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

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Date:   Wed, 26 Jun 2019 19:54:08 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     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>,
        Steven Rostedt <rostedt@...dmis.org>,
        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, Jun 26, 2019 at 7:47 PM 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.

I like it if that works :)
Definitely cleaner.
May be a bit more linker script magic would be necessary,
but hopefully not.
And no need to rely on gcc style of mangling static vars.