bpf: Fix issue in verifying allow_ptr_leaks

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix issue in verifying allow_ptr_leaks

After we converted the capabilities of our networking-bpf program from

cap_sys_admin to cap_net_admin+cap_bpf, our networking-bpf program

failed to start. Because it failed the bpf verifier, and the error log

is "R3 pointer comparison prohibited".

A simple reproducer as follows,

SEC("cls-ingress")

int ingress(struct __sk_buff *skb)

{

struct iphdr *iph = (void *)(long)skb->data + sizeof(struct ethhdr);

if ((long)(iph + 1) > (long)skb->data_end)

return TC_ACT_STOLEN;

return TC_ACT_OK;

}

Per discussion with Yonghong and Alexei [1], comparison of two packet

pointers is not a pointer leak. This patch fixes it.

Our local kernel is 6.1.y and we expect this fix to be backported to

6.1.y, so stable is CCed.

[1]. https://lore.kernel.org/bpf/CAADnVQ+Nmspr7Si+pxWn8zkE7hX-7s93ugwC+94aXSy4uQ9vBg@mail.gmail.com/