linux-kernel - Re: [PATCH] arm64: vmlinux.lds.S: keep .entry.tramp.text section

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Message-ID: <CAK8P3a07nPbbP095LJ84Z5hP3Ce53fb_+0Zav8PUA+7R_92gMw@mail.gmail.com>
Date:   Tue, 16 Mar 2021 17:39:27 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Fangrui Song <maskray@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        David Brazdil <dbrazdil@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: vmlinux.lds.S: keep .entry.tramp.text section

On Tue, Mar 16, 2021 at 5:27 PM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Tue, Mar 16, 2021 at 10:45:32AM +0000, Catalin Marinas wrote:
> > On Fri, Feb 26, 2021 at 08:32:57PM -0800, Fangrui Song wrote:
> > > On 2021-02-26, Kees Cook wrote:
> > > > On Fri, Feb 26, 2021 at 03:03:39PM +0100, Arnd Bergmann wrote:
> > > > > From: Arnd Bergmann <arnd@...db.de>
> > > > >
> > > > > When building with CONFIG_LD_DEAD_CODE_DATA_ELIMINATION,
> > > > > I sometimes see an assertion
> > > > >
> > > > >  ld.lld: error: Entry trampoline text too big
> > > >
> > > > Heh, "too big" seems a weird report for having it discarded. :)
> > > >
> > > > Any idea on this Fangrui?
> > > >
> > > > ( I see this is https://github.com/ClangBuiltLinux/linux/issues/1311 )
> > >
> > > This diagnostic is from an ASSERT in arch/arm64/kernel/vmlinux.lds
> > >
> > >   ASSERT((__entry_tramp_text_end - __entry_tramp_text_start) == (1 << 16),
> > >    "Entry trampoline text too big")
> >
> > Can we not change the ASSERT to be <= PAGE_SIZE instead?
>
> Ah, that won't work as I suspect we still need the trampoline section.
>
> Arnd, do you know why this section disappears? I did a simple test with
> defconfig + LD_DEAD_CODE_DATA_ELIMINATION and the trampoline section is
> still around.

If I remember correctly, this showed up when CONFIG_ARM_SDE_INTERFACE
is disabled, which dropped the only reference into this section.
If that doesn't make sense, I can try digging through the old build logs to
reproduce the problem.

    Arnd