Re: [RFC] bpf: verifier check for dead branch

[Jiri Olsa]

On Mon, Aug 10, 2020 at 10:16:12AM -0700, Yonghong Song wrote:

SNIP

> 
> Thanks for the test case. I can reproduce the issue. The following
> is why this happens in llvm.
> the pseudo IR code looks like
>    data = skb->data
>    data_end = skb->data_end
>    comp = data + 42 > data_end
>    ip = select "comp" nullptr "data + some offset"
>          <=== select return one of nullptr or "data + some offset" based on
> "comp"
>    if comp   // original skb_shorter condition
>       ....
>    ...
>       = ip
> 
> In llvm, bpf backend "select" actually inlined "comp" to generate proper
> control flow. Therefore, comp is computed twice like below
>    data = skb->data
>    data_end = skb->data_end
>    if (data + 42 > data_end) {
>       ip = nullptr; goto block1;
>    } else {
>       ip = data + some_offset;
>       goto block2;
>    }
>    ...
>    if (data + 42 > data_end) // original skb_shorter condition
> 
> The issue can be workarounded the source. Just check data + 42 > data_end
> and if failure return. Then you will be able to assign
> a value to "ip" conditionally.

is the change below what you mean? it produces the same code for me:

        diff --git a/tools/testing/selftests/bpf/progs/verifier-cond-repro.c 
b/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        index 2f11027d7e67..9c401bd00ab7 100644
        --- a/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        +++ b/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        @@ -41,12 +41,10 @@ static INLINE struct iphdr *get_iphdr (struct 
__sk_buff *skb)
                struct ethhdr *eth;
         
                if (skb_shorter(skb, ETH_IPV4_UDP_SIZE))
        -               goto out;
        +               return NULL;
         
                eth = (void *)(long)skb->data;
                ip = (void *)(eth + 1);
        -
        -out:
                return ip;
         }
         

I also tried this one:

        diff --git a/tools/testing/selftests/bpf/progs/verifier-cond-repro.c 
b/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        index 2f11027d7e67..00ff06fe6fdd 100644
        --- a/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        +++ b/tools/testing/selftests/bpf/progs/verifier-cond-repro.c
        @@ -57,7 +57,7 @@ int my_prog(struct __sk_buff *skb)
                __u8 proto = 0;
         
                if (!(ip = get_iphdr(skb)))
        -               goto out;
        +               return -1;
         
                proto = ip->protocol;

it did just slight change in generated code - added 'w0 = -1'
before the second condition

> 
> Will try to fix this issue in llvm12 as well.
> Thanks!

great, could you please CC me on the changes?

thanks a lot!
jirka