[PATCH] riscv/stacktrace: fix stackframe without ra on the top

[Chen Huang]

When a function doesn't have a callee, then it will not push ra
into the stack, such as lkdtm_BUG() function:

addi	sp,sp,-16
sd	s0,8(sp)
addi	s0,sp,16
ebreak

Then we use pt_regs as a parameter to walk_stackframe(), for the
struct stackframe use {fp,ra} to get information from stack, it
will get the wrong value. And the call trace will be:

[<ffffffe00066c56c>] lkdtm_BUG+0x6/0x8
---[ end trace 18da3fbdf08e25d5 ]---

It should be that:
[<ffffffe00066c56c>] lkdtm_BUG+0x6/0x8
[<ffffffe0008b24a4>] lkdtm_do_action+0x14/0x1c
[<ffffffe00066c372>] direct_entry+0xc0/0x10a
[<ffffffe000439f86>] full_proxy_write+0x42/0x6a
[<ffffffe000309626>] vfs_write+0x7e/0x214
[<ffffffe00030992a>] ksys_write+0x98/0xc0
[<ffffffe000309960>] sys_write+0xe/0x16
[<ffffffe0002014bc>] ret_from_syscall+0x0/0x2
---[ end trace 61917f3d9a9fadcd ]---

Signed-off-by: Chen Huang <chenhuang5 at huawei.com>
---
 arch/riscv/kernel/stacktrace.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
index 595342910c3f..d1d4c18335c4 100644
--- a/arch/riscv/kernel/stacktrace.c
+++ b/arch/riscv/kernel/stacktrace.c
@@ -57,7 +57,14 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 		/* Unwind stack frame */
 		frame = (struct stackframe *)fp - 1;
 		sp = fp;
-		fp = frame->fp;
+		if (regs && (frame->fp & 0x7) && (pc == regs->epc)) {
+			fp = frame->ra;
+			pc = regs->ra;
+			continue;
+		} else {
+			fp = frame->fp;
+		}
+
 		pc = ftrace_graph_ret_addr(current, NULL, frame->ra,
 					   (unsigned long *)(fp - 8));
 	}
-- 
2.17.1