Re: [PATCH bpf-next v5 1/6] arm64: ftrace: Add ftrace direct call support

[Mark Rutland <mark.rutland@arm.com>]

On Thu, May 26, 2022 at 10:48:05PM +0800, Xu Kuohai wrote:
> On 5/26/2022 6:06 PM, Mark Rutland wrote:
> > On Thu, May 26, 2022 at 05:45:03PM +0800, Xu Kuohai wrote:
> >> On 5/25/2022 9:38 PM, Mark Rutland wrote:
> >>> On Wed, May 18, 2022 at 09:16:33AM -0400, Xu Kuohai wrote:
> >>>> Add ftrace direct support for arm64.
> >>>>
> >>>> 1. When there is custom trampoline only, replace the fentry nop to a
> >>>>    jump instruction that jumps directly to the custom trampoline.
> >>>>
> >>>> 2. When ftrace trampoline and custom trampoline coexist, jump from
> >>>>    fentry to ftrace trampoline first, then jump to custom trampoline
> >>>>    when ftrace trampoline exits. The current unused register
> >>>>    pt_regs->orig_x0 is used as an intermediary for jumping from ftrace
> >>>>    trampoline to custom trampoline.
> >>>
> >>> For those of us not all that familiar with BPF, can you explain *why* you want
> >>> this? The above explains what the patch implements, but not why that's useful.
> >>>
> >>> e.g. is this just to avoid the overhead of the ops list processing in the
> >>> regular ftrace code, or is the custom trampoline there to allow you to do
> >>> something special?
> >>
> >> IIUC, ftrace direct call was designed to *remove* the unnecessary
> >> overhead of saving regs completely [1][2].
> > 
> > Ok. My plan is to get rid of most of the register saving generally, so I think
> > that aspect can be solved without direct calls.
> Looking forward to your new solution.

For the register saving rework, I have a WIP branch on my kernel.org repo:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/ftrace/minimal-regs
  git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm64/ftrace/minimal-regs

I'm working on that at the moment along with a per-callsite ops implementaiton
that would avoid most of the need for custom trampolines (and work with branch
range limitations):

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/ftrace/per-callsite-ops
  git://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git arm64/ftrace/per-callsite-ops

> >> [1]
> >> https://lore.kernel.org/all/20191022175052.frjzlnjjfwwfov64@ast-mbp.dhcp.thefacebook.com/
> >> [2] https://lore.kernel.org/all/20191108212834.594904349@goodmis.org/
> >>
> >> This patch itself is just a variant of [3].
> >>
> >> [3] https://lore.kernel.org/all/20191108213450.891579507@goodmis.org/
> >>
> >>>
> >>> There is another patch series on the list from some of your colleagues which
> >>> uses dynamic trampolines to try to avoid that ops list overhead, and it's not
> >>> clear to me whether these are trying to solve the largely same problem or
> >>> something different. That other thread is at:
> >>>
> >>>   https://lore.kernel.org/linux-arm-kernel/20220316100132.244849-1-bobo.shaobowang@huawei.com/
> >>>
> >>> ... and I've added the relevant parties to CC here, since there doesn't seem to
> >>> be any overlap in the CC lists of the two threads.
> >>
> >> We're not working to solve the same problem. The trampoline introduced
> >> in this series helps us to monitor kernel function or another bpf prog
> >> with bpf, and also helps us to use bpf prog like a normal kernel
> >> function pointer.
> > 
> > Ok, but why is it necessary to have a special trampoline?
> > 
> > Is that *just* to avoid overhead, or do you need to do something special that
> > the regular trampoline won't do?
> > 
> 
> Sorry for not explaining the problem. The main bpf prog accepts only a
> single argument 'ctx' in r1, so to allow kernel code to call bpf prog
> transparently, we need a trampoline to convert native calling convention
> into BPF calling convention [1].
> 
> [1] https://lore.kernel.org/bpf/20191114185720.1641606-5-ast@kernel.org/

Thanks for the pointer; I'll go page that in.

> For example,
> 
> SEC("struct_ops/dctcp_state")
> void BPF_PROG(dctcp_state, struct sock *sk, __u8 new_state)
> {
>     // do something
> }
> 
> The above bpf prog will be compiled to something like this:
> 
> dctcp_state:
>     r2 = *(u64 *)(r1 + 8)  // new_state
>     r1 = *(u64 *)(r1 + 0)  // sk
>     ...
> 
> It accepts only one argument 'ctx' in r1, and loads the actual arugment
> 'sk' and 'new_state' from r1 + 0 and r1 + 8, resepectively. So before
> calling this prog, we need to construct 'ctx' and store its address to r1.
> 
> >>>
> >>> In that other thread I've suggested a general approach we could follow at:
> >>>   
> >>>   https://lore.kernel.org/linux-arm-kernel/YmGF%2FOpIhAF8YeVq@lakrids/
> >>
> >> Is it possible for a kernel function to take a long jump to common
> >> trampoline when we get a huge kernel image?
> > 
> > It is possible, but only where the kernel Image itself is massive and the .text
> > section exceeeds 128MiB, at which point other things break anyway. Practically
> > speaking, this doesn't happen for production kernels, or reasonable test
> > kernels.
> 
> So even for normal kernel functions, we need some way to construct and
> destruct long jumps atomically and safely.

My point was that case is unrealistic for production kernels, and utterly
broken anyway (and as below I intend to make ftrace detect this and mark itself
as broken).

FWIW, an allmodconfig kernel built with GCC 12.1.0 has a ~30MB .text segment,
so for realistic kernels we have plenty of headroom for normal functions to
reach the in-kernel trampoline.

> > I've been meaning to add some logic to detect this at boot time and idsable
> > ftrace (or at build time), since live patching would also be broken in that
> > case.
> >>>> As noted in that thread, I have a few concerns which equally apply here:
> >>>
> >>> * Due to the limited range of BL instructions, it's not always possible to
> >>>   patch an ftrace call-site to branch to an arbitrary trampoline. The way this
> >>>   works for ftrace today relies upon knowingthe set of trampolines at
> >>>   compile-time, and allocating module PLTs for those, and that approach cannot
> >>>   work reliably for dynanically allocated trampolines.
> >>
> >> Currently patch 5 returns -ENOTSUPP when long jump is detected, so no
> >> bpf trampoline is constructed for out of range patch-site:
> >>
> >> if (is_long_jump(orig_call, image))
> >> 	return -ENOTSUPP;
> > 
> > Sure, my point is that in practice that means that (from the user's PoV) this
> > may randomly fail to work, and I'd like something that we can ensure works
> > consistently.
> > 
> 
> OK, should I suspend this work until you finish refactoring ftrace?

Yes; I'd appreciate if we could hold on this for a bit.

I think with some ground work we can avoid most of the painful edge cases and
might be able to avoid the need for custom trampolines.

Thanks,
Mark.

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