RISCV64: Fix 'bt' output when no ra on the stack top

Same as the Linux commit f766f77a74f5 ("riscv/stacktrace: Fix
stack output without ra on the stack top").

When a function doesn't have a callee, then it will not
push ra into the stack, such as lkdtm functions, so
correct the FP of the second frame and use pt_regs to get
the right PC of the second frame.

Before this patch, the `bt -f` outputs only the first frame with
the wrong PC and FP of next frame:
```
crash> bt -f
PID: 1        TASK: ff600000000e0000  CPU: 1    COMMAND: "sh"
 #0 [ff20000000013cf0] lkdtm_EXCEPTION at ffffffff805303c0
    [PC: ffffffff805303c0 RA: ff20000000013d10 SP: ff20000000013cf0 SIZE: 16] <- wrong next PC
    ff20000000013cf0: 0000000000000001 ff20000000013d10 <- next FP
    ff20000000013d00: ff20000000013d40
crash>
```
After this patch, the `bt` outputs the full frames:
```
crash> bt
PID: 1        TASK: ff600000000e0000  CPU: 1    COMMAND: "sh"
 #0 [ff20000000013cf0] lkdtm_EXCEPTION at ffffffff805303c0
 crash-utility#1 [ff20000000013d00] lkdtm_do_action at ffffffff8052fe36
 crash-utility#2 [ff20000000013d10] direct_entry at ffffffff80530018
 crash-utility#3 [ff20000000013d40] full_proxy_write at ffffffff80305044
 crash-utility#4 [ff20000000013d80] vfs_write at ffffffff801b68b4
 crash-utility#5 [ff20000000013e30] ksys_write at ffffffff801b6c4a
 crash-utility#6 [ff20000000013e80] __riscv_sys_write at ffffffff801b6cc4
 crash-utility#7 [ff20000000013e90] do_trap_ecall_u at ffffffff80836798
crash>
```

Acked-by: Kazuhito Hagio <[email protected]>
Signed-off-by: Song Shuai <[email protected]>

riscv64.c

@@ -747,11 +747,14 @@ riscv64_back_trace_cmd(struct bt_info *bt)
 {
 	struct riscv64_unwind_frame current, previous;
 	struct stackframe curr_frame;
+	struct riscv64_register * regs;
 	int level = 0;
 
 	if (bt->flags & BT_REGS_NOT_FOUND)
 		return;
 
+	regs = (struct riscv64_register *) bt->machdep;
+
 	current.pc = bt->instptr;
 	current.sp = bt->stkptr;
 	current.fp = bt->frameptr;
@@ -788,8 +791,16 @@ riscv64_back_trace_cmd(struct bt_info *bt)
 		    sizeof(curr_frame), "get stack frame", RETURN_ON_ERROR))
 			return;
 
-		previous.pc = curr_frame.ra;
-		previous.fp = curr_frame.fp;
+		/* correct PC and FP of the second frame when the first frame has no callee */
+
+		if (regs && (regs->regs[RISCV64_REGS_EPC] == current.pc) && curr_frame.fp & 0x7){
+			previous.pc = regs->regs[RISCV64_REGS_RA];
+			previous.fp = curr_frame.ra;
+		} else {
+			previous.pc = curr_frame.ra;
+			previous.fp = curr_frame.fp;
+		}
+
 		previous.sp = current.fp;
 
 		riscv64_dump_backtrace_entry(bt, symbol, &current, &previous, level++);