ebpf: issue with clang

[Eric Leblond <eric@regit.org>]

Hello,

I'm working on integrating ebpf cluster load balancing for AF_PACKET
and I've got some problem to get real code inside the EBPF filter.

I've tried different command lines in the build process. One of them
is:
clang-3.9 -Wall -O2 -emit-llvm -c hash_ports.c -o - | llc-3.9 -march=bpf -filetype=obj -o hash_ports.bpf

If I use that one, then the generated code is almost void. If I remove
the -O2 then I've got a generated code that fails during load. When not
using -O2, I manage to load a trivial filter (return of static value).
 
The C code is the following (a derivative of http-simple-filter.c used
for testing):

int filter(struct __sk_buff *skb) {
	uint8_t *cursor = 0;
	struct ethernet_t *ethernet = cursor_advance(cursor, sizeof(*ethernet));
	
	if (!(ethernet->type == (unsigned int)0x0800)) {
		goto DEFAULT;	
	}
	struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));
	if (ip->nextp != IP_TCP) {
		goto DEFAULT;
	}
    return 1;
DEFAULT:
    return 0;
}

And error is as follows

bpf: Permission denied
bpf verifier:
0: (7b) *(u64 *)(r10 -16) = r1
1: (b7) r1 = 0
2: (7b) *(u64 *)(r10 -24) = r1
3: (7b) *(u64 *)(r10 -40) = r1
4: (79) r2 = *(u64 *)(r10 -24)
5: (07) r2 += 14
6: (7b) *(u64 *)(r10 -24) = r2
7: (79) r2 = *(u64 *)(r10 -40)
8: (7b) *(u64 *)(r10 -48) = r2
9: (7b) *(u64 *)(r10 -32) = r2
10: (71) r3 = *(u8 *)(r2 +12)
R2 invalid mem access 'inv'

This has been tested with a 4.6.0 kernel and a 4.5.x kernel.

What did I miss here ?

BR,
-- 

Eric Leblond <eric@regit.org>
Blog: https://home.regit.org/