netdev - [PATCH net-next] uapi: in6: restore visibility of most IPv6 socket options

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Message-ID: <20250609143933.1654417-1-kuba@kernel.org>
Date: Mon,  9 Jun 2025 07:39:33 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: davem@...emloft.net
Cc: netdev@...r.kernel.org,
	edumazet@...gle.com,
	pabeni@...hat.com,
	andrew+netdev@...n.ch,
	horms@...nel.org,
	Jakub Kicinski <kuba@...nel.org>,
	stephen@...workplumber.org,
	hannes@...essinduktion.org,
	willemb@...gle.com
Subject: [PATCH net-next] uapi: in6: restore visibility of most IPv6 socket options

A decade ago commit 6d08acd2d32e ("in6: fix conflict with glibc")
hid the definitions of IPV6 options, because GCC was complaining
about duplicates. The commit did not list the warnings seen, but
trying to recreate them now I think they are (building iproute2):

In file included from ./include/uapi/rdma/rdma_user_cm.h:39,
                 from rdma.h:16,
                 from res.h:9,
                 from res-ctx.c:7:
../include/uapi/linux/in6.h:171:9: warning: ‘IPV6_ADD_MEMBERSHIP’ redefined
  171 | #define IPV6_ADD_MEMBERSHIP     20
      |         ^~~~~~~~~~~~~~~~~~~
In file included from /usr/include/netinet/in.h:37,
                 from rdma.h:13:
/usr/include/bits/in.h:233:10: note: this is the location of the previous definition
  233 | # define IPV6_ADD_MEMBERSHIP    IPV6_JOIN_GROUP
      |          ^~~~~~~~~~~~~~~~~~~
../include/uapi/linux/in6.h:172:9: warning: ‘IPV6_DROP_MEMBERSHIP’ redefined
  172 | #define IPV6_DROP_MEMBERSHIP    21
      |         ^~~~~~~~~~~~~~~~~~~~
/usr/include/bits/in.h:234:10: note: this is the location of the previous definition
  234 | # define IPV6_DROP_MEMBERSHIP   IPV6_LEAVE_GROUP
      |          ^~~~~~~~~~~~~~~~~~~~

Compilers don't complain about redefinition if the defines
are identical, but here we have the kernel using the literal
value, and glibc using an indirection (defining to a name
of another define, with the same numerical value).

Problem is, the commit in question hid all the IPV6 socket
options, and glibc has a pretty sparse list. For instance
it lacks Flow Label related options. Willem called this out
in commit 3fb321fde22d ("selftests/net: ipv6 flowlabel"):

  /* uapi/glibc weirdness may leave this undefined */
  #ifndef IPV6_FLOWINFO
  #define IPV6_FLOWINFO 11
  #endif

More interestingly some applications (socat) use
a #ifdef IPV6_FLOWINFO to gate compilation of thier
rudimentary flow label support. (For added confusion
socat misspells it as IPV4_FLOWINFO in some places.)

Hide only the two defines we know glibc has a problem
with. If we discover more warnings we can hide more
but we should avoid covering the entire block of
defines for "IPV6 socket options".

Signed-off-by: Jakub Kicinski <kuba@...nel.org>
---
CC: stephen@...workplumber.org
CC: hannes@...essinduktion.org
CC: willemb@...gle.com
---
 include/uapi/linux/in6.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/in6.h b/include/uapi/linux/in6.h
index ff8d21f9e95b..5a47339ef7d7 100644
--- a/include/uapi/linux/in6.h
+++ b/include/uapi/linux/in6.h
@@ -152,7 +152,6 @@ struct in6_flowlabel_req {
 /*
  *	IPV6 socket options
  */
-#if __UAPI_DEF_IPV6_OPTIONS
 #define IPV6_ADDRFORM		1
 #define IPV6_2292PKTINFO	2
 #define IPV6_2292HOPOPTS	3
@@ -169,8 +168,10 @@ struct in6_flowlabel_req {
 #define IPV6_MULTICAST_IF	17
 #define IPV6_MULTICAST_HOPS	18
 #define IPV6_MULTICAST_LOOP	19
+#if __UAPI_DEF_IPV6_OPTIONS
 #define IPV6_ADD_MEMBERSHIP	20
 #define IPV6_DROP_MEMBERSHIP	21
+#endif
 #define IPV6_ROUTER_ALERT	22
 #define IPV6_MTU_DISCOVER	23
 #define IPV6_MTU		24
@@ -203,7 +204,6 @@ struct in6_flowlabel_req {
 #define IPV6_IPSEC_POLICY	34
 #define IPV6_XFRM_POLICY	35
 #define IPV6_HDRINCL		36
-#endif

 /*
  * Multicast:
-- 
2.49.0