Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64

[Mark Rutland]

On Mon, Sep 26, 2022 at 03:40:20PM +0100, Catalin Marinas wrote:
> On Thu, Sep 22, 2022 at 08:01:16PM +0200, Daniel Borkmann wrote:
> > On 9/13/22 6:27 PM, Xu Kuohai wrote:
> > > This series adds ftrace direct call for arm64, which is required to attach
> > > bpf trampoline to fentry.
> > > 
> > > Although there is no agreement on how to support ftrace direct call on arm64,
> > > no patch has been posted except the one I posted in [1], so this series
> > > continues the work of [1] with the addition of long jump support. Now ftrace
> > > direct call works regardless of the distance between the callsite and custom
> > > trampoline.
> > > 
> > > [1] https://lore.kernel.org/bpf/20220518131638.3401509-2-xukuohai@xxxxxxxxxx/
> > > 
> > > v2:
> > > - Fix compile and runtime errors caused by ftrace_rec_arch_init
> > > 
> > > v1: https://lore.kernel.org/bpf/20220913063146.74750-1-xukuohai@xxxxxxxxxxxxxxx/
> > > 
> > > Xu Kuohai (4):
> > >    ftrace: Allow users to disable ftrace direct call
> > >    arm64: ftrace: Support long jump for ftrace direct call
> > >    arm64: ftrace: Add ftrace direct call support
> > >    ftrace: Fix dead loop caused by direct call in ftrace selftest
> > 
> > Given there's just a tiny fraction touching BPF JIT and most are around core arm64,
> > it probably makes sense that this series goes via Catalin/Will through arm64 tree
> > instead of bpf-next if it looks good to them. Catalin/Will, thoughts (Ack + bpf-next
> > could work too, but I'd presume this just results in merge conflicts)?
> 
> I think it makes sense for the series to go via the arm64 tree but I'd
> like Mark to have a look at the ftrace changes first.

>From a quick scan, I still don't think this is quite right, and as it stands I
believe this will break backtracing (as the instructions before the function
entry point will not be symbolized correctly, getting in the way of
RELIABLE_STACKTRACE). I think I was insufficiently clear with my earlier
feedback there, as I have a mechanism in mind that wa a little simpler.

I'll try to reply with some more detail tomorrow, but I don't think this is the
right approach, and as mentioned previously (and e.g. at LPC) I'd strongly
prefer to *not* implement direct calls, so that we can have more consistent
entry/exit handling.

Thanks,
Mark.