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>

3 files changed, 89 insertions, 9 deletions

diff --git a/arch/sh/kernel/process.c b/arch/sh/kernel/process.c
index f3e2631be144..7347f6afa030 100644
--- a/arch/sh/kernel/process.c
+++ b/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);
 }
diff --git a/arch/sh/kernel/traps.c b/arch/sh/kernel/traps.c
index 3762d9dc2046..ec110157992d 100644
--- a/arch/sh/kernel/traps.c
+++ b/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
diff --git a/include/asm-sh/bug.h b/include/asm-sh/bug.h
index 1b4fc52a59e8..2f89dd06d0cd 100644
--- a/include/asm-sh/bug.h
+++ b/include/asm-sh/bug.h
@@ -1,19 +1,54 @@
 #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 */