[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.s...@gmail.com>
Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
---
 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];