linux-kernel - Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

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Message-ID: <CAADnVQKrL3LZaRcgoTdGN-csPt=eyujPbw9qoxgv9tPYPmZiZA@mail.gmail.com>
Date:   Wed, 2 Aug 2023 12:48:14 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Florent Revest <revest@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 3/9] bpf/btf: Add a function to search a member of a struct/union

On Wed, Aug 2, 2023 at 11:38 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 2 Aug 2023 11:24:12 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>
> > This is a non starter.
> > bpf progs expect arch dependent 'struct pt_regs *' and we cannot change that.
>
> If the progs are compiled into native code, isn't there optimizations that
> could be done? That is, if ftrace_regs is available, and the bpf program is
> just using the subset of pt_regs, is it possible that it could be compiled
> to use ftrace_regs?
>
> Forgive my ignorance on how BPF programs turn into executables when running
> in the kernel.

Right. It's possible for the verifier to do an offset rewrite,
forbid certain access, always return 0 on load from certain offset,
and so on.
It's all non trivial amount of work.
ftrace_partial_regs() from ftrace_regs into pt_regs is so much simpler.