linux-kernel - Re: linux-next: build failure after merge of the net-next tree

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Date:   Fri, 12 Jan 2018 07:56:10 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     David Miller <davem@...emloft.net>, sfr@...b.auug.org.au,
        netdev@...r.kernel.org, ast@...nel.org, linux-next@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: linux-next: build failure after merge of the net-next tree

On Fri, Jan 12, 2018 at 11:45:42AM +0100, Daniel Borkmann wrote:
> On 01/12/2018 05:21 AM, Alexei Starovoitov wrote:
> > On Thu, Jan 11, 2018 at 10:11:45PM -0500, David Miller wrote:
> >> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> >> Date: Wed, 10 Jan 2018 17:58:54 -0800
> >>
> >>> On Thu, Jan 11, 2018 at 11:53:55AM +1100, Stephen Rothwell wrote:
> >>>> Hi all,
> >>>>
> >>>> After merging the net-next tree, today's linux-next build (x86_64
> >>>> allmodconfig) failed like this:
> >>>>
> >>>> kernel/bpf/verifier.o: In function `bpf_check':
> >>>> verifier.c:(.text+0xd86e): undefined reference to `bpf_patch_call_args'
> >>>>
> >>>> Caused by commit
> >>>>
> >>>>   1ea47e01ad6e ("bpf: add support for bpf_call to interpreter")
> >>>>
> >>>> interacting with commit
> >>>>
> >>>>   290af86629b2 ("bpf: introduce BPF_JIT_ALWAYS_ON config")
> >>>>
> >>>> from the bpf and net trees.
> >>>>
> >>>> I have just reverted commit 290af86629b2 for today.  A better solution
> >>>> would be nice (lie fixing this in a merge between the net-next and net
> >>>> trees).
> >>>
> >>> that's due to 'endif' from 290af86629b2 needs to be moved above
> >>> bpf_patch_call_args() definition.
> >>
> >> That doesn't fix it, because then you'd need to expose
> >> interpreters_args as well and obviously that can't be right.
> >>
> >> Instead, we should never call bpf_patch_call_args() when JIT always on
> >> is enabled.  So if we fail to JIT the subprogs we should fail
> >> immediately.
> > 
> > right, as I was trying to say one extra hunk would be needed for net-next.
> > I was reading this patch:
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index a2b211262c25..ca80559c4ec3 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5267,7 +5267,11 @@ static int fixup_call_args(struct bpf_verifier_env *env)
> >                 depth = get_callee_stack_depth(env, insn, i);
> >                 if (depth < 0)
> >                         return depth;
> > +#ifdef CONFIG_BPF_JIT_ALWAYS_ON
> > +               return -ENOTSUPP;
> > +#else
> >                 bpf_patch_call_args(insn, depth);
> > +#endif
> >         }
> >         return 0;
> > 
> > but below should be fine too.
> > Will test it asap.
> > 
> >> This is the net --> net-next merge resolution I am about to use to fix
> >> this:
> >>
> >> ...
> >>  +static int fixup_call_args(struct bpf_verifier_env *env)
> >>  +{
> >>  +	struct bpf_prog *prog = env->prog;
> >>  +	struct bpf_insn *insn = prog->insnsi;
> >> - 	int i, depth;
> >> ++	int i, depth, err;
> >>  +
> >> - 	if (env->prog->jit_requested)
> >> - 		if (jit_subprogs(env) == 0)
> >> ++	err = 0;
> 
> Looks fine to me. The only thing I was wondering was whether we should
> set err = -ENOTSUPP here above, but actually that is unnecessary. Say,
> if for some reason we would missed to set prog->jit_requested bit under
> CONFIG_BPF_JIT_ALWAYS_ON, we would return 0 here even if we would have
> calls in the prog. But that also means for bpf_prog_load() that right
> after bpf_check() returned, we would go into bpf_prog_select_runtime()
> since prog->bpf_func is still NULL at that point, and bpf_int_jit_compile()
> from there wouldn't do anything either since prog->jit_requested was
> not set in the first place, therefore we return with -ENOTSUPP from
> there. So the resolution looks fine to me, we can leave it as is.

jit_subprogs() can fail, so err = -ENOTSUPP is necessary.