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

[Masami Hiramatsu (Google)]

On Mon, 30 May 2022 14:38:31 +0200
Jiri Olsa <olsajiri at gmail.com> wrote:

> 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

Yes, ftrace_regs already provides partial filled pt_regs (which registers
are valid is arch-dependent). Thus, my idea is changing fprobe's handler
interface to expose ftrace_regs instead of pt_regs, and the BPF handler
will extract the internal pt_regs.
If the BPF compiler can list which registers will be accessed form the
user program, the kernel side can filter it.
I think similar feature can be done in the kprobe-event (new fprobe event?).

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat at kernel.org>