From: Maciej Żenczykowski <maze@xxxxxxxxxx> Android has long had an extension to IDLETIMER to send netlink messages to userspace, see: https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/include/uapi/linux/netfilter/xt_IDLETIMER.h#42 Note: this is idletimer target rev 1, there is no rev 0 in the Android common kernel sources, see registration at: https://android.googlesource.com/kernel/common/+/refs/heads/android-mainline/net/netfilter/xt_IDLETIMER.c#483 When we compare that to upstream's new idletimer target rev 1: https://git.kernel.org/pub/scm/linux/kernel/git/pablo/nf-next.git/tree/include/uapi/linux/netfilter/xt_IDLETIMER.h#n46 We immediately notice that these two rev 1 structs are the same size and layout, and that while timer_type and send_nl_msg are differently named and serve a different purpose, they're at the same offset. This makes them impossible to tell apart - and thus one cannot know in a mixed Android/vanilla environment whether one means timer_type or send_nl_msg. Since this is iptables/netfilter uapi it introduces a problem between iptables (vanilla vs Android) userspace and kernel (vanilla vs Android) if the two don't match each other. Additionally when at some point in the future Android picks up 5.7+ it's not at all clear how to resolve the resulting merge conflict. Furthermore, since upgrading the kernel on old Android phones is pretty much impossible there does not seem to be an easy way out of this predicament. The only thing I've been able to come up with is some super disgusting kernel version >= 5.7 check in the iptables binary to flip between different struct layouts. By adding a dummy field to the vanilla Linux kernel header file we can force the two structs to be compatible with each other. Long term I think I would like to deprecate send_nl_msg out of Android entirely, but I haven't quite been able to figure out exactly how we depend on it. It seems to be very similar to sysfs notifications but with some extra info. Currently it's actually always enabled whenever Android uses the IDLETIMER target, so we could also probably entirely remove it from the uapi in favour of just always enabling it, but again we can't upgrade old kernels already in the field. (Also note that this doesn't change the structure's size, as it is simply fitting into the pre-existing padding, and that since 5.7 hasn't been released yet, there's still time to make this uapi visible change) Cc: Manoj Basapathi <manojbm@xxxxxxxxxxxxxx> Cc: Subash Abhinov Kasiviswanathan <subashab@xxxxxxxxxxxxxx> Signed-off-by: Maciej Żenczykowski <maze@xxxxxxxxxx> Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx> --- include/uapi/linux/netfilter/xt_IDLETIMER.h | 1 + net/netfilter/xt_IDLETIMER.c | 3 +++ 2 files changed, 4 insertions(+) diff --git a/include/uapi/linux/netfilter/xt_IDLETIMER.h b/include/uapi/linux/netfilter/xt_IDLETIMER.h index 434e6506abaa..49ddcdc61c09 100644 --- a/include/uapi/linux/netfilter/xt_IDLETIMER.h +++ b/include/uapi/linux/netfilter/xt_IDLETIMER.h @@ -48,6 +48,7 @@ struct idletimer_tg_info_v1 { char label[MAX_IDLETIMER_LABEL_SIZE]; + __u8 send_nl_msg; /* unused: for compatibility with Android */ __u8 timer_type; /* for kernel module internal use only */ diff --git a/net/netfilter/xt_IDLETIMER.c b/net/netfilter/xt_IDLETIMER.c index 75bd0e5dd312..7b2f359bfce4 100644 --- a/net/netfilter/xt_IDLETIMER.c +++ b/net/netfilter/xt_IDLETIMER.c @@ -346,6 +346,9 @@ static int idletimer_tg_checkentry_v1(const struct xt_tgchk_param *par) pr_debug("checkentry targinfo%s\n", info->label); + if (info->send_nl_msg) + return -EOPNOTSUPP; + ret = idletimer_tg_helper((struct idletimer_tg_info *)info); if(ret < 0) { -- 2.11.0
