Re: [PATCH RFC bpf-next 01/10] bpf: Prepare relo_core.c for kernel duty.

From: Matteo Croce <mcroce@linux.microsoft.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	John Fastabend <john.fastabend@gmail.com>,
	Lorenz Bauer <lmb@cloudflare.com>,
	Matteo Croce <mcroce@microsoft.com>, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH RFC bpf-next 01/10] bpf: Prepare relo_core.c for kernel duty.
Date: Wed, 29 Sep 2021 14:32:37 +0200	[thread overview]
Message-ID: <CAFnufp3hx0CaF=ukCXY3UJj0omVX+5WWk0=-QuENvTPGye_sKA@mail.gmail.com> (raw)
In-Reply-To: <CAADnVQ+ajFPKfP+Q5WQFztfZ+05uGgbuQk3H8_9OTny=0vku=g@mail.gmail.com>

On Tue, Sep 28, 2021 at 10:35 PM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Tue, Sep 28, 2021 at 10:11 AM Matteo Croce
> <mcroce@linux.microsoft.com> wrote:
> >
> > On Tue, 28 Sep 2021 09:37:30 -0700
> > Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> >
> > > On Tue, Sep 28, 2021 at 04:45:15PM +0200, Matteo Croce wrote:
> > > > On Fri, 17 Sep 2021 14:57:12 -0700
> > > > Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
> > > >
> > > > > From: Alexei Starovoitov <ast@kernel.org>
> > > > >
> > > > > Make relo_core.c to be compiled with kernel and with libbpf.
> > > > >
> > > > > Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> > > > > ---
> > > >
> > > > I give it a try with a sample co-re program.
> > >
> > > Thanks for testing!
> > >
> >
> > I just found an error during the relocations.
> > It was hiding because of bpf_core_apply_relo() always returning
> > success[1].
> >
> > I have a BPF with the following programs:
> >
> > #if 0
> > SEC("tp_btf/xdp_devmap_xmit")
> > int BPF_PROG(tp_xdp_devmap_xmit_multi, const struct net_device *from_dev,
> >              const struct net_device *to_dev, int sent, int drops, int err)
> >
> > {
> >         randmap(from_dev->ifindex);
> >         return 0;
> > }
> > #endif
> >
> > SEC("fentry/eth_type_trans")
> > int BPF_PROG(fentry_eth_type_trans, struct sk_buff *skb,
> >              struct net_device *dev, unsigned short protocol)
> > {
> >         randmap(dev->ifindex + skb->len);
> >         return 0;
> > }
> >
> > SEC("fexit/eth_type_trans")
> > int BPF_PROG(fexit_eth_type_trans, struct sk_buff *skb,
> >              struct net_device *dev, unsigned short protocol)
> > {
> >         randmap(dev->ifindex + skb->len);
> >         return 0;
> > }
> >
> >
> > randmap() just writes the value into a random map. If I keep #if 0
> > everything works, if I remove it so to build tp_btf/xdp_devmap_xmit
> > too, I get this:
> >
> > [ 3619.229378] libbpf: prog 'prog_name': relo #0: kind <byte_off> (0), spec is [24] STRUCT net_device.ifindex (0:17 @ offset 208)
> > [ 3619.229384] libbpf: prog 'prog_name': relo #0: matching candidate #0 [2617] STRUCT net_device.ifindex (0:17 @ offset 208)
> > [ 3619.229538] libbpf: prog 'prog_name': relo #0: patched insn #0 (LDX/ST/STX) off 208 -> 208
> > [ 3619.230278] libbpf: prog 'prog_name': relo #0: kind <byte_off> (0), spec is [87] STRUCT sk_buff.len (0:5 @ offset 104)
> > [ 3619.230282] libbpf: prog 'prog_name': relo #0: matching candidate #0 [2660] STRUCT sk_buff.len (0:5 @ offset 104)
> > [ 3619.230393] libbpf: prog 'prog_name': relo #0: trying to relocate unrecognized insn #0, code:0x85, src:0x0, dst:0x0, off:0x0, imm:0x7
> > [ 3619.230562] libbpf: prog 'prog_name': relo #0: failed to patch insn #0: -22
> >
> > The program in tp_btf/xdp_devmap_xmit makes the relocations into
> > another section fail, note that sk_buff.len is used in the fentry
> > program.
> >
> > Ideas?
>
> I'll take a look. Could you provide the full .c file?

Sure. I put everything in this repo:

https://gist.github.com/teknoraver/2855e0f8770d1363b57d683fa32bccc3

tp_btf/xdp_devmap_xmit is the program which lets the other fail.

-- 
per aspera ad upstream