Fix root mounting with no mount options

The "trivial conversion" in commit cccaa5e ("init: use do_mount()
instead of ksys_mount()") was totally broken, since it didn't handle the
case of a NULL mount data pointer.  And while I had "tested" it (and
presumably Dominik had too) that bug was hidden by me having options.

Cc: Dominik Brodowski <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Reported-by: Ondřej Jirman <[email protected]>
Reported-by: Guenter Roeck <[email protected]>
Reported-by: Naresh Kamboju <[email protected]>
Reported-and-tested-by: Borislav Petkov <[email protected]>
Tested-by: Chris Clayton <[email protected]>
Tested-by: Eric Biggers <[email protected]>
Tested-by: Geert Uytterhoeven <[email protected]>
Tested-by: Guido Günther <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: torvalds

init/do_mounts.c

@@ -391,17 +391,19 @@ static int __init do_mount_root(const char *name, const char *fs,
 				 const int flags, const void *data)
 {
 	struct super_block *s;
-	char *data_page;
-	struct page *p;
+	struct page *p = NULL;
+	char *data_page = NULL;
 	int ret;
 
-	/* do_mount() requires a full page as fifth argument */
-	p = alloc_page(GFP_KERNEL);
-	if (!p)
-		return -ENOMEM;
-
-	data_page = page_address(p);
-	strncpy(data_page, data, PAGE_SIZE - 1);
+	if (data) {
+		/* do_mount() requires a full page as fifth argument */
+		p = alloc_page(GFP_KERNEL);
+		if (!p)
+			return -ENOMEM;
+		data_page = page_address(p);
+		/* zero-pad. do_mount() will make sure it's terminated */
+		strncpy(data_page, data, PAGE_SIZE);
+	}
 
 	ret = do_mount(name, "/root", fs, flags, data_page);
 	if (ret)
@@ -417,7 +419,8 @@ static int __init do_mount_root(const char *name, const char *fs,
 	       MAJOR(ROOT_DEV), MINOR(ROOT_DEV));
 
 out:
-	put_page(p);
+	if (p)
+		put_page(p);
 	return ret;
 }