sh: BUG() handling through trapa vector.

Previously we haven't been doing anything with verbose BUG() reporting,
and we've been relying on the oops path for handling BUG()'s, which is
rather sub-optimal.

This switches BUG handling to use a fixed trapa vector (#0x3e) where we
construct a small bug frame post trapa instruction to get the context
right. This also makes it trivial to wire up a DIE_BUG for the atomic
die chain, which we couldn't really do before.
Signed-off-by: Paul Mundt <lethal@linux-sh.org>

arch/sh/kernel/process.c

@@ -498,6 +498,16 @@ asmlinkage void break_point_trap_software(unsigned long r4, unsigned long r5,
 {
 	struct pt_regs *regs = RELOC_HIDE(&__regs, 0);
 
+	/* Rewind */
 	regs->pc -= 2;
+
+#ifdef CONFIG_BUG
+	if (__kernel_text_address(instruction_pointer(regs))) {
+		u16 insn = *(u16 *)instruction_pointer(regs);
+		if (insn == TRAPA_BUG_OPCODE)
+			handle_BUG(regs);
+	}
+#endif
+
 	force_sig(SIGTRAP, current);
 }

arch/sh/kernel/traps.c

@@ -19,6 +19,7 @@
 #include <linux/kallsyms.h>
 #include <linux/io.h>
 #include <linux/debug_locks.h>
+#include <linux/limits.h>
 #include <asm/system.h>
 #include <asm/uaccess.h>
 
@@ -129,6 +130,40 @@ static int die_if_no_fixup(const char * str, struct pt_regs * regs, long err)
 	return -EFAULT;
 }
 
+#ifdef CONFIG_BUG
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+static inline void do_bug_verbose(struct pt_regs *regs)
+{
+	struct bug_frame f;
+	long len;
+
+	if (__copy_from_user(&f, (const void __user *)regs->pc,
+			     sizeof(struct bug_frame)))
+		return;
+
+	len = __strnlen_user(f.file, PATH_MAX) - 1;
+	if (unlikely(len < 0 || len >= PATH_MAX))
+		f.file = "<bad filename>";
+	len = __strnlen_user(f.func, PATH_MAX) - 1;
+	if (unlikely(len < 0 || len >= PATH_MAX))
+		f.func = "<bad function>";
+
+	printk(KERN_ALERT "kernel BUG in %s() at %s:%d!\n",
+	       f.func, f.file, f.line);
+}
+#else
+static inline void do_bug_verbose(struct pt_regs *regs)
+{
+}
+#endif /* CONFIG_DEBUG_BUGVERBOSE */
+#endif /* CONFIG_BUG */
+
+void handle_BUG(struct pt_regs *regs)
+{
+	do_bug_verbose(regs);
+	die("Kernel BUG", regs, TRAPA_BUG_OPCODE & 0xff);
+}
+
 /*
  * handle an instruction that does an unaligned memory access by emulating the
  * desired behaviour

include/asm-sh/bug.h

 #ifndef __ASM_SH_BUG_H
 #define __ASM_SH_BUG_H
 
-
 #ifdef CONFIG_BUG
-/*
- * Tell the user there is some problem.
- */
-#define BUG() do { \
-	printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
-	*(volatile int *)0 = 0; \
+
+struct bug_frame {
+	unsigned short	opcode;
+	unsigned short	line;
+	const char	*file;
+	const char	*func;
+};
+
+struct pt_regs;
+
+extern void handle_BUG(struct pt_regs *);
+
+#define TRAPA_BUG_OPCODE	0xc33e	/* trapa #0x3e */
+
+#ifdef CONFIG_DEBUG_BUGVERBOSE
+
+#define BUG()						\
+do {							\
+	__asm__ __volatile__ (				\
+		".align	2\n\t"				\
+		".short	%O0\n\t"			\
+		".short	%O1\n\t"			\
+		".long	%O2\n\t"			\
+		".long	%O3\n\t"			\
+		:					\
+		: "n" (TRAPA_BUG_OPCODE),		\
+		  "i" (__LINE__), "X" (__FILE__),	\
+		  "X" (__FUNCTION__));			\
+} while (0)
+
+#else
+
+#define BUG()					\
+do {						\
+	__asm__ __volatile__ (			\
+		".align	2\n\t"			\
+		".short	%O0\n\t"		\
+		:				\
+		: "n" (TRAPA_BUG_OPCODE));	\
 } while (0)
 
+#endif /* CONFIG_DEBUG_BUGVERBOSE */
+
 #define HAVE_ARCH_BUG
-#endif
+
+#endif /* CONFIG_BUG */
 
 #include <asm-generic/bug.h>
 
-#endif
+#endif /* __ASM_SH_BUG_H */