On Tue, 28 Sep 2021 09:37:30 -0700
Alexei Starovoitov [off-list ref] wrote:

On Tue, Sep 28, 2021 at 04:45:15PM +0200, Matteo Croce wrote:

quoted

On Fri, 17 Sep 2021 14:57:12 -0700
Alexei Starovoitov [off-list ref] wrote:

quoted

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?

[1] https://lore.kernel.org/bpf/20210927200410.460e014f@linux.microsoft.com/ (local)

Regards,
-- 
per aspera ad upstream