[PATCH] riscv: eliminate unreliable __builtin_frame_address(1)

[Changbin Du]

I tried different pieces of code which uses __builtin_frame_address(1)
(with both gcc version 7.5.0 and 10.3.0) to verify whether it works as
expected on riscv64. The result is negative.

What the compiler had generated is as below:
31                      fp = (unsigned long)__builtin_frame_address(1);
   0xffffffff80006024 <+200>:   ld      s1,0(s0)

It takes '0(s0)' as the address of frame 1 (caller), but the actual address
should be '-16(s0)'.

          |       ...       | <-+
          +-----------------+   |
          | return address  |   |
          | previous fp     |   |
          | saved registers |   |
          | local variables |   |
  $fp --> |       ...       |   |
          +-----------------+   |
          | return address  |   |
          | previous fp --------+
          | saved registers |
  $sp --> | local variables |
          +-----------------+

This leads the kernel can not dump the full stack trace on riscv.

[    7.222126][    T1] Call Trace:
[    7.222804][    T1] [<ffffffff80006058>] dump_backtrace+0x2c/0x3a

This problem is not exposed on most riscv builds just because the '0(s0)'
occasionally is the address frame 2 (caller's caller), if only ra and fp
are stored in frame 1 (caller).

          |       ...       | <-+
          +-----------------+   |
          | return address  |   |
  $fp --> | previous fp     |   |
          +-----------------+   |
          | return address  |   |
          | previous fp --------+
          | saved registers |
  $sp --> | local variables |
          +-----------------+

This could be a *bug* of gcc that should be fixed. But as noted in gcc
manual "Calling this function with a nonzero argument can have
unpredictable effects, including crashing the calling program.", let's
remove the '__builtin_frame_address(1)' in backtrace code.

With this fix now it can show full stack trace:
[   10.444838][    T1] Call Trace:
[   10.446199][    T1] [<ffffffff8000606c>] dump_backtrace+0x2c/0x3a
[   10.447711][    T1] [<ffffffff800060ac>] show_stack+0x32/0x3e
[   10.448710][    T1] [<ffffffff80a005c0>] dump_stack_lvl+0x58/0x7a
[   10.449941][    T1] [<ffffffff80a005f6>] dump_stack+0x14/0x1c
[   10.450929][    T1] [<ffffffff804c04ee>] ubsan_epilogue+0x10/0x5a
[   10.451869][    T1] [<ffffffff804c092e>] __ubsan_handle_load_invalid_value+0x6c/0x78
[   10.453049][    T1] [<ffffffff8018f834>] __pagevec_release+0x62/0x64
[   10.455476][    T1] [<ffffffff80190830>] truncate_inode_pages_range+0x132/0x5be
[   10.456798][    T1] [<ffffffff80190ce0>] truncate_inode_pages+0x24/0x30
[   10.457853][    T1] [<ffffffff8045bb04>] kill_bdev+0x32/0x3c
...

Signed-off-by: Changbin Du <changbin.du at gmail.com>
---
 arch/riscv/kernel/stacktrace.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/kernel/stacktrace.c b/arch/riscv/kernel/stacktrace.c
index 201ee206fb57..14d2b53ec322 100644
--- a/arch/riscv/kernel/stacktrace.c
+++ b/arch/riscv/kernel/stacktrace.c
@@ -22,15 +22,16 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 			     bool (*fn)(void *, unsigned long), void *arg)
 {
 	unsigned long fp, sp, pc;
+	int level = 0;
 
 	if (regs) {
 		fp = frame_pointer(regs);
 		sp = user_stack_pointer(regs);
 		pc = instruction_pointer(regs);
 	} else if (task == NULL || task == current) {
-		fp = (unsigned long)__builtin_frame_address(1);
-		sp = (unsigned long)__builtin_frame_address(0);
-		pc = (unsigned long)__builtin_return_address(0);
+		fp = (unsigned long)__builtin_frame_address(0);
+		sp = sp_in_global;
+		pc = (unsigned long)walk_stackframe;
 	} else {
 		/* task blocked in __switch_to */
 		fp = task->thread.s[0];
@@ -42,7 +43,7 @@ void notrace walk_stackframe(struct task_struct *task, struct pt_regs *regs,
 		unsigned long low, high;
 		struct stackframe *frame;
 
-		if (unlikely(!__kernel_text_address(pc) || !fn(arg, pc)))
+		if (unlikely(!__kernel_text_address(pc) || (level++ >= 1 && !fn(arg, pc))))
 			break;
 
 		/* Validate frame pointer */
-- 
2.32.0