On Mon, 13 Jul 2020 23:39:21 +0000
guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> This patch adds support for kprobes on ftrace call sites to avoids
> much of the overhead with regular kprobes. Try it with simple
> steps:
>
> 1. Get _do_fork ftrace call site.
> Dump of assembler code for function _do_fork:
> 0xffffffe00020af64 <+0>: addi sp,sp,-128
> 0xffffffe00020af66 <+2>: sd s0,112(sp)
> 0xffffffe00020af68 <+4>: sd ra,120(sp)
> 0xffffffe00020af6a <+6>: addi s0,sp,128
> 0xffffffe00020af6c <+8>: sd s1,104(sp)
> 0xffffffe00020af6e <+10>: sd s2,96(sp)
> 0xffffffe00020af70 <+12>: sd s3,88(sp)
> 0xffffffe00020af72 <+14>: sd s4,80(sp)
> 0xffffffe00020af74 <+16>: sd s5,72(sp)
> 0xffffffe00020af76 <+18>: sd s6,64(sp)
> 0xffffffe00020af78 <+20>: sd s7,56(sp)
> 0xffffffe00020af7a <+22>: mv s4,a0
> 0xffffffe00020af7c <+24>: mv a0,ra
> 0xffffffe00020af7e <+26>: nop <<<<<<<< here!
> 0xffffffe00020af82 <+30>: nop
> 0xffffffe00020af86 <+34>: ld s3,0(s4)
>
> 2. Set _do_fork+26 as the kprobe.
> echo 'p:myprobe _do_fork+26 dfd=%a0 filename=%a1 flags=%a2 mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
> echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> cat /sys/kernel/debug/tracing/trace
> tracer: nop
>
> entries-in-buffer/entries-written: 3/3 #P:1
>
> _-----=> irqs-off
> / _----=> need-resched
> | / _---=> hardirq/softirq
> || / _--=> preempt-depth
> ||| / delay
> TASK-PID CPU# |||| TIMESTAMP FUNCTION
> | | | |||| | |
> sh-87 [000] .... 551.557031: myprobe: (_do_fork+0x1a/0x2e6) dfd=0xffffffe00020af7e filename=0xffffffe00020b34e flags=0xffffffe00101e7c0 mode=0x20af86ffffffe0
>
> cat /sys/kernel/debug/kprobes/list
> ffffffe00020af7e k _do_fork+0x1a [FTRACE]
> ^^^^^^
Hmm, this seems fentry is not supported on RISC-V yet. But anyway,
it will be useful for users (if they can find the offset).
Reviewed-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Thank you,
>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> Cc: Paul Walmsley <paul.walmsley@xxxxxxxxxx>
> Cc: Björn Töpel <bjorn.topel@xxxxxxxxx>
> Cc: Zong Li <zong.li@xxxxxxxxxx>
> Cc: Pekka Enberg <penberg@xxxxxxxxxx>
> ---
> arch/riscv/Kconfig | 1 +
> arch/riscv/kernel/probes/Makefile | 1 +
> arch/riscv/kernel/probes/ftrace.c | 52 +++++++++++++++++++++++++++++++++++++++
> 3 files changed, 54 insertions(+)
> create mode 100644 arch/riscv/kernel/probes/ftrace.c
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index a41b785..0e9f5eb 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -60,6 +60,7 @@ config RISCV
> select HAVE_FUTEX_CMPXCHG if FUTEX
> select HAVE_GENERIC_VDSO if MMU && 64BIT
> select HAVE_KPROBES
> + select HAVE_KPROBES_ON_FTRACE
> select HAVE_KRETPROBES
> select HAVE_PCI
> select HAVE_PERF_EVENTS
> diff --git a/arch/riscv/kernel/probes/Makefile b/arch/riscv/kernel/probes/Makefile
> index cb62991..7f0840d 100644
> --- a/arch/riscv/kernel/probes/Makefile
> +++ b/arch/riscv/kernel/probes/Makefile
> @@ -1,5 +1,6 @@
> # SPDX-License-Identifier: GPL-2.0
> obj-$(CONFIG_KPROBES) += kprobes.o decode-insn.o simulate-insn.o
> obj-$(CONFIG_KPROBES) += kprobes_trampoline.o
> +obj-$(CONFIG_KPROBES_ON_FTRACE) += ftrace.o
> obj-$(CONFIG_UPROBES) += uprobes.o decode-insn.o simulate-insn.o
> CFLAGS_REMOVE_simulate-insn.o = $(CC_FLAGS_FTRACE)
> diff --git a/arch/riscv/kernel/probes/ftrace.c b/arch/riscv/kernel/probes/ftrace.c
> new file mode 100644
> index 00000000..e0fe58a
> --- /dev/null
> +++ b/arch/riscv/kernel/probes/ftrace.c
> @@ -0,0 +1,52 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#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;
> +
> + 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);
> + instruction_pointer_set(regs, ip);
> +
> + __this_cpu_write(current_kprobe, p);
> + kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> + if (!p->pre_handler || !p->pre_handler(p, regs)) {
> + /*
> + * Emulate singlestep (and also recover regs->pc)
> + * as if there is a nop
> + */
> + instruction_pointer_set(regs,
> + (unsigned long)p->addr + MCOUNT_INSN_SIZE);
> + if (unlikely(p->post_handler)) {
> + kcb->kprobe_status = KPROBE_HIT_SSDONE;
> + p->post_handler(p, regs, 0);
> + }
> + instruction_pointer_set(regs, orig_ip);
> + }
> +
> + /*
> + * If pre_handler returns !0, it changes regs->pc. We have to
> + * skip emulating post_handler.
> + */
> + __this_cpu_write(current_kprobe, NULL);
> + }
> +}
> +NOKPROBE_SYMBOL(kprobe_ftrace_handler);
> +
> +int arch_prepare_kprobe_ftrace(struct kprobe *p)
> +{
> + p->ainsn.api.insn = NULL;
> + return 0;
> +}
> --
> 2.7.4
>
--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>
