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

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Date:   Tue, 16 Mar 2021 16:27:45 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>, Arnd Bergmann <arnd@...nel.org>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        David Brazdil <dbrazdil@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: vmlinux.lds.S: keep .entry.tramp.text section

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.

-- 
Catalin