linux-kernel - Re: linux-next: Tree for Jul 2 (objtool)

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Date:   Wed, 3 Jul 2019 16:03:53 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: linux-next: Tree for Jul 2 (objtool)

On Wed, Jul 03, 2019 at 02:46:14PM -0500, Josh Poimboeuf wrote:
> On Wed, Jul 03, 2019 at 02:45:28PM -0500, Josh Poimboeuf wrote:
> > On Wed, Jul 03, 2019 at 10:23:21AM -0700, Randy Dunlap wrote:
> > > On 7/3/19 9:44 AM, Josh Poimboeuf wrote:
> > > > On Tue, Jul 02, 2019 at 11:47:02AM -0700, Randy Dunlap wrote:
> > > >> On 7/2/19 2:51 AM, Stephen Rothwell wrote:
> > > >>> Hi all,
> > > >>>
> > > >>> Changes since 20190701:
> > > >>>
> > > >>
> > > >> on x86_64:
> > > >>
> > > >> kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x22: can't find switch jump table
> > > > 
> > > > I don't see it on current linux-next/master.  Can you share the config
> > > > and .o?
> > > > 
> > > 
> > > This warning happened in 6 of 10 builds for 20190702.
> > > 
> > > Today (20190703) I don't see it at all, but I see this instead:
> > > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0xa3: sibling call from callable instruction with modified stack frame
> > > 
> > > Kernel (rand)config file and .o file for 20190702 are attached.
> > 
> > Thanks, I'm able to recreate the sibling call warning with a randconfig
> > on today's linux-next.
> > 
> > GCC is doing some funky stuff.  With my randconfig, ___bpf_prog_run()
> > has 166 indirect calls to its jump table.  Seems (a bit!) excessive.
> 
> s/calls/jumps/ obviously
> 
> > 
> > I'll need to look into it.

Hm, it looks like disabling CONFIG_RETPOLINE causes GCC to produce
vastly different code here.  Without retpolines, it's not shy at all
about indirect jumps.

-- 
Josh