/dev/mem: dont allow seek to last page

So as to return a uniform error -EOVERFLOW instead of a random one:

# kmem-seek 0xfffffffffffffff0
seek /dev/kmem: Device or resource busy
# kmem-seek 0xfffffffffffffff1
seek /dev/kmem: Block device required

Suggested by OGAWA Hirofumi.

Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/mem.c

@@ -708,13 +708,20 @@ static loff_t memory_lseek(struct file * file, loff_t offset, int orig)
 
 	mutex_lock(&file->f_path.dentry->d_inode->i_mutex);
 	switch (orig) {
-		case 0:
-			file->f_pos = offset;
-			ret = file->f_pos;
-			force_successful_syscall_return();
+		case SEEK_CUR:
+			offset += file->f_pos;
+			if ((unsigned long long)offset <
+			    (unsigned long long)file->f_pos) {
+				ret = -EOVERFLOW;
 				break;
-		case 1:
-			file->f_pos += offset;
+			}
+		case SEEK_SET:
+			/* to avoid userland mistaking f_pos=-9 as -EBADF=-9 */
+			if ((unsigned long long)offset >= ~0xFFFULL) {
+				ret = -EOVERFLOW;
+				break;
+			}
+			file->f_pos = offset;
 			ret = file->f_pos;
 			force_successful_syscall_return();
 			break;