linux-kernel - Re: [PATCH 3/4] s390: enable text relative kallsyms for 64-bit targets

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Message-ID: <20160120101751.GG3395@osiris>
Date:	Wed, 20 Jan 2016 11:17:51 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-s390@...r.kernel.org,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	mpe@...erman.id.au, Michal Marek <mmarek@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 3/4] s390: enable text relative kallsyms for 64-bit
 targets

On Wed, Jan 20, 2016 at 11:04:24AM +0100, Ard Biesheuvel wrote:
> On 20 January 2016 at 10:43, Heiko Carstens <heiko.carstens@...ibm.com> wrote:
> > On Wed, Jan 20, 2016 at 10:05:37AM +0100, Ard Biesheuvel wrote:
> >> This enables the newly introduced text-relative kallsyms support when
> >> building 64-bit targets. This cuts the size of the kallsyms address
> >> table in half, reducing the memory footprint of the kernel .rodata
> >> section by about 250 KB for a defconfig build.
> >>
> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> >> ---
> >>
> >> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> >> index dbeeb3a049f2..588160fd1db0 100644
> >> --- a/arch/s390/Kconfig
> >> +++ b/arch/s390/Kconfig
> >> @@ -149,6 +149,7 @@ config S390
> >>       select HAVE_REGS_AND_STACK_ACCESS_API
> >>       select HAVE_SYSCALL_TRACEPOINTS
> >>       select HAVE_VIRT_CPU_ACCOUNTING
> >> +     select KALLSYMS_TEXT_RELATIVE if 64BIT
> >
> > Please remove the "if 64BIT" since s390 is always 64BIT in the meantime.
> > Tested on s390 and everything seems still to work ;)
> >
> > Acked-by: Heiko Carstens <heiko.carstens@...ibm.com>
> >
> 
> Thanks! Did you take a look at /proc/kallsyms, by any chance? It
> should look identical with and without these patches

Close to identical, since the generated code and offsets change a bit with
your new config option enabled and disabled. But only those parts that are
linked behind kernel/kallsyms.c.

However I did run a couple of ftrace, kprobes tests and enforced call
backtraces. Everything still works.

So it looks all good.