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

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Message-ID: <CALCETrUNvg88_VrwE+9RKnSgts=3zVcXbX6tuNLgUxJU436CrQ@mail.gmail.com>
Date:	Fri, 15 Aug 2014 12:02:51 -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 11:56 AM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> 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...

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.

--Andy
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