netdev - Re: [RFC PATCH bpf-next] bpf: change syscall_nr type to int in struct syscall_tp

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Date: Fri, 13 Oct 2023 12:43:18 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Artem Savkov <asavkov@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org, 
	netdev@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>, 
	linux-rt-users@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [RFC PATCH bpf-next] bpf: change syscall_nr type to int in struct syscall_tp_t

On Fri, Oct 13, 2023 at 7:00 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Fri, 13 Oct 2023 08:01:34 +0200
> Artem Savkov <asavkov@...hat.com> wrote:
>
> > > But looking at [0] and briefly reading some of the discussions you,
> > > Steven, had. I'm just wondering if it would be best to avoid
> > > increasing struct trace_entry altogether? It seems like preempt_count
> > > is actually a 4-bit field in trace context, so it doesn't seem like we
> > > really need to allocate an entire byte for both preempt_count and
> > > preempt_lazy_count. Why can't we just combine them and not waste 8
> > > extra bytes for each trace event in a ring buffer?
> > >
> > >   [0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?id=b1773eac3f29cbdcdfd16e0339f1a164066e9f71
> >
> > I agree that avoiding increase in struct trace_entry size would be very
> > desirable, but I have no knowledge whether rt developers had reasons to
> > do it like this.
> >
> > Nevertheless I think the issue with verifier running against a wrong
> > struct still needs to be addressed.
>
> Correct. My Ack is based on the current way things are done upstream.
> It was just that linux-rt showed the issue, where the code was not as
> robust as it should have been. To me this was a correctness issue, not
> an issue that had to do with how things are done in linux-rt.

I think we should at least add some BUILD_BUG_ON() that validates
offsets in syscall_tp_t matches the ones in syscall_trace_enter and
syscall_trace_exit, to fail more loudly if there is any mismatch in
the future. WDYT?

>
> As for the changes in linux-rt, they are not upstream yet. I'll have my
> comments on that code when that happens.

Ah, ok, cool. I'd appreciate you cc'ing bpf@...r.kernel.org in that
discussion, thank you!

>
> -- Steve