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

[Alexei Starovoitov]

On Wed, Aug 2, 2023 at 6:56 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> On Tue, 1 Aug 2023 20:40:54 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Wed, 2 Aug 2023 09:21:46 +0900
> > Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx> wrote:
> >
> > > > Then use kprobes. When I asked Masami what the difference between fprobes
> > > > and kprobes was, he told me that it would be that it would no longer rely
> > > > on the slower FTRACE_WITH_REGS. But currently, it still does.
> > >
> > > kprobes needs to keep using pt_regs because software-breakpoint exception
> > > handler gets that. And fprobe is used for bpf multi-kprobe interface,
> > > but I think it can be optional.
> > >
> > > So until user-land tool supports the ftrace_regs, you can just disable
> > > using fprobes if CONFIG_DYNAMIC_FTRACE_WITH_REGS=n
> >
> > I'm confused. I asked about the difference between kprobes on ftrace
> > and fprobes, and you said it was to get rid of the requirement of
> > FTRACE_WITH_REGS.
> >
> >  https://lore.kernel.org/all/20230120205535.98998636329ca4d5f8325bc3@xxxxxxxxxx/
>
> Yes, it is for enabling fprobe (and fprobe-event) on more architectures.
> I don't think it's possible to change everything at once. So, it will be
> changed step by step. At the first step, I will replace pt_regs with
> ftrace_regs, and make bpf_trace.c and fprobe_event depends on
> FTRACE_WITH_REGS.
>
> At this point, we can split the problem into two, how to move bpf on
> ftrace_regs and how to move fprobe-event on ftrace_regs. fprobe-event
> change is not hard because it is closing in the kernel and I can do it.
> But for BPF, I need to ask BPF user-land tools to support ftrace_regs.
>
> >
> > >
> > > Then you can safely use
> > >
> > > struct pt_regs *regs = ftrace_get_regs(fregs);
> > >
> > > I think we can just replace the CONFIG_FPROBE ifdefs with
> > > CONFIG_DYNAMIC_FTRACE_WITH_REGS in kernel/trace/bpf_trace.c
> > > And that will be the first version of using ftrace_regs in fprobe.
> >
> > But it is still slow. The FTRACE_WITH_REGS gives us the full pt_regs
> > and saves all registers including flags, which is a very slow operation
> > (and noticeable in profilers).
>
> Yes, to solve this part, we need to work with BPF user-land people.
> I guess the BPF is accessing registers from pt_regs with fixed offset
> which is calculated from pt_regs layout in the user-space.

This is a non starter.
bpf progs expect arch dependent 'struct pt_regs *' and we cannot change that.