linux-kernel - Re: [PATCH] arm64/kprobes: Add support for KPROBES_ON

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Date:   Mon, 7 Nov 2022 15:32:24 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Jianlin Lv <iecedge@...il.com>, corbet@....net,
        catalin.marinas@....com, rostedt@...dmis.org, mingo@...hat.com,
        naveen.n.rao@...ux.ibm.com, anil.s.keshavamurthy@...el.com,
        davem@...emloft.net, mhiramat@...nel.org, arnd@...db.de,
        zhengzengkai@...wei.com, jianlv@...y.com,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64/kprobes: Add support for KPROBES_ON_FTRACE

On Mon, Nov 07, 2022 at 02:49:31PM +0000, Will Deacon wrote:
> [+Mark R]
> 
> On Thu, Jul 28, 2022 at 02:02:50AM +0000, Jianlin Lv wrote:
> > This is the arm64 version of ftrace-based kprobes to avoid the overhead
> > with regular kprobes, by using the ftrace infrastructure.
> > 
> > Signed-off-by: Jianlin Lv <iecedge@...il.com>
> > ---
> >  .../debug/kprobes-on-ftrace/arch-support.txt  |  2 +-
> >  arch/arm64/Kconfig                            |  1 +
> >  arch/arm64/kernel/probes/Makefile             |  1 +
> >  arch/arm64/kernel/probes/kprobes-ftrace.c     | 81 +++++++++++++++++++
> >  include/linux/kprobes.h                       |  2 +
> >  kernel/kprobes.c                              |  4 +-
> >  6 files changed, 88 insertions(+), 3 deletions(-)
> >  create mode 100644 arch/arm64/kernel/probes/kprobes-ftrace.c
> 
> Sorry for the slow reply on this, but I think this deserved to be split
> into two patches: the first one reworking the core check_ftrace_location()
> logic to work properly with branch-and-link style architectures, and the
> second one adding support for arm64.

I'd prefer we don't do this at all; there a bunch of issues with kprobes *not*
taking an exception, since we get a dodgy not-quite-real pt_regs, and to clean
up the existing issues the plan is:

1) Move ftrace over to ftrace_regs
2) Implement fprobes using ftrace_regs
3) Remove kretprobes

... and regular kprobes will need to take an exception (via BRK) to get a real
pt_regs, so that can't be optimized to use ftrace.

Thanks,
Mark.