[PATCH] ARM: Use generic BUG() handler

[sjg@chromium.org (Simon Glass)]

From: Simon Glass <sjg@google.com>

ARM uses its own BUG() handler which makes its output slightly different
from other archtectures.

One of the problems is that the ARM implementation doesn't report the function
with the BUG() in it, but always reports the PC being in __bug(). The generic
implementation doesn't have this problem.

Currently we get something like:

kernel BUG at fs/proc/breakme.c:35!
Unable to handle kernel NULL pointer dereference at virtual address 00000000
...
PC is at __bug+0x20/0x2c

With this patch it displays:

kernel BUG at fs/proc/breakme.c:35!
Internal error: Oops - undefined instruction: 0 [#1] PREEMPT SMP
...
PC is at write_breakme+0xd0/0x1b4

This implementation uses an undefined instruction to implement BUG, and sets up
a bug table containing the relevant information. Many version of gcc do not
support %c properly for ARM (inserting a # when they shouldn't) so we work
around this using distasteful macro magic.

Also the backtrace the message now appears even when CONFIG_ARM_UNWIND is
defined.

You can fall back to the previous BUG implementation by making GENERIC_BUG
default to n in arch/arm/Kconfig.

Change-Id: I07d77c832e816f5ad2390e25f466ddf750adecf4

Signed-off-by: Simon Glass <sjg@chromium.org>
---
 arch/arm/Kconfig              |    4 +++
 arch/arm/include/asm/bug.h    |   57 +++++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/traps.c       |   24 +++++++++++++++++
 arch/arm/kernel/vmlinux.lds.S |   12 ++++++++
 4 files changed, 97 insertions(+), 0 deletions(-)

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 166efa2..7c8f11c 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -191,6 +191,10 @@ config VECTORS_BASE
 	help
 	  The base address of exception vectors.
 
+config GENERIC_BUG
+	def_bool y
+	depends on BUG
+
 source "init/Kconfig"
 
 source "kernel/Kconfig.freezer"
diff --git a/arch/arm/include/asm/bug.h b/arch/arm/include/asm/bug.h
index 4d88425..3f8a798 100644
--- a/arch/arm/include/asm/bug.h
+++ b/arch/arm/include/asm/bug.h
@@ -3,6 +3,61 @@
 
 
 #ifdef CONFIG_BUG
+
+#ifdef CONFIG_GENERIC_BUG
+
+/*
+ * Use a suitable undefined instruction to use for ARM/Thumb2 bug handling.
+ * We need to be careful not to conflict with those used by other modules and
+ * the register_undef_hook() system.
+ */
+#ifdef CONFIG_THUMB2_KERNEL
+#define BUG_INSTR_VALUE 0xde02
+#define BUG_INSTR_TYPE ".hword "
+#else
+#define BUG_INSTR_VALUE 0xe7f001f2
+#define BUG_INSTR_TYPE ".word "
+#endif
+
+
+#define BUG() _BUG(__FILE__, __LINE__, BUG_INSTR_VALUE)
+#define _BUG(file, line, value) __BUG(file, line, value)
+
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+
+/*
+ * The extra indirection is to ensure that the __FILE__ string comes through
+ * OK. Many version of gcc do not support the asm %c parameter which would be
+ * preferable to this unpleasantness. We use mergeable string sections to
+ * avoid multiple copies of the string appearing in the kernel image.
+ */
+
+#define __BUG(__file, __line, __value)				\
+do {								\
+	BUILD_BUG_ON(sizeof(struct bug_entry) != 12);		\
+	asm volatile("1:\t" BUG_INSTR_TYPE #__value "\n"	\
+		".pushsection .rodata.str, \"aMS\", 1\n"	\
+		"2:\t.asciz " #__file "\n" 			\
+		".popsection\n" 				\
+		".pushsection __bug_table,\"a\"\n"		\
+		"3:\t.word 1b, 2b\n"				\
+		"\t.hword " #__line ", 0\n"			\
+		".popsection");					\
+	unreachable();						\
+} while (0)
+
+#else  /* not CONFIG_DEBUG_BUGVERBOSE */
+
+#define __BUG(__file, __line, __value)				\
+do {								\
+	asm volatile(BUG_INSTR_TYPE #__value);			\
+	unreachable();						\
+} while (0)
+#endif  /* CONFIG_DEBUG_BUGVERBOSE */
+
+
+#else  /* not CONFIG_GENERIC_BUG */
+
 #ifdef CONFIG_DEBUG_BUGVERBOSE
 extern void __bug(const char *file, int line) __attribute__((noreturn));
 
@@ -16,6 +71,8 @@ extern void __bug(const char *file, int line) __attribute__((noreturn));
 
 #endif
 
+#endif  /* CONFIG_GENERIC_BUG */
+
 #define HAVE_ARCH_BUG
 #endif
 
diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index ee57640..1199cfe 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -21,6 +21,7 @@
 #include <linux/kdebug.h>
 #include <linux/module.h>
 #include <linux/kexec.h>
+#include <linux/bug.h>
 #include <linux/delay.h>
 #include <linux/init.h>
 
@@ -163,6 +164,7 @@ static void dump_instr(const char *lvl, struct pt_regs *regs)
 #ifdef CONFIG_ARM_UNWIND
 static inline void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk)
 {
+	printk("Backtrace: ");
 	unwind_backtrace(regs, tsk);
 }
 #else
@@ -271,6 +273,8 @@ void die(const char *str, struct pt_regs *regs, int err)
 	spin_lock_irq(&die_lock);
 	console_verbose();
 	bust_spinlocks(1);
+	if (!user_mode(regs))
+		report_bug(regs->ARM_pc, regs);
 	ret = __die(str, err, thread, regs);
 
 	if (regs && kexec_should_crash(thread->task))
@@ -302,6 +306,26 @@ void arm_notify_die(const char *str, struct pt_regs *regs,
 	}
 }
 
+#ifdef CONFIG_GENERIC_BUG
+
+int is_valid_bugaddr(unsigned long pc)
+{
+#ifdef CONFIG_THUMB2_KERNEL
+	unsigned short bkpt;
+#else
+	unsigned long bkpt;
+#endif
+
+	if (pc < PAGE_OFFSET)
+		return 0;
+	if (probe_kernel_address((unsigned *)pc, bkpt))
+		return 0;
+
+	return bkpt == BUG_INSTR_VALUE;
+}
+
+#endif
+
 static LIST_HEAD(undef_hook);
 static DEFINE_SPINLOCK(undef_lock);
 
diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 6146279..4f22346 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -80,6 +80,18 @@ SECTIONS
 
 	PERCPU(PAGE_SIZE)
 
+	/*
+	* .exit.text is discarded@runtime, not link time, to deal with
+	*  references from bug_table
+	*/
+	.exit.text : AT(ADDR(.exit.text)) {
+		EXIT_TEXT
+	}
+
+	.exit.data : AT(ADDR(.exit.data)) {
+		EXIT_DATA
+	}
+
 #ifndef CONFIG_XIP_KERNEL
 	. = ALIGN(PAGE_SIZE);
 	__init_end = .;
-- 
1.7.3.1