[PATCH v2] arm: backtrace-clang: fix wrong sp usage for unwinding

show_stack() can be called for any task, but c_backtrace is always
unwinding frames based on "sp" register.

Which leads to printing backtrace of current task instead of
required task.

Try with normal TC:
==================
       for_each_process(p){
               sched_show_task(p);
       }

[7.433271] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
[7.433633] Call trace:
[7.433640] [<80113418>] (dump_backtrace) from [<80113510>] (show_stack+0x14/0x18)
..
[7.433676] [<8016749c>] (sched_show_task) from [<803cb324>] (meminfo_proc_show+0x6c/0x930)
[7.434019]  r5:8158e300 r4:8b4882d0
[7.434024] [<803cb324>] (meminfo_proc_show) from [<80365788>] (seq_read_iter+0x148/0x4bc)
[7.434045] [<80365788>] (seq_read_iter) from [<803c044c>] (proc_reg_read_iter+0xb8/0xc4)
[7.434060] [<803c044c>] (proc_reg_read_iter) from [<80377104>] (copy_splice_read+0x228/0x308)

It should unwind frame based on passed "fp".
(CONFIG_UNWINDER_FRAME_POINTER=y)

Extra sp manipulation is needed only when c_backtrace()
is called from dump_backtrace() with "frame == fp" i.e.
NULL regs and not current task. (Suggested by Ard Biesheuvel)

Otherwise passed "fp" should be used to unwind backtraces.

With fix:
=========
[13.933732] task:kthreadd        state:S stack:0     pid:2     tgid:2     ppid:0      task_flags:0x208040 flags:0x00000000
[13.934165] Call trace:
[13.934604] [<80afa1a8>] (schedule) from [<8015426c>] (kthreadd+0x124/0x208)
[13.934654]  r10:8100bbf0 r4:8116f440
[13.934664] [<8015426c>] (kthreadd) from [<8010010c>] (ret_from_fork+0x14/0x28)
[13.934691] Exception stack(0xf081df9c to 0xf081dfe4)

Signed-off-by: Onkarnath <[email protected]>
Signed-off-by: Maninder Singh <[email protected]>
---
v1 -> v2: don't ignore sp manipulation completely, instead use it for NULL regs.

 arch/arm/lib/backtrace-clang.S | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
index 290c52a60fc6..bf9738070af7 100644
--- a/arch/arm/lib/backtrace-clang.S
+++ b/arch/arm/lib/backtrace-clang.S
@@ -107,8 +107,10 @@ ENDPROC(c_backtrace)
 
 /*
  * Switches the current frame to be the frame for dump_stack.
+ * (Only in case of current task using current registers)
  */
-		add	frame, sp, #24		@ switch to false frame
+		cmp	frame, fp
+		addeq	frame, sp, #24		@ switch to false frame
 for_each_frame:	tst	frame, mask		@ Check for address exceptions
 		bne	no_frame
 
-- 
2.34.1