linux-kernel - Re: [PATCH v3 2/4] objtool: Add support for C jump tables

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Message-ID: <20190627085804.070a9541@gandalf.local.home>
Date:   Thu, 27 Jun 2019 08:58:04 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>,
        Kairui Song <kasong@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3 2/4] objtool: Add support for C jump tables

On Wed, 26 Jun 2019 21:47:00 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Wed, Jun 26, 2019 at 06:42:40PM -0700, Alexei Starovoitov wrote:
> > > @@ -1035,9 +1038,18 @@ static struct rela *find_switch_table(struct objtool_file *file,
> > >
> > >                 /*
> > >                  * Make sure the .rodata address isn't associated with a
> > > -                * symbol.  gcc jump tables are anonymous data.
> > > +                * symbol.  GCC jump tables are anonymous data.
> > > +                *
> > > +                * Also support C jump tables which are in the same format as
> > > +                * switch jump tables.  Each jump table should be a static
> > > +                * local const array named "jump_table" for objtool to
> > > +                * recognize it.  
> > 
> > Nacked-by: Alexei Starovoitov <ast@...nel.org>
> > 
> > It's not acceptable for objtool to dictate kernel naming convention.  
> 
> Abrasive nack notwithstanding, I agree it's not ideal.
> 
> How about the following approach instead?  This is the only other way I
> can think of to annotate a jump table so that objtool can distinguish
> it:
> 
> #define __annotate_jump_table __section(".jump_table.rodata")
> 
> Then bpf would just need the following:
> 
> -	static const void *jumptable[256] = {
> +	static const void __annotate_jump_table *jumptable[256] = {
> 
> This would be less magical and fragile than my original approach.
> 
> I think the jump table would still be placed with all the other rodata,
> like before, because the vmlinux linker script recognizes the section
> ".rodata" suffix and bundles them all together.
> 

After finally getting a chance to skim through this lovely thread, I
was going to suggest exactly this. This is the way we usually handle
"special" data.

As it appears that Alexei is good with this approach, please go this
route.

Thanks!

-- Steve