bpf: make error reporting in bpf_warn_invalid_xdp_action more clear

Differ between illegal XDP action code and just driver
unsupported one to provide better feedback when we throw
a one-time warning here. Reason is that with 814abfabef3c
("xdp: add bpf_redirect helper function") not all drivers
support the new XDP return code yet and thus they will
fall into their 'default' case when checking for return
codes after program return, which then triggers a
bpf_warn_invalid_xdp_action() stating that the return
code is illegal, but from XDP perspective it's not.

I decided not to place something like a XDP_ACT_MAX define
into uapi i) given we don't have this either for all other
program types, ii) future action codes could have further
encoding there, which would render such define unsuitable
and we wouldn't be able to rip it out again, and iii) we
rarely add new action codes.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 0848df2..3a50a9b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -3609,7 +3609,11 @@ static bool xdp_is_valid_access(int off, int size,
 
 void bpf_warn_invalid_xdp_action(u32 act)
 {
-	WARN_ONCE(1, "Illegal XDP return value %u, expect packet loss\n", act);
+	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);
 }
 EXPORT_SYMBOL_GPL(bpf_warn_invalid_xdp_action);