 at 12:50:16AM -0800, Fangrui Song wrote:
> > On 2022-02-18, Ard Biesheuvel wrote:
> > > On Fri, 18 Feb 2022 at 09:12, Fangrui Song <maskray@google.com> wrote:
> > > >
> > > > On ELF, (NOLOAD) sets the section type to SHT_NOBITS[1]. It is conceptually
> > > > inappropriate for .plt and .text.* sections which are always
> > > > SHT_PROGBITS.
> > > >
> > > > In GNU ld, if PLT entries are needed, .plt will be SHT_PROGBITS anyway
> > > > and (NOLOAD) will be essentially ignored. In ld.lld, since
> > > > https://reviews.llvm.org/D118840 ("[ELF] Support (TYPE=<value>) to
> > > > customize the output section type"), ld.lld will report a `section type
> > > > mismatch` error. Just remove (NOLOAD) to fix the error.
> > > >
> > > > [1] https://lld.llvm.org/ELF/linker_script.html As of today, "The
> > > > section should be marked as not loadable" on
> > > > https://sourceware.org/binutils/docs/ld/Output-Section-Type.html is
> > > > outdated for ELF.
> > >
> > > This patch lacks a SOB line.
> > >
> > > With one added,
> > >
> > > Acked-by: Ard Biesheuvel <ardb@kernel.org>
> >
> > Ah, yes. Sorry, I haven't sent a kernel patch for a while...
> >
> > Reported-by: Nathan Chancellor <nathan@kernel.org>
> > Signed-off-by: Fangrui Song <maskray@google.com>
> > Acked-by: Ard Biesheuvel <ardb@kernel.org>
>
> I am assuming this patch is the solution we want, as opposed to Ard's
> suggestion of renaming these sections at
> https://reviews.llvm.org/D118840 (unless that was a tangential
> suggestion).
>

Renaming will not make the problem go away. It will only clear up the
confusion regarding the contents of these sections, which are not in
fact linker emitted PLT entries, but branching veneers emitted by the
kernel's module loader.

> I have verified that modules still load. Additionally, this needs to go
> to stable so that groups who upgrade their toolchain to a revision that
> include the LLD patch don't get broken as well.
>
> Cc: stable@vger.kernel.org
> Tested-by: Nathan Chancellor <nathan@kernel.org>
>
>
> > > > ---
> > > >  arch/arm64/include/asm/module.lds.h | 6 +++---
> > > >  1 file changed, 3 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/arch/arm64/include/asm/module.lds.h b/arch/arm64/include/asm/module.lds.h
> > > > index a11ccadd47d2..094701ec5500 100644
> > > > --- a/arch/arm64/include/asm/module.lds.h
> > > > +++ b/arch/arm64/include/asm/module.lds.h
> > > > @@ -1,8 +1,8 @@
> > > >  SECTIONS {
> > > >  #ifdef CONFIG_ARM64_MODULE_PLTS
> > > > -       .plt 0 (NOLOAD) : { BYTE(0) }
> > > > -       .init.plt 0 (NOLOAD) : { BYTE(0) }
> > > > -       .text.ftrace_trampoline 0 (NOLOAD) : { BYTE(0) }
> > > > +       .plt 0 : { BYTE(0) }
> > > > +       .init.plt 0 : { BYTE(0) }
> > > > +       .text.ftrace_trampoline 0 : { BYTE(0) }
> > > >  #endif
> > > >
> > > >  #ifdef CONFIG_KASAN_SW_TAGS
> > > > --
> > > > 2.35.1.265.g69c8d7142f-goog
> > > >
> >