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Date:   Thu, 22 Aug 2019 12:23:58 +0530
From:   "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Subject: Re: [PATCH v4] arm64: implement KPROBES_ON_FTRACE
To:     Catalin Marinas <catalin.marinas@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Jisheng Zhang <Jisheng.Zhang@synaptics.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Will Deacon <will@kernel.org>
Cc:     "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20190822113421.52920377@xhacker.debian>
In-Reply-To: <20190822113421.52920377@xhacker.debian>
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Jisheng Zhang wrote:
> KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
> eliminates the need for a trap, as well as the need to emulate or
> single-step instructions.
> 
> Tested on berlin arm64 platform.
> 
> ~ # mount -t debugfs debugfs /sys/kernel/debug/
> ~ # cd /sys/kernel/debug/
> /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events
> 
> before the patch:
> 
> /sys/kernel/debug # cat kprobes/list
> ffffff801009fe28  k  _do_fork+0x0    [DISABLED]
> 
> after the patch:
> 
> /sys/kernel/debug # cat kprobes/list
> ffffff801009ff54  k  _do_fork+0x4    [DISABLED][FTRACE]
> 
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>

This looks good to me. Except for a small confirmation below:
Reviewed-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>


> ---
> KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
> eliminates the need for a trap, as well as the need to emulate or
> single-step instructions.
> 
> Applied after arm64 FTRACE_WITH_REGS:
> http://lists.infradead.org/pipermail/linux-arm-kernel/2019-August/674404.html
> 
> Changes since v3:
>   - move kprobe_lookup_name() and arch_kprobe_on_func_entry to ftrace.c since
>     we only want to choose the ftrace entry for KPROBES_ON_FTRACE.
>   - only choose ftrace entry if (addr && !offset)
> 
> Changes since v2:
>   - remove patch1, make it a single cleanup patch
>   - remove "This patch" in the change log
>   - implement arm64's kprobe_lookup_name() and arch_kprobe_on_func_entry instead
>     of patching the common kprobes code
> 
> Changes since v1:
>   - make the kprobes/x86: use instruction_pointer and instruction_pointer_set
>     as patch1
>   - add Masami's ACK to patch1
>   - add some description about KPROBES_ON_FTRACE and why we need it on
>     arm64
>   - correct the log before the patch
>   - remove the consolidation patch, make it as TODO
>   - only adjust kprobe's addr when KPROBE_FLAG_FTRACE is set
>   - if KPROBES_ON_FTRACE, ftrace_call_adjust() the kprobe's addr before
>     calling ftrace_location()
>   - update the kprobes-on-ftrace/arch-support.txt in doc
> 
> 
>  .../debug/kprobes-on-ftrace/arch-support.txt  |  2 +-
>  arch/arm64/Kconfig                            |  1 +
>  arch/arm64/kernel/probes/Makefile             |  1 +
>  arch/arm64/kernel/probes/ftrace.c             | 84 +++++++++++++++++++
>  4 files changed, 87 insertions(+), 1 deletion(-)
>  create mode 100644 arch/arm64/kernel/probes/ftrace.c
> 
> diff --git a/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> index 68f266944d5f..e8358a38981c 100644
> --- a/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> +++ b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> @@ -9,7 +9,7 @@
>      |       alpha: | TODO |
>      |         arc: | TODO |
>      |         arm: | TODO |
> -    |       arm64: | TODO |
> +    |       arm64: |  ok  |
>      |         c6x: | TODO |
>      |        csky: | TODO |
>      |       h8300: | TODO |
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 663392d1eae2..928700f15e23 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -167,6 +167,7 @@ config ARM64
>  	select HAVE_STACKPROTECTOR
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_KPROBES
> +	select HAVE_KPROBES_ON_FTRACE
>  	select HAVE_KRETPROBES
>  	select HAVE_GENERIC_VDSO
>  	select IOMMU_DMA if IOMMU_SUPPORT
> diff --git a/arch/arm64/kernel/probes/Makefile b/arch/arm64/kernel/probes/Makefile
> index 8e4be92e25b1..4020cfc66564 100644
> --- a/arch/arm64/kernel/probes/Makefile
> +++ b/arch/arm64/kernel/probes/Makefile
> @@ -4,3 +4,4 @@ obj-$(CONFIG_KPROBES)		+= kprobes.o decode-insn.o	\
>  				   simulate-insn.o
>  obj-$(CONFIG_UPROBES)		+= uprobes.o decode-insn.o	\
>  				   simulate-insn.o
> +obj-$(CONFIG_KPROBES_ON_FTRACE)	+= ftrace.o
> diff --git a/arch/arm64/kernel/probes/ftrace.c b/arch/arm64/kernel/probes/ftrace.c
> new file mode 100644
> index 000000000000..5989c57660f3
> --- /dev/null
> +++ b/arch/arm64/kernel/probes/ftrace.c
> @@ -0,0 +1,84 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * Dynamic Ftrace based Kprobes Optimization
> + *
> + * Copyright (C) Hitachi Ltd., 2012
> + * Copyright (C) 2019 Jisheng Zhang <jszhang@kernel.org>
> + *		      Synaptics Incorporated
> + */
> +
> +#include <linux/kprobes.h>
> +
> +/* Ftrace callback handler for kprobes -- called under preepmt disabed */
> +void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> +			   struct ftrace_ops *ops, struct pt_regs *regs)
> +{
> +	struct kprobe *p;
> +	struct kprobe_ctlblk *kcb;
> +
> +	/* Preempt is disabled by ftrace */
> +	p = get_kprobe((kprobe_opcode_t *)ip);
> +	if (unlikely(!p) || kprobe_disabled(p))
> +		return;
> +
> +	kcb = get_kprobe_ctlblk();
> +	if (kprobe_running()) {
> +		kprobes_inc_nmissed_count(p);
> +	} else {
> +		unsigned long orig_ip = instruction_pointer(regs);
> +		/* Kprobe handler expects regs->pc = pc + 4 as breakpoint hit */
> +		instruction_pointer_set(regs, ip + sizeof(kprobe_opcode_t));

Just want to make sure that you've confirmed that this is what happens 
with a regular trap/brk based kprobe on ARM64. The reason for setting 
the instruction pointer here is to ensure that it is set to the same 
value as would be set if there was a trap/brk instruction at the ftrace 
location. This ensures that the kprobe pre handler sees the same value 
regardless.

Further changes to the instruction pointer are to achieve the same 
effect for kprobe post handlers.


- Naveen