[PATCH RFC 3/3] arm64: unwind stack on exception

[Caleb Connolly]

We already build arm64 images with frame pointers. Let's finally make
use of them in tandem with the new symbol lookup support by unwinding
the stack when an exception occurs, producing a backtrace similar to
those emitted by Linux.

In addition, introduce a dedicated unwind_stack() function which can be
called from anywhere to print a backtrace.

Signed-off-by: Caleb Connolly <caleb.connolly at linaro.org>
---
 arch/arm/include/asm/ptrace.h |  4 +++
 arch/arm/lib/interrupts_64.c  | 76 +++++++++++++++++++++++++++++++++++++++++++
 lib/Kconfig                   |  6 ++--
 3 files changed, 84 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/ptrace.h b/arch/arm/include/asm/ptrace.h
index a836f6cc60db..f42cfb33e918 100644
--- a/arch/arm/include/asm/ptrace.h
+++ b/arch/arm/include/asm/ptrace.h
@@ -22,8 +22,12 @@
 
 #ifdef __KERNEL__
 extern void show_regs(struct pt_regs *);
 
+#if CONFIG_IS_ENABLED(SYMBOL_LOOKUP)
+void unwind_stack(void);
+#endif
+
 #define predicate(x)	(x & 0xf0000000)
 #define PREDICATE_ALWAYS	0xe0000000
 
 #endif
diff --git a/arch/arm/lib/interrupts_64.c b/arch/arm/lib/interrupts_64.c
index b3024ba514ec..24edeb0de51e 100644
--- a/arch/arm/lib/interrupts_64.c
+++ b/arch/arm/lib/interrupts_64.c
@@ -3,15 +3,17 @@
  * (C) Copyright 2013
  * David Feng <fenghua at phytium.com.cn>
  */
 
+#include <asm-generic/sections.h>
 #include <asm/esr.h>
 #include <asm/global_data.h>
 #include <asm/ptrace.h>
 #include <irq_func.h>
 #include <linux/compiler.h>
 #include <efi_loader.h>
 #include <semihosting.h>
+#include <symbols.h>
 
 DECLARE_GLOBAL_DATA_PTR;
 
 int interrupt_init(void)
@@ -80,8 +82,77 @@ static void dump_instr(struct pt_regs *regs)
 		printf(i == 0 ? "(%08x) " : "%08x ", addr[i]);
 	printf("\n");
 }
 
+#if IS_ENABLED(CONFIG_SYMBOL_LOOKUP)
+
+static void print_sym(unsigned long symaddr, const char *symname, unsigned long offset)
+{
+	if (symaddr > (unsigned long)__bss_end)
+		printf("\t<%#016lx> ???\n", symaddr);
+	else
+		printf("\t<%#016lx> %s+%#lx\n", symaddr, symname, offset);
+}
+
+/* Stack frames automatically generated by compiler emitted code */
+struct stack_frame {
+	struct stack_frame *prev_ptr; /* Alays a bigger address on ARM64 */
+	unsigned long lr;
+	char data[];
+};
+
+static void __unwind_stack(unsigned long lr, unsigned long x29)
+{
+	char symname[KSYM_NAME_LEN+1] = { 0 };
+	unsigned long addr, offset;
+	struct stack_frame *fp, *last_fp;
+
+	/*
+	 * Either the place where unwind_stack() was called, or the
+	 * instruction that caused an exception
+	 */
+	symbols_lookup(lr, &addr, &offset, symname);
+	print_sym(addr, symname, offset);
+
+	fp = (struct stack_frame *)x29;
+	while (fp && fp->prev_ptr > fp) {
+		symbols_lookup(fp->lr, &addr, &offset, symname);
+		/*
+		 * _start is used over gd->relocaddr because it will be correct
+		 * if U-Boot was built as relocatable and loaded at an arbitrary
+		 * address (both pre and post-relocation).
+		 */
+		print_sym(addr + (unsigned long)_start, symname, offset);
+		last_fp = fp;
+		fp = (struct stack_frame *)(u64)fp->prev_ptr;
+	}
+}
+
+/**
+ * unwind_stack() - Unwind the callstack and print a backtrace.
+ *
+ * This function can be called for debugging purposes to walk backwards through
+ * stack frames, printing the function name and offset of the branch instruction.
+ *
+ * It reads out the link register (x30) which contains the return address of the
+ * caller, and x29 which contains the frame pointer of the caller.
+ */
+void unwind_stack(void)
+{
+	unsigned long x29, x30;
+
+	asm("mov %0, x29" : "=r" (x29));
+	asm("mov %0, x30" : "=r" (x30));
+
+	__unwind_stack(x30, x29);
+}
+
+#else
+
+#define __unwind_stack(lr, x29) do {} while (0)
+
+#endif
+
 void show_regs(struct pt_regs *regs)
 {
 	int i;
 
@@ -95,8 +166,13 @@ void show_regs(struct pt_regs *regs)
 		printf("x%-2d: %016lx x%-2d: %016lx\n",
 		       i, regs->regs[i], i+1, regs->regs[i+1]);
 	printf("\n");
 	dump_instr(regs);
+
+	if (IS_ENABLED(CONFIG_SYMBOL_LOOKUP)) {
+		printf("\nBacktrace:\n");
+		__unwind_stack(regs->elr, regs->regs[29]);
+	}
 }
 
 /*
  * Try to "emulate" a semihosting call in the event that we don't have a
diff --git a/lib/Kconfig b/lib/Kconfig
index 06a78f83b7d6..092b780aeeb8 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -979,11 +979,13 @@ config VPL_OF_LIBFDT_ASSUME_MASK
 	  unsafe execution. See FDT_ASSUME_PERFECT, etc. in libfdt_internal.h
 
 config SYMBOL_LOOKUP
 	bool "Enable symbol lookup"
+	default n
 	help
-	  This enables support for looking up symbol names from addresses. The
-	  primary usecase for this is improved debugging support.
+	  This enables support for looking up symbol names from addresses. When
+	  enabled U-Boot will print stack traces with function names when an
+	  exception occurs.
 
 menu "System tables"
 	depends on (!EFI && !SYS_COREBOOT) || (ARM && EFI_LOADER)
 

-- 
2.45.2