[RFC v2 4/4] arm64: ftrace: add a stack frame for exception handler

[AKASHI Takahiro]

On arm64, an exception handler use the same stack as in non-exception
contexts, but doesn't create a stack frame for elx_xx entry, only updating
sp register. This behavior results in save_stace_trace() missing a function
that is the one when an exception happens.

This patch creates a stack frame for this case, and puts an additional
entry for the function  in a stack trace list.

Signed-off-by: AKASHI Takahiro <takahiro.akashi at linaro.org>
---
 arch/arm64/kernel/entry.S      |    4 ++++
 arch/arm64/kernel/stacktrace.c |   17 +++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index f860bfd..aacb6c6 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -107,6 +107,8 @@
 	str	x21, [sp, #S_SYSCALLNO]
 	.endif
 
+	/* create a stack frame for stack tracer */
+	mov	x29, sp
 	/*
 	 * Registers that may be useful after this macro is invoked:
 	 *
@@ -737,3 +739,5 @@ ENTRY(sys_rt_sigreturn_wrapper)
 	mov	x0, sp
 	b	sys_rt_sigreturn
 ENDPROC(sys_rt_sigreturn_wrapper)
+
+ENTRY(end_of_vectors)
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index d1790eb..22ce7c9 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -25,6 +25,10 @@
 #include <asm/stacktrace.h>
 
 #define S_FRAME_SIZE sizeof(struct pt_regs) /* asm-offsets.h */
+#define S_FP offsetof(struct pt_regs, regs[29])
+#define S_LR offsetof(struct pt_regs, regs[30])
+
+extern unsigned int *vectors, *end_of_vectors;
 
 /*
  * AArch64 PCS assigns the frame pointer to x29.
@@ -50,6 +54,19 @@ int notrace unwind_frame(struct stackframe *frame)
 	if (fp < low || fp > high - 0x18 || fp & 0xf)
 		return -EINVAL;
 
+	if ((frame->pc >= (unsigned long)&vectors) &&
+			(frame->pc < (unsigned long)&end_of_vectors)) {
+		/*
+		 * Expection handler does not use a normal format of
+		 * stack frame, but allocates struct pt_regs.
+		 */
+		frame->sp = frame->sp + S_FRAME_SIZE;
+		frame->fp = *(unsigned long *)(fp + S_FP);
+		frame->pc = *(unsigned long *)(fp + S_LR);
+
+		return 0;
+	}
+
 	frame->sp = fp + 0x10;
 	frame->fp = *(unsigned long *)(fp);
 	/*
-- 
1.7.9.5