netdev - Re: [PATCH bpf] bpf: fix "unresolved symbol" build error with resolve

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Date:   Wed, 30 Sep 2020 15:59:41 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Yonghong Song <yhs@...com>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>,
        Michal Kubecek <mkubecek@...e.cz>
Subject: Re: [PATCH bpf] bpf: fix "unresolved symbol" build error with
 resolve_btfids

On Wed, Sep 30, 2020 at 03:50:10PM -0700, Yonghong Song wrote:
> 
> 
> On 9/30/20 1:58 PM, Martin KaFai Lau wrote:
> > On Wed, Sep 30, 2020 at 09:41:09AM -0700, Yonghong Song wrote:
> > > Michal reported a build failure likes below:
> > >     BTFIDS  vmlinux
> > >     FAILED unresolved symbol tcp_timewait_sock
> > >     make[1]: *** [/.../linux-5.9-rc7/Makefile:1176: vmlinux] Error 255
> > > 
> > > This error can be triggered when config has CONFIG_NET enabled
> > > but CONFIG_INET disabled. In this case, there is no user of
> > > structs inet_timewait_sock and tcp_timewait_sock and hence vmlinux BTF
> > > types are not generated for these two structures.
> > > 
> > > To fix the problem, omit the above two types for BTF_SOCK_TYPE_xxx
> > > macro if CONFIG_INET is not defined.
> > > 
> > > Fixes: fce557bcef11 ("bpf: Make btf_sock_ids global")
> > > Reported-by: Michal Kubecek <mkubecek@...e.cz>
> > > Signed-off-by: Yonghong Song <yhs@...com>
> > > ---
> > >   include/linux/btf_ids.h | 20 ++++++++++++++++----
> > >   1 file changed, 16 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > > index 4867d549e3c1..d9a1e18d0921 100644
> > > --- a/include/linux/btf_ids.h
> > > +++ b/include/linux/btf_ids.h
> > > @@ -102,24 +102,36 @@ asm(							\
> > >    * skc_to_*_sock() helpers. All these sockets should have
> > >    * sock_common as the first argument in its memory layout.
> > >    */
> > > -#define BTF_SOCK_TYPE_xxx \
> > > +
> > > +#define __BTF_SOCK_TYPE_xxx \
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_INET, inet_sock)			\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_INET_CONN, inet_connection_sock)	\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_INET_REQ, inet_request_sock)	\
> > > -	BTF_SOCK_TYPE(BTF_SOCK_TYPE_INET_TW, inet_timewait_sock)	\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_REQ, request_sock)			\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_SOCK, sock)				\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_SOCK_COMMON, sock_common)		\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_TCP, tcp_sock)			\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_TCP_REQ, tcp_request_sock)		\
> > > -	BTF_SOCK_TYPE(BTF_SOCK_TYPE_TCP_TW, tcp_timewait_sock)		\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_TCP6, tcp6_sock)			\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_UDP, udp_sock)			\
> > >   	BTF_SOCK_TYPE(BTF_SOCK_TYPE_UDP6, udp6_sock)
> > > +#define __BTF_SOCK_TW_TYPE_xxx \
> > > +	BTF_SOCK_TYPE(BTF_SOCK_TYPE_INET_TW, inet_timewait_sock)	\
> > > +	BTF_SOCK_TYPE(BTF_SOCK_TYPE_TCP_TW, tcp_timewait_sock)
> > > +
> > > +#ifdef CONFIG_INET
> > > +#define BTF_SOCK_TYPE_xxx						\
> > > +	__BTF_SOCK_TYPE_xxx						\
> > > +	__BTF_SOCK_TW_TYPE_xxx
> > > +#else
> > > +#define BTF_SOCK_TYPE_xxx	__BTF_SOCK_TYPE_xxx
> > BTF_SOCK_TYPE_xxx is used in BTF_ID_LIST_GLOBAL(btf_sock_ids) in filter.c
> > which does not include BTF_SOCK_TYPE_TCP_TW.
> > However, btf_sock_ids[BTF_SOCK_TYPE_TCP_TW] is still used
> > in bpf_skc_to_tcp_timewait_sock_proto.
> > 
> > > +#endif
> > > +
> > >   enum {
> > >   #define BTF_SOCK_TYPE(name, str) name,
> > > -BTF_SOCK_TYPE_xxx
> > > +__BTF_SOCK_TYPE_xxx
> > > +__BTF_SOCK_TW_TYPE_xxx
> > BTF_SOCK_TYPE_TCP_TW is at the end of this enum.
> > 
> > Would btf_sock_ids[BTF_SOCK_TYPE_TCP_TW] always be 0?
> 
> No. If CONFIG_INET is y, the above BTF_SOCK_TYPE_xxx contains
>   __BTF_SOCK_TW_TYPE_xxx
> and
>   btf_sock_ids[BTF_SOCK_TYPE_TCP_TW] will be calculated properly.
> 
> But if CONFIG_INET is n, then BTF_SOCK_TYPE_xxx will not contain
>    __BTF_SOCK_TW_TYPE_xxx
> so btf_sock_ids[BTF_SOCK_TYPE_TCP_TW] will have default value 0
> as btf_sock_ids is a global.
I could be missing something here.
Why btf_sock_ids[BTF_SOCK_TYPE_TCP_TW] must be 0?
How does BTF_ID_LIST_GLOBAL() know there is a
btf_sock_ids[BTF_SOCK_TYPE_TCP_TW]?
I would expect at least BTF_ID_UNUSED is required.
Otherwise, the value will be whatever follow the last
btf_sock_ids[BTF_SOCK_TYPE_UDP6].

> 
> Will send v2 to add some comments to make it easy to understand.
> 
> > 
> > >   #undef BTF_SOCK_TYPE
> > >   MAX_BTF_SOCK_TYPE,
> > >   };
> > > -- 
> > > 2.24.1
> > >