netdev - Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num

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Message-ID: <20180328133830.3d5d74d3@gandalf.local.home>
Date:   Wed, 28 Mar 2018 13:38:30 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <ast@...com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        netdev <netdev@...r.kernel.org>,
        kernel-team <kernel-team@...com>,
        linux-api <linux-api@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH v7 bpf-next 06/10] tracepoint: compute num_args at build
 time

On Wed, 28 Mar 2018 10:10:34 -0700
Alexei Starovoitov <ast@...com> wrote:


> > and have:
> >
> > 	u64 tp_offset = (u64)tp - (u64)_sdata;
> >
> > 	if (WARN_ON(tp_offset > UINT_MAX)
> > 		return -EINVAL;
> >
> > 	 btp->tp_offset = (u32)tp_offset;  
> 
> above math has to be build time constant, so warn_on likely
> won't work.

Right, it would require a BUILD_BUG_ON.

> imo the whole thing is too fragile and obscure.
> I suggest to compress this 8 bytes * num_of_tracepoints later.
> Especially would be good to do it in one way for
> bpf_raw_event_map, ftrace and other places.

Fair enough. We can defer this shrinkage to another time. I only
suggested it here over your concern for the added bloat.

-- Steve