netdev - Re: BTFIDS: FAILED unresolved symbol udp6

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Date:   Sun, 3 Jan 2021 00:06:54 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Qais Yousef <qais.yousef@....com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: BTFIDS: FAILED unresolved symbol udp6_sock

On Sat, Jan 02, 2021 at 02:25:34PM -0800, Andrii Nakryiko wrote:

SNIP

> >
> > so your .config has
> >   CONFIG_CRYPTO_DEV_BCM_SPU=y
> >
> > and that defines 'struct device_private' which
> > clashes with the same struct defined in drivers/base/base.h
> >
> > so several networking structs will be doubled, like net_device:
> >
> >         $ bpftool btf dump file ../vmlinux.config | grep net_device\' | grep STRUCT
> >         [2731] STRUCT 'net_device' size=2240 vlen=133
> >         [113981] STRUCT 'net_device' size=2240 vlen=133
> >
> > each is using different 'struct device_private' when it's unwinded
> >
> > and that will confuse BTFIDS logic, becase we have multiple structs
> > with the same name, and we can't be sure which one to pick
> >
> > perhaps we should check on this in pahole and warn earlier with
> > better error message.. I'll check, but I'm not sure if pahole can
> > survive another hastab ;-)
> >
> > Andrii, any ideas on this? ;-)
> 
> It's both unavoidable and correct from the C type system's
> perspective, so there is nothing for pahole to warn about. We used to
> have (for a long time) a similar clash with two completely different
> ring_buffer structs. Eventually they just got renamed to avoid
> duplication of related structs (task_struct and tons of other). But
> both BTF dedup and CO-RE relocation algorithms are designed to handle
> this correctly, ...

AFAIU it's all correctly dedulicated, but still all structs that
contain (at some point) 'struct device_private' will appear twice
in BTF data.. each with different 'struct device_private'

> ... so perhaps BTFIDS should be able to handle this as
> well?

hm, BTFIDS sees BTF data with two same struct names and has no
way to tell which one to use

unless we have some annotation data for BTF types I don't
see a way to handle this correctly.. but I think we can
detect this directly in BTFIDS and print more accurate error
message

as long as we dont see this on daily basis, I think that better
error message + following struct rename is good solution

> 
> >
> > easy fix is the patch below that renames the bcm's structs,
> > it makes the kernel to compile.. but of course the new name
> > is probably wrong and we should push this through that code
> > authors
> 
> In this case, I think renaming generic device_private name is a good
> thing regardless.

ok, I'll send the change

jirka