netdev - Re: kernel/bpf/core.o: warning: objtool: ___bpf_prog_run.cold()+0x7: call without frame pointer save/setup

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Message-ID: <20190827145839.dsm6at6hp7rwwrjo@treble>
Date:   Tue, 27 Aug 2019 09:58:39 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     He Zhe <zhe.he@...driver.com>
Cc:     ast@...nel.org, daniel@...earbox.net, kafai@...com,
        songliubraving@...com, yhs@...com, ndesaulniers@...gle.com,
        miguel.ojeda.sandonis@...il.com, luc.vanoostenryck@...il.com,
        schwidefsky@...ibm.com, gregkh@...uxfoundation.org, mst@...hat.com,
        gor@...ux.ibm.com, andreyknvl@...gle.com,
        liuxiaozhou@...edance.com, yamada.masahiro@...ionext.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: kernel/bpf/core.o: warning: objtool: ___bpf_prog_run.cold()+0x7:
 call without frame pointer save/setup

On Tue, Aug 27, 2019 at 10:43:27AM +0800, He Zhe wrote:
> 
> 
> On 8/26/19 11:18 PM, Josh Poimboeuf wrote:
> > On Mon, Aug 26, 2019 at 10:42:53PM +0800, He Zhe wrote:
> >> Hi All,
> >>
> >> Since 3193c0836f20 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()"),
> >> We have got the following warning,
> >> kernel/bpf/core.o: warning: objtool: ___bpf_prog_run.cold()+0x7: call without frame pointer save/setup
> >>
> >> If reverting the above commit, we will get the following warning,
> >> kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x8b9: sibling call from callable instruction with modified stack frame
> >> if CONFIG_RETPOLINE=n, and no warning if CONFIG_RETPOLINE=y
> > Can you please share the following:
> >
> > - core.o file
> 
> Attached.
> 
> >
> > The following would also be helpful for me to try to recreate it:
> >
> > - config file
> > - compiler version
> > - kernel version
> 
> I pasted them in the other reply.

Thanks.  I was able to recreate.  I reduced it to:

void a(b);
__attribute__((optimize(""))) c(void) { a(); }

Apparently '__attribute__((optimize()))' is overwriting GCC cmdline
flags, including -fno-omit-frame-pointer.  I had assumed it would append
instead of replace.

I'm guessing this is a GCC "feature" instead of a bug.  I'll need to
follow up.

-- 
Josh