On 6/16/21 5:05 PM, Vincent Li wrote:
Hi BPF Experts,
I had a problem that verifier report "R1 invalid mem access 'inv'" when
I attempted to rewrite packet destination ethernet MAC address in Cilium
tunnel mode, I opened an issue
with detail here https://github.com/cilium/cilium/issues/16571:
I have couple of questions in general to try to understand the compiler,
BPF byte code, and the verifier.
1 Why the BPF byte code changes so much with my simple C code change
a: BPF byte code before C code change:
0000000000006068 <LBB12_410>:
3085: bf a2 00 00 00 00 00 00 r2 = r10
; tunnel = map_lookup_elem(&TUNNEL_MAP, key);
3086: 07 02 00 00 78 ff ff ff r2 += -136
3087: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
3089: 85 00 00 00 01 00 00 00 call 1
; if (!tunnel)
3090: 15 00 06 01 00 00 00 00 if r0 == 0 goto +262 <LBB12_441>
; key.tunnel_id = seclabel;
3091: 18 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r2 = 0 ll
3093: 67 02 00 00 20 00 00 00 r2 <<= 32
3094: 77 02 00 00 20 00 00 00 r2 >>= 32
3095: b7 01 00 00 06 00 00 00 r1 = 6
3096: 15 02 02 00 01 00 00 00 if r2 == 1 goto +2 <LBB12_413>
3097: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
00000000000060d8 <LBB12_413>:
; return __encap_and_redirect_with_nodeid(ctx, tunnel->ip4,
seclabel, monitor);
b: BPF byte code after C code change:
the C code diff change:
diff --git a/bpf/lib/encap.h b/bpf/lib/encap.h
index dfd87bd82..19199429d 100644
--- a/bpf/lib/encap.h
+++ b/bpf/lib/encap.h
@@ -187,6 +187,8 @@ encap_and_redirect_lxc(struct __ctx_buff *ctx, __u32
tunnel_endpoint,
struct endpoint_key *key, __u32 seclabel, __u32
monitor)
{
struct endpoint_key *tunnel;
+#define VTEP_MAC { .addr = { 0xce, 0x72, 0xa7, 0x03, 0x88, 0x58 } }
+ union macaddr vtep_mac = VTEP_MAC;
if (tunnel_endpoint) {
#ifdef ENABLE_IPSEC
@@ -221,6 +223,8 @@ encap_and_redirect_lxc(struct __ctx_buff *ctx, __u32
tunnel_endpoint,
seclabel);
}
#endif
+ if (eth_store_daddr(ctx, (__u8 *) &vtep_mac.addr, 0) < 0)
+ return DROP_WRITE_ERROR;
return __encap_and_redirect_with_nodeid(ctx, tunnel->ip4,
seclabel, monitor);
}
the result BPF byte code
0000000000004468 <LBB3_274>:
2189: bf a2 00 00 00 00 00 00 r2 = r10
; tunnel = map_lookup_elem(&TUNNEL_MAP, key);
2190: 07 02 00 00 50 ff ff ff r2 += -176
2191: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
2193: 85 00 00 00 01 00 00 00 call 1
2194: bf 07 00 00 00 00 00 00 r7 = r0
2195: 79 a6 48 ff 00 00 00 00 r6 = *(u64 *)(r10 - 184)
; if (!tunnel)
2196: 55 07 94 00 00 00 00 00 if r7 != 0 goto +148 <LBB3_289>
00000000000044a8 <LBB3_275>:
; __u8 new_ttl, ttl = ip4->ttl;
2197: 79 a1 38 ff 00 00 00 00 r1 = *(u64 *)(r10 - 200)
2198: 71 13 16 00 00 00 00 00 r3 = *(u8 *)(r1 + 22)
; if (ttl <= 1)
2199: 25 03 01 00 01 00 00 00 if r3 > 1 goto +1 <LBB3_277>
2200: 05 00 20 ff 00 00 00 00 goto -224 <LBB3_253>
You can see that:
before change: <LBB12_410>
after change <LBB3_274>
is different that <LBB12_410> has instructions 3091, 3092... but
<LBB3_274> end with instruction 2196
before change: <LBB12_413> follows <LBB12_410>
after change: <LBB3_275> follows <LBB3_274>
<LBB12_413> and <LBB3_275> is very much different
and <LBB3_275> instruction 2198 is the one with "R1 invalid mem access
'inv'"
Why <LBB3_275> follows <LBB3_274> ? from C code, <LBB3_275> is not close
to <LBB3_274>.
The cilium code has a lot of functions inlined and after inlining, clang
may do reordering based on its internal heuristics. It is totally
possible slight code change may cause generated codes quite different.
Regarding to
> and <LBB3_275> instruction 2198 is the one with "R1 invalid mem access
> 'inv'"
> 00000000000044a8 <LBB3_275>:
> ; __u8 new_ttl, ttl = ip4->ttl;
> 2197: 79 a1 38 ff 00 00 00 00 r1 = *(u64 *)(r10 - 200)
> 2198: 71 13 16 00 00 00 00 00 r3 = *(u8 *)(r1 + 22)
> ; if (ttl <= 1)
> 2199: 25 03 01 00 01 00 00 00 if r3 > 1 goto +1 <LBB3_277>
> 2200: 05 00 20 ff 00 00 00 00 goto -224 <LBB3_253>
Looks like "ip4" is spilled on the stack. and maybe the stack
save/restore for "ip4" does not preserve its original type.
This mostly happens to old kernels, I think.
If you have verifier log, it may help identify issues easily.
2, Can I assume the verifier is to simulate the order of BPF byte
code execution in run time, like if without any jump or goto in
<LBB3_274>, <LBB3_275> will be executed after <LBB3_274>?
The verifier will try to verify both paths, jumping to LBB3_289
or fall back to LBB3_275.
Enterprise Network Engineer
F5 Networks Inc
https://www.youtube.com/c/VincentLi
