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

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Date:   Wed, 2 Aug 2023 14:28:50 -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 1:12 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 2 Aug 2023 12:48:14 -0700
> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>
> > 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.
>
> Sure, and the copy could be the solution we have in the near future, but if
> we could optimize it in the future, then perhaps it would be worth doing it.
>
> Also, how are the bpf programs referencing the pt_regs?

Typically through macros that abstract arch differences away in
tools/lib/bpf/bpf_tracing.h
PT_REGS_PARM1
PT_REGS_PARM1_CORE
PT_REGS_PARM1_SYSCALL

pt_regs at syscall entry is special, since syscall calling convention
is different from the rest of the kernel.
ftrace_regs cannot help with that either.

> Could a ftrace_regs
> API be added too?

Potentially yes, but I don't see the value.
bpf users are slowly migrating to fentry/fexit that has accurate
args and return value and much faster.
kprobes are still heavily used, of course.
multi-kprobe (with fprobe_ips underneath) is a new addition that is
also very important to some users.

> If the verifier sees that the program is using
> ftrace_regs, it could then use the lighter weight fprobes for access,
> otherwise it falls back to the kprobe version.
>
> -- Steve