[PATCH] bootconfig: Extend the magic check range to the preceding 3 bytes

[Masami Hiramatsu]

Since Grub may align the size of initrd to 4 if user pass
initrd from cpio, we have to check the preceding 3 bytes as well.

Fixes: 85c46b78da58 ("bootconfig: Add bootconfig magic word for indicating bootconfig explicitly")
Reported-by: Chen Yu <yu.chen.surf@xxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
---
 init/main.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/init/main.c b/init/main.c
index 130376ec10ba..20baced721ad 100644
--- a/init/main.c
+++ b/init/main.c
@@ -269,14 +269,24 @@ static void * __init get_boot_config_from_initrd(u32 *_size, u32 *_csum)
 	u32 size, csum;
 	char *data;
 	u32 *hdr;
+	int i;
 
 	if (!initrd_end)
 		return NULL;
 
 	data = (char *)initrd_end - BOOTCONFIG_MAGIC_LEN;
-	if (memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
-		return NULL;
+	/*
+	 * Since Grub may align the size of initrd to 4, we must
+	 * check the preceding 3 bytes as well.
+	 */
+	for (i = 0; i < 4; i++) {
+		if (!memcmp(data, BOOTCONFIG_MAGIC, BOOTCONFIG_MAGIC_LEN))
+			goto found;
+		data--;
+	}
+	return NULL;
 
+found:
 	hdr = (u32 *)(data - 8);
 	size = hdr[0];
 	csum = hdr[1];