[PATCH -next 7/7] riscv: stacktrace: Fix unwinding on __kretporbe_trampoline

[Chen Zhongjin]

When unwinding on __kretprobe_trampoline, the caller of traced function
will be skipped because unwinder doesn't read the saved pt_regs.

Things going like this:

caller's caller		|       ...                 |<---+
caller			+---------------------------+    |
			| ra caller's caller        |    |
			| s0 of caller's caller     |    |
			|       ...                 |    |
probed func returned	+---------------------------+    |
__kretprobe_trampoline	| pt_regs:                  |    |
			| epc caller                |    |
                        | ra  __kretprobe_trampoline|    |
			|       ...                 |    |
                        | s0 of caller              | {ra, fp}
			|       ...                 |

Since from caller to __kretprobe_trampoline, the {ra, fp} are not
changed, unwinder will go directly to caller's caller.

Now we can have an ENCODED_FRAME_POINTER on stack and read the pt_regs,
kretporbe will set the epc to correct_ret_addr so that we can unwind
to the correct caller.

Stacktrace before this patch:

 Call Trace:
  ...
  [<ffffffff800d5d48>] __kretprobe_trampoline_handler+0xc2/0x13e
  [<ffffffff808b766c>] trampoline_probe_handler+0x30/0x46
  [<ffffffff800070de>] __kretprobe_trampoline+0x52/0x92
  [<ffffffff0163809c>] kprobe_init+0x9c/0x1000 [kprobe_unwind]
  [<ffffffff800027c8>] do_one_initcall+0x4c/0x1f2
  ...

Stacktrace after this patch:

 Call Trace:
  ...
  [<ffffffff800d5d48>] __kretprobe_trampoline_handler+0xc2/0x13e
  [<ffffffff808b766c>] trampoline_probe_handler+0x30/0x46
  [<ffffffff800070de>] __kretprobe_trampoline+0x52/0x92
+ [<ffffffff01633076>] the_caller+0x2c/0x38 [kprobe_unwind]
  [<ffffffff0163809c>] kprobe_init+0x9c/0x1000 [kprobe_unwind]
  [<ffffffff800027c8>] do_one_initcall+0x4c/0x1f2
  ...

Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
---
 arch/riscv/include/asm/stacktrace.h           | 4 ++++
 arch/riscv/kernel/probes/kprobes_trampoline.S | 8 ++++++++
 arch/riscv/kernel/stacktrace.c                | 5 +++++
 3 files changed, 17 insertions(+)

diff --git a/arch/riscv/include/asm/stacktrace.h b/arch/riscv/include/asm/stacktrace.h
index a39e4ef1dbd5..506c7c38b6cb 100644
--- a/arch/riscv/include/asm/stacktrace.h
+++ b/arch/riscv/include/asm/stacktrace.h
@@ -16,6 +16,10 @@ struct unwind_state {
 	unsigned long sp;
 	unsigned long pc;
 	struct pt_regs *regs;
+#ifdef CONFIG_KRETPROBES
+	struct llist_node *kr_cur;
+	struct task_struct *task;
+#endif
 };
 
 extern void dump_backtrace(struct pt_regs *regs, struct task_struct *task,
diff --git a/arch/riscv/kernel/probes/kprobes_trampoline.S b/arch/riscv/kernel/probes/kprobes_trampoline.S
index 7bdb09ded39b..3c0677a714a6 100644
--- a/arch/riscv/kernel/probes/kprobes_trampoline.S
+++ b/arch/riscv/kernel/probes/kprobes_trampoline.S
@@ -6,6 +6,8 @@
 
 #include <asm/asm.h>
 #include <asm/asm-offsets.h>
+#include <asm/frame.h>
+#include <asm/csr.h>
 
 	.text
 	.altmacro
@@ -79,6 +81,12 @@ ENTRY(__kretprobe_trampoline)
 	addi sp, sp, -(PT_SIZE_ON_STACK)
 	save_all_base_regs
 
+#ifdef CONFIG_FRAME_POINTER
+	li s0, SR_PP
+	REG_S s0, PT_STATUS(sp)
+	ENCODE_FRAME_POINTER
+#endif
+
 	move a0, sp /* pt_regs */
 
 	call trampoline_probe_handler
diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
index 976dc298ab3b..53edc685ca18 100644
--- a/arch/riscv/kernel/stacktrace.c
+++ b/arch/riscv/kernel/stacktrace.c
@@ -11,6 +11,7 @@
 #include <linux/sched/task_stack.h>
 #include <linux/stacktrace.h>
 #include <linux/ftrace.h>
+#include <linux/kprobes.h>
 
 #include <asm/stacktrace.h>
 
@@ -123,6 +124,10 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
 		state.sp = task->thread.sp;
 		state.pc = task->thread.ra;
 	}
+#ifdef CONFIG_KRETPROBES
+	state.kr_cur = NULL;
+	state.task = task;
+#endif
 
 	unwind(&state, consume_entry, cookie);
 }
-- 
2.17.1