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

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Message-ID: <202106021231.A1B7A10@keescook>
Date:   Wed, 2 Jun 2021 12:32:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Arnd Bergmann <arnd@...nel.org>, Fangrui Song <maskray@...gle.com>,
        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 07:09:23PM +0000, Catalin Marinas wrote:
> On Tue, Mar 16, 2021 at 05:39:27PM +0100, Arnd Bergmann wrote:
> > 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.
> 
> I suspected this as well but still worked for me when disabling it.
> 
> Anyway, I don't think identifying the exact option is necessary. With
> CONFIG_UNMAP_KERNEL_AT_EL0=y we need this section around even if only
> __entry_tramp_text_start/end are referenced.
> 
> In this case we happened to detect this issue because of the ASSERT in
> vmlinux.lds.S but I wonder what else the linker drops with this dead
> code elimination that we may not notice (it seems to remove about 500KB
> from the resulting image in my test).
> 
> I'll push these two patches to -next for wider coverage before deciding
> on mainline (though the option may not get much testing as it's hidden
> behind EXPERT and default n).

I don't see this in -next? Catalin, do you want me to pick it up as part
of my collecting various linker fixes?

-Kees

-- 
Kees Cook