[RFC PATCH -next v2 3/4] arm64/ftrace: support dynamically allocated trampolines

[Jiri Olsa]

On Mon, May 30, 2022 at 10:03:10AM +0900, Masami Hiramatsu wrote:
> (Cc: BPF ML)
> 
> On Wed, 25 May 2022 13:17:30 +0100
> Mark Rutland <mark.rutland at arm.com> wrote:
> 
> > On Thu, May 12, 2022 at 09:02:31PM +0900, Masami Hiramatsu wrote:
> > > On Wed, 11 May 2022 11:12:07 -0400
> > > Steven Rostedt <rostedt at goodmis.org> wrote:
> > > 
> > > > On Wed, 11 May 2022 23:34:50 +0900
> > > > Masami Hiramatsu <mhiramat at kernel.org> wrote:
> > > > 
> > > > > OK, so fregs::regs will have a subset of pt_regs, and accessibility of
> > > > > the registers depends on the architecture. If we can have a checker like
> > > > > 
> > > > > ftrace_regs_exist(fregs, reg_offset)
> > > > 
> > > > Or something. I'd have to see the use case.
> > > > 
> > > > > 
> > > > > kprobe on ftrace or fprobe user (BPF) can filter user's requests.
> > > > > I think I can introduce a flag for kprobes so that user can make a
> > > > > kprobe handler only using a subset of registers. 
> > > > > Maybe similar filter code is also needed for BPF 'user space' library
> > > > > because this check must be done when compiling BPF.
> > > > 
> > > > Is there any other case without full regs that the user would want anything
> > > > other than the args, stack pointer and instruction pointer?
> > > 
> > > For the kprobes APIs/events, yes, it needs to access to the registers
> > > which is used for local variables when probing inside the function body.
> > > However at the function entry, I think almost no use case. (BTW, pstate
> > > is a bit special, that may show the actual processor-level status
> > > (context), so for the debugging, user might want to read it.)
> > 
> > As before, if we really need PSTATE we *must* take an exception to get a
> > reliable snapshot (or to alter the value). So I'd really like to split this
> > into two cases:
> > 
> > * Where users *really* need PSTATE (or arbitrary GPRs), they use kprobes. That
> >   always takes an exception and they can have a complete, real struct pt_regs.
> > 
> > * Where users just need to capture a function call boundary, they use ftrace.
> >   That uses a trampoline without taking an exception, and they get the minimal
> >   set of registers relevant to the function call boundary (which does not
> >   include PSTATE or most GPRs).
> 
> I totally agree with this idea. The x86 is a special case, since the
> -fentry option puts a call on the first instruction of the function entry,
> I had to reuse the ftrace instead of swbp for kprobes.
> But on arm64 (and other RISCs), we can use them properly.
> 
> My concern is that the eBPF depends on kprobe (pt_regs) interface, thus
> I need to ask them that it is OK to not accessable to some part of
> pt_regs (especially, PSTATE) if they puts probes on function entry
> with ftrace (fprobe in this case.)
> 
> (Jiri and BPF developers)
> Currently fprobe is only enabled on x86 for "multiple kprobes" BPF
> interface, but in the future, it will be enabled on arm64. And at
> that point, it will be only accessible to the regs for function
> arguments. Is that OK for your use case? And will the BPF compiler

I guess from practical POV registers for arguments and ip should be enough,
but whole pt_regs was already exposed to programs, so people can already use
any of them.. not sure it's good idea to restrict it

> be able to restrict the user program to access only those registers
> when using the "multiple kprobes"?

pt-regs pointer is provided to kprobe programs, I guess we could provide copy
of that with just available values

jirka