Hello,
We are implementing a basic version of a router. In order to perform the
routing process we implemented a bounded loop that iterates over all
routing table entries looking for a match.
We also used the #pragma directives in order to let clang automatically
unroll the loop, unfortunately when we try to inject the module (using
hover) we get the "back-edge from insn 37 to 13" error, that according
to my understanding means that there are loops in the code.
We have tried different versions of the pragma directives [1], but in
all the cases we continue to get the same error.
The only workaround that we have found so far is to manually unroll the
loop, but this is awful as it complicates the code readability and
maintainability.
This is a portion of the code that gives the error:
#include <linux/ip.h>
#include <linux/bpf.h>
#define BPF_TRACE
#define ROUTING_TABLE_DIM 4
/*Routing Table Entry*/
struct rt_entry {
u32 network; //network: e.g. 192.168.1.0
u32 netmask; //netmask: e.g. 255.255.255.0
u32 port; //port of the router
};
BPF_TABLE("array", u32, struct rt_entry, routing_table, ROUTING_TABLE_DIM);
static int handle_rx(void *skb, struct metadata *md) {
u8 *cursor = 0;
struct ip_t *ip = cursor_advance(cursor, sizeof(*ip));
int i = 0;
struct rt_entry *rt_entry_p = 0;
u32 ip_dst_masked = 0;
//#pragma unroll
#pragma clang loop unroll(full)
for (i = 0; i < ROUTING_TABLE_DIM; i++) {
rt_entry_p = routing_table.lookup(&i);
if (rt_entry_p) {
ip_dst_masked = ip->dst & rt_entry_p->netmask;
if (ip_dst_masked == rt_entry_p->network) {
goto FORWARD;
}
}
}
DROP:
return RX_DROP;
FORWARD:
pkt_redirect(skb,md, rt_entry_p->port);
return RX_REDIRECT;
}
Do you have any idea why clang is not unrolling that loop?
Thanks in advance,
Mauricio Vásquez B
[1] http://clang.llvm.org/docs/LanguageExtensions.html#loop-unrolling
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