netdev - Re: [PATCH RFC 0/4] Add support to directly attach BPF program to ftrace

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Date:   Fri, 26 Jul 2019 15:49:30 -0400
From:   Joel Fernandes <joelaf@...gle.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        LKML <linux-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>
Subject: Re: [PATCH RFC 0/4] Add support to directly attach BPF program to ftrace

On Fri, Jul 26, 2019 at 3:18 PM Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Fri, Jul 26, 2019 at 11:39:56AM -0700, Alexei Starovoitov wrote:
[snip]
> > > For bpf program:
> > > https://android.googlesource.com/platform/system/bpfprogs/+/908f6cd718fab0de7a944f84628c56f292efeb17%5E%21/
> >
> > what is unsafe_bpf_map_update_elem() in there?
> > The verifier comment sounds odd.
> > Could you describe the issue you see with the verifier?
>
> Will dig out the verifier issue I was seeing. I was just trying to get a
> prototype working so I did not go into verifier details much.

This is actually slightly old code, the actual function name is
bpf_map_update_elem_unsafe() .
 https://android.googlesource.com/platform/system/bpf/+/refs/heads/master/progs/include/bpf_helpers.h#39

This function came about because someone added a DEFINE_BPF_MAP macro
which defines BPF map accessors based on the type of the key and
value. So that's the "safe" variant:
https://android.googlesource.com/platform/system/bpf/+/refs/heads/master/progs/include/bpf_helpers.h#54
(added in commit
https://android.googlesource.com/platform/system/bpf/+/6564b8eac46fc27dde807a39856386d98d2471c3)

So the "safe" variant of the bpf_map_update_elem for us became a map
specific version with a prototype:
static inline __always_inline __unused int
bpf_##the_map##_update_elem(TypeOfKey* k, TypeOfValue* v, unsigned
long long flags)

Since I had not upgraded my BPF program to the "safe" variant, I had
to use the internal "unsafe" variant of the API (if that makes
sense..).

thanks Alexei!

- Joel