netdev - Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be attached to events

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Date:	Fri, 15 Aug 2014 12:16:11 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux API <linux-api@...r.kernel.org>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Kees Cook <keescook@...omium.org>,
	Network Development <netdev@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be
 attached to events

On Fri, Aug 15, 2014 at 12:02 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>
>> correct. eBPF program would be using 8-byte read on 64-bit kernel
>> and 4-byte read on 32-bit kernel. Same with access to ptrace fields
>> and pretty much all other fields in the kernel. The program will be
>> different on different kernels.
>> Say, this bpf_context struct doesn't exist at all. The programs would
>> still need to be different to walk in-kernel data structures...
>
> Hmm.  I guess this isn't so bad.
>
> What's the actual difficulty with using u64?  ISTM that, if the clang
> front-end can't deal with u64, there's a bigger problem.  Or is it
> something else I don't understand.

clang/llvm has no problem with u64 :)
This bpf_context struct for tracing is trying to answer the question:
 'what's the most convenient way to access tracepoint arguments
from a script'.
When kernel code has something like:
 trace_kfree_skb(skb, net_tx_action);
the script needs to be able to access this 'skb' and 'net_tx_action'
values through _single_ data structure.
In this proposal they are ctx->arg1 and ctx->arg2.
I've considered having different bpf_context's for every event, but
the complexity explodes. I need to hack all event definitions and so on.
imo it's better to move complexity to userspace, so program author
or high level language abstracts these details.
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