[PATCH v2 6/6] riscv: Add KPROBES_ON_FTRACE supported

guoren
2020-07-09T02:22:40Z

From: Guo Ren <guoren@linux.alibaba.com>

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]
^^^^^^

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Björn Töpel <bjorn.topel@gmail.com>
Cc: Zong Li <zong.li@sifive.com>
Cc: Pekka Enberg <penberg@kernel.org>
---
arch/riscv/Kconfig | 1 +
arch/riscv/kernel/probes/Makefile | 1 +
arch/riscv/kernel/probes/ftrace.c | 53 +++++++++++++++++++++++++++++++++++++++
3 files changed, 55 insertions(+)
create mode 100644 arch/riscv/kernel/probes/ftrace.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index f927a91..91bfc6c 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -58,6 +58,7 @@ 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..08fc1cc
--- /dev/null
+++ b/arch/riscv/kernel/probes/ftrace.c
@@ -0,0 +1,53 @@
+// 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 {
+ /*
+ * The regs->epc hasn't been saved by SAVE_ALL in mcount-dyn.S
+ * So no need to resume it, just for kprobe handler.
+ */
+ 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);
+ }
+ }
+
+ /*
+ * 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