Re: WARNING in bpf_warn_invalid_xdp_action

[Eric Dumazet <eric.dumazet@gmail.com>]

On 1/22/20 1:01 PM, syzbot wrote:
> syzbot has bisected this bug to:
> 
> commit 58956317c8de52009d1a38a721474c24aef74fe7
> Author: David Ahern <dsahern@gmail.com>
> Date:   Fri Dec 7 20:24:57 2018 +0000
> 
>     neighbor: Improve garbage collection
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=124a5985e00000
> start commit:   d0f41851 net, ip_tunnel: fix namespaces move
> git tree:       net
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=114a5985e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=164a5985e00000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d9290aeb7e6cf1c4
> dashboard link: https://syzkaller.appspot.com/bug?extid=8ce4113dadc4789fac74
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11f99369e00000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13d85601e00000
> 
> Reported-by: syzbot+8ce4113dadc4789fac74@syzkaller.appspotmail.com
> Fixes: 58956317c8de ("neighbor: Improve garbage collection")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> 

bisection looks bogus...

It would be nice to have alternative helpers to conveniently replace some WARN_ON/WARN_ONCE/...
and not having to hand-code stuff like :

diff --git a/net/core/filter.c b/net/core/filter.c
index 538f6a735a19f017df8e10149cb578107ddc8cbb..633988f7c81b3b4f015d827ccb485e8b227ad20b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6913,11 +6913,15 @@ static bool xdp_is_valid_access(int off, int size,
 
 void bpf_warn_invalid_xdp_action(u32 act)
 {
+       static bool __section(.data.once) warned;
        const u32 act_max = XDP_REDIRECT;
 
-       WARN_ONCE(1, "%s XDP return value %u, expect packet loss!\n",
-                 act > act_max ? "Illegal" : "Driver unsupported",
-                 act);
+       if (!warned) {
+               warned = true;
+               pr_err("%s XDP return value %u, expect packet loss!\n",
+                      act > act_max ? "Illegal" : "Driver unsupported", act);
+               dump_stack();
+       }
 }
 EXPORT_SYMBOL_GPL(bpf_warn_invalid_xdp_action);