ARM: 8992/1: Fix unwind_frame for clang-built kernels

Since clang does not push pc and sp in function prologues, the current
implementation of unwind_frame does not work. By using the previous
frame's lr/fp instead of saved pc/sp we get valid unwinds on clang-built
kernels.

The bounds check on next frame pointer must be changed as well since
there are 8 less bytes between frames.

This fixes /proc/<pid>/stack.

Link: https://github.com/ClangBuiltLinux/linux/issues/912Reported-by: NMiles Chen <miles.chen@mediatek.com>
Tested-by: NMiles Chen <miles.chen@mediatek.com>
Cc: stable@vger.kernel.org
Reviewed-by: NNick Desaulniers <ndesaulniers@google.com>
Signed-off-by: NNathan Huckleberry <nhuck@google.com>
Signed-off-by: NRussell King <rmk+kernel@armlinux.org.uk>

arch/arm/kernel/stacktrace.c

@@ -22,6 +22,19 @@
  * A simple function epilogue looks like this:
  *	ldm	sp, {fp, sp, pc}
  *
+ * When compiled with clang, pc and sp are not pushed. A simple function
+ * prologue looks like this when built with clang:
+ *
+ *	stmdb	{..., fp, lr}
+ *	add	fp, sp, #x
+ *	sub	sp, sp, #y
+ *
+ * A simple function epilogue looks like this when built with clang:
+ *
+ *	sub	sp, fp, #x
+ *	ldm	{..., fp, pc}
+ *
+ *
  * Note that with framepointer enabled, even the leaf functions have the same
  * prologue and epilogue, therefore we can ignore the LR value in this case.
  */
@@ -34,6 +47,16 @@ int notrace unwind_frame(struct stackframe *frame)
 	low = frame->sp;
 	high = ALIGN(low, THREAD_SIZE);
 
+#ifdef CONFIG_CC_IS_CLANG
+	/* check current frame pointer is within bounds */
+	if (fp < low + 4 || fp > high - 4)
+		return -EINVAL;
+
+	frame->sp = frame->fp;
+	frame->fp = *(unsigned long *)(fp);
+	frame->pc = frame->lr;
+	frame->lr = *(unsigned long *)(fp + 4);
+#else
 	/* check current frame pointer is within bounds */
 	if (fp < low + 12 || fp > high - 4)
 		return -EINVAL;
@@ -42,6 +65,7 @@ int notrace unwind_frame(struct stackframe *frame)
 	frame->fp = *(unsigned long *)(fp - 12);
 	frame->sp = *(unsigned long *)(fp - 8);
 	frame->pc = *(unsigned long *)(fp - 4);
+#endif
 
 	return 0;
 }