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 11:56:12 -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 11:50 AM, Andy Lutomirski <luto@...capital.net> wrote:
> On Aug 15, 2014 10:36 AM, "Alexei Starovoitov" <ast@...mgrid.com> wrote:
>>
>> On Fri, Aug 15, 2014 at 10:20 AM, Andy Lutomirski <luto@...capital.net> wrote:
>> > The downside of this approach is that compat support might be
>> > difficult or impossible.
>>
>> Would do you mean by compat? 32-bit programs on 64-bit kernels?
>> There is no such concept for eBPF. All eBPF programs are always
>> operating on 64-bit registers.
>
> Doesn't the eBPF program need to know sizeof(long) to read these
> fields correctly?  Or am I misunderstanding what the code does?

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...
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