[PATCH] x86_64: Display meaningful part of filename during BUG()

When building in a separate objtree, file names produced by BUG() & Co. can
get fairly long; printing only the first 50 characters may thus result in
(almost) no useful information. The following change makes it so that rather
the last 50 characters of the filename get printed.

Signed-Off-By: Jan Beulich <[e-mail]>

Signed-off-by: Andi Kleen <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/x86_64/kernel/traps.c

···
 void handle_BUG(struct pt_regs *regs)
 { 
 	struct bug_frame f;
-	char tmp;
+	long len;
+	const char *prefix = "";
 
 	if (user_mode(regs))
 		return; 
···
 	if (f.filename >= 0 ||
 	    f.ud2[0] != 0x0f || f.ud2[1] != 0x0b) 
 		return;
-	if (__get_user(tmp, (char *)(long)f.filename))
+	len = __strnlen_user((char *)(long)f.filename, PATH_MAX) - 1;
+	if (len < 0 || len >= PATH_MAX)
 		f.filename = (int)(long)"unmapped filename";
+	else if (len > 50) {
+		f.filename += len - 50;
+		prefix = "...";
+	}
 	printk("----------- [cut here ] --------- [please bite here ] ---------\n");
-	printk(KERN_ALERT "Kernel BUG at %.50s:%d\n", (char *)(long)f.filename, f.line);
+	printk(KERN_ALERT "Kernel BUG at %s%.50s:%d\n", prefix, (char *)(long)f.filename, f.line);
 } 
 
 #ifdef CONFIG_BUG

arch/x86_64/lib/usercopy.c

···
  * Return 0 on exception, a value greater than N if too long
  */
 
-long strnlen_user(const char __user *s, long n)
+long __strnlen_user(const char __user *s, long n)
 {
 	long res = 0;
 	char c;
-
-	if (!access_ok(VERIFY_READ, s, n))
-		return 0;
 
 	while (1) {
 		if (res>n)
···
 		res++;
 		s++;
 	}
+}
+
+long strnlen_user(const char __user *s, long n)
+{
+	if (!access_ok(VERIFY_READ, s, n))
+		return 0;
+	return __strnlen_user(s, n);
 }
 
 long strlen_user(const char __user *s)

include/asm-x86_64/uaccess.h

···
 long strncpy_from_user(char *dst, const char __user *src, long count);
 long __strncpy_from_user(char *dst, const char __user *src, long count);
 long strnlen_user(const char __user *str, long n);
+long __strnlen_user(const char __user *str, long n);
 long strlen_user(const char __user *str);
 unsigned long clear_user(void __user *mem, unsigned long len);
 unsigned long __clear_user(void __user *mem, unsigned long len);