linux-kernel - Re: [PATCH] bpf: Tweak BPF jump table optimizations for objtool compatibility

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Date:   Tue, 5 May 2020 14:31:51 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] bpf: Tweak BPF jump table optimizations for objtool
 compatibility

On Tue, May 05, 2020 at 12:14:05PM -0700, Alexei Starovoitov wrote:
> > 
> > Hi,
> > 
> > I see the objtool warning:
> > kernel/bpf/core.o: warning: objtool: ___bpf_prog_run()+0x33: call without frame pointer save/setup
> > 
> > when using:
> > gcc (SUSE Linux) 9.3.1 20200406 [revision 6db837a5288ee3ca5ec504fbd5a765817e556ac2]
> > 
> > with the attached config file.
> 
> Thanks Randy. I reproduced it.

This problem isn't a mystery, it's caused by __attribute__((optimize)).

The only real solution is to revert

  3193c0836f20 ("bpf: Disable GCC -fgcse optimization for ___bpf_prog_run()")

Once you do that (and disable retpolines) then you should see the
problem described in my other email.

-- 
Josh