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

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Message-ID: <CAE40pdf0pNYyazjpdkzxNJi7iC4LOOr_XEu078OUqP_uoCXnHg@mail.gmail.com>
Date:	Thu, 14 Aug 2014 14:20:05 -0700
From:	Brendan Gregg <brendan.d.gregg@...il.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Steven Rostedt <rostedt@...dmis.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Chema Gonzalez <chema@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>, linux-api@...r.kernel.org,
	netdev@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be
 attached to events

On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov <ast@...mgrid.com> wrote:
[...]
> +/* For tracing filters save first six arguments of tracepoint events.
> + * On 64-bit architectures argN fields will match one to one to arguments passed
> + * to tracepoint events.
> + * On 32-bit architectures u64 arguments to events will be seen into two
> + * consecutive argN, argN+1 fields. Pointers, u32, u16, u8, bool types will
> + * match one to one
> + */
> +struct bpf_context {
> +       unsigned long arg1;
> +       unsigned long arg2;
> +       unsigned long arg3;
> +       unsigned long arg4;
> +       unsigned long arg5;
> +       unsigned long arg6;
> +       unsigned long ret;
> +};

While this works, the argN+1 shift for 32-bit is a gotcha to learn.
Lets say arg1 was 64-bit, and my program only examined arg2. I'd need
two programs, one for 64-bit (using arg2) and 32-bit (arg3). If there
was a way not to shift arguments, I could have one program for both.
Eg, additional arg1hi, arg2hi, ... for the higher order u32s.

Brendan
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