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

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Message-ID: <20231013100023.5b0943ec@rorschach.local.home>
Date:   Fri, 13 Oct 2023 10:00:23 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Artem Savkov <asavkov@...hat.com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.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, 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.

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

-- Steve