yonghong-song wrote:
> what veristat says before/after?
I tried with latest bpf-next and latest llvm trunk. The following is the
difference
with/without this patch:
```
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$ ./veristat *.bpf.o -o
csv > old.csv
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$ ./veristat *.bpf.o -o
csv > new.csv
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$ ./veristat -e
file,prog,insns -C old.csv new.csv -f 'insns_diff!=0'
File Program Insns (A)
Insns (B) Insns (DIFF)
------------------------------- ---------------------------------- ---------
--------- -------------
cgroup_skb_sk_lookup_kern.bpf.o ingress_lookup 96
106 +10 (+10.42%)
cgroup_tcp_skb.bpf.o client_egress 94
104 +10 (+10.64%)
cgroup_tcp_skb.bpf.o client_egress_srv 115
125 +10 (+8.70%)
cgroup_tcp_skb.bpf.o client_ingress 115
125 +10 (+8.70%)
cgroup_tcp_skb.bpf.o client_ingress_srv 94
104 +10 (+10.64%)
cgroup_tcp_skb.bpf.o server_egress 115
125 +10 (+8.70%)
cgroup_tcp_skb.bpf.o server_egress_srv 94
104 +10 (+10.64%)
cgroup_tcp_skb.bpf.o server_ingress 129
139 +10 (+7.75%)
cgroup_tcp_skb.bpf.o server_ingress_srv 137
147 +10 (+7.30%)
verifier_array_access.bpf.o an_array_with_a_constant_too_small 7
9 +2 (+28.57%)
[~/work/bpf-next/tools/testing/selftests/bpf (master)]$
```
For cgroup_tcp_skb.c, e.g.,
```
SEC("cgroup_skb/egress")
int server_egress_srv(struct __sk_buff *skb)
{
struct tcphdr tcph;
if (!needed_tcp_pkt(skb, &tcph))
return 1;
...
```
the tcphdr is larger so it costs more with initializing to 0.
The same for cgroup_skb_sk_lookup_kern.c.
For verifier_array_access.c,
```
SEC("socket")
__description("invalid map access into an array using constant smaller than
key_size")
__failure __msg("R0 invalid mem access 'map_value_or_null'")
unsigned int an_array_with_a_constant_too_small(void)
{
__u32 __attribute__((aligned(8))) key;
struct test_val *val;
/* Mark entire key as STACK_MISC */
bpf_probe_read_user(&key, sizeof(key), NULL);
...
}
```
the 'key' is initialized.
>
> re: selftests
>
> there should be a flag to disable this, so the tests can remain as-is (with
> only Makefile change).
Will do.
>
> There is also -ftrivial-auto-var-init-max-size= flag. What is the default?
> Looks like: char buf[64]; will be zero inited as well?
The default is 0 (-ftrivial-auto-var-init-max-size=0) which means there is no
limitation.
yes, char buf[64] and all buf[64] will be initialized to 0 if the compiler does
not know whehter any buf[i](i = 0...63) is used or not.
>That will probably hurt performance/verification for cases like: char
>comm[16]; bpf_get_current_comm(comm, ...) ?
I checked. In almost all cases in bpf selftest progs, comm[16] is a field in a
map, so it is not impacted.
https://github.com/llvm/llvm-project/pull/125601
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