Re: [PATCH bpf] bpf: fix "unresolved symbol" build error with resolve_btfids

[Yonghong Song <yhs@fb.com>]

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@suse.cz>
>> Signed-off-by: Yonghong Song <yhs@fb.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.

Will send v2 to add some comments to make it easy to understand.

> 
>>   #undef BTF_SOCK_TYPE
>>   MAX_BTF_SOCK_TYPE,
>>   };
>> -- 
>> 2.24.1
>>