[PATCH] do_write_mem() return value check

From: BlaisorBlade <blaisorblade_spam@yahoo.it>, and me

- remove unused `file *' arg from do_write_mem()

- Add checking for copy_from_user() failures in do_write_mem()

- Return correct value from kmem writes() when a fault is encountered.  A
  write()-style syscall's return values are:

   0 when nothing was written and there was no error (someone tried to
   write zero bytes)

   >0: the number of bytes copied, whether or not there was an error.
   Userspace detects errors by noting that the write() return value is less
   than was requested.

   <0: there was an error and no bytes were copied

drivers/char/mem.c

@@ -102,10 +102,11 @@ static inline int valid_phys_addr_range(unsigned long addr, size_t *count)
 }
 #endif
 
-static ssize_t do_write_mem(struct file * file, void *p, unsigned long realp,
+static ssize_t do_write_mem(void *p, unsigned long realp,
 			    const char * buf, size_t count, loff_t *ppos)
 {
 	ssize_t written;
+	unsigned long copied;
 
 	written = 0;
 #if defined(__sparc__) || (defined(__mc68000__) && defined(CONFIG_MMU))
@@ -120,8 +121,14 @@ static ssize_t do_write_mem(struct file * file, void *p, unsigned long realp,
 		written+=sz;
 	}
 #endif
-	if (copy_from_user(p, buf, count))
+	copied = copy_from_user(p, buf, count);
+	if (copied) {
+		ssize_t ret = written + (count - copied);
+
+		if (ret)
+			return ret;
 		return -EFAULT;
+	}
 	written += count;
 	*ppos += written;
 	return written;
@@ -171,7 +178,7 @@ static ssize_t write_mem(struct file * file, const char * buf,
 
 	if (!valid_phys_addr_range(p, &count))
 		return -EFAULT;
-	return do_write_mem(file, __va(p), p, buf, count, ppos);
+	return do_write_mem(__va(p), p, buf, count, ppos);
 }
 
 static int mmap_mem(struct file * file, struct vm_area_struct * vma)
@@ -275,15 +282,19 @@ static ssize_t write_kmem(struct file * file, const char * buf,
 	unsigned long p = *ppos;
 	ssize_t wrote = 0;
 	ssize_t virtr = 0;
+	ssize_t written;
 	char * kbuf; /* k-addr because vwrite() takes vmlist_lock rwlock */
 
 	if (p < (unsigned long) high_memory) {
+
 		wrote = count;
 		if (count > (unsigned long) high_memory - p)
 			wrote = (unsigned long) high_memory - p;
 
-		wrote = do_write_mem(file, (void*)p, p, buf, wrote, ppos);
-
+		written = do_write_mem((void*)p, p, buf, wrote, ppos);
+		if (written != wrote)
+			return written;
+		wrote = written;
 		p += wrote;
 		buf += wrote;
 		count -= wrote;
@@ -292,15 +303,21 @@ static ssize_t write_kmem(struct file * file, const char * buf,
 	if (count > 0) {
 		kbuf = (char *)__get_free_page(GFP_KERNEL);
 		if (!kbuf)
-			return -ENOMEM;
+			return wrote ? wrote : -ENOMEM;
 		while (count > 0) {
 			int len = count;
 
 			if (len > PAGE_SIZE)
 				len = PAGE_SIZE;
-			if (len && copy_from_user(kbuf, buf, len)) {
-				free_page((unsigned long)kbuf);
-				return -EFAULT;
+			if (len) {
+				written = copy_from_user(kbuf, buf, len);
+				if (written != len) {
+					ssize_t ret;
+
+					free_page((unsigned long)kbuf);
+					ret = wrote + virtr + (len - written);
+					return ret ? ret : -EFAULT;
+				}
 			}
 			len = vwrite(kbuf, (char *)p, len);
 			count -= len;