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

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On 9/30/20 3:59 PM, Martin KaFai Lau wrote:
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@xxxxxxx>
Signed-off-by: Yonghong Song <yhs@xxxxxx>
---
   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].

Right, I realized later on when I prepare v2. Will send patch v2 shortly.



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


   #undef BTF_SOCK_TYPE
   MAX_BTF_SOCK_TYPE,
   };
--
2.24.1




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