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 12:18:53 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
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 12:16 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> 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.

I still don't understand why making them long instead of u64 is
helpful, though.  I feel like I'm missing obvious here.

-- 
Andy Lutomirski
AMA Capital Management, LLC
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