ebpf: Ensure the bounds of log buffer

eBPF verifier rejects programs which are not checking the bounds of the
log buffer before writing any arguments. This change ensures that
written log arguments.

In practice, it means that doing this kind of checks is not going to be
needed in eBPF program code anymore:

33a1aee2ea/echo-ebpf/src/main.rs (L47)

Tested on:

876f8b4551

Signed-off-by: Michal Rostecki <vadorovsky@gmail.com>

ebpf/aya-log-ebpf-macros/src/expand.rs

@@ -92,6 +92,9 @@ pub(crate) fn log(args: LogArgs, level: Option<TokenStream>) -> Result<TokenStre
             let write_args = quote! {{
                 use ::aya_log_ebpf::WriteToBuf;
                 Ok::<_, ()>(record_len) #( .and_then(|record_len| {
+                    if record_len >= buf.buf.len() {
+                        return Err(());
+                    }
                     { #formatting_exprs }.write(&mut buf.buf[record_len..]).map(|len| record_len + len)
                 }) )*
             }};