shim: Properly generate absolute paths from relative image paths

The generate_path_from_image_path() doesn't properly handle the case when
shim is invoked using a relative path (e.g: from the EFI shell). In that
function, always the last component is stripped from absolute file path
to calculate the dirname, and this is concatenated with the image path.

But if the path is a relative one, the function will wrongly concatenate
the dirname with the relative image path, i.e:

 Shell> FS0:
 FS0:\> cd EFI
 FS0:\EFI\> BOOT\BOOTX64.EFI
 Failed to open \EFI\BOOT\BOOT\BOOTX64.EFI - Not found
 Failed to load image \EFI\BOOT\BOOT\BOOTX64.EFI: Not found
 start_image() returned Not found

Calculate the image path basename and concatenate that with the dirname.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Maran Wilson maran.wilson@oracle.com
Tested-by: Maran Wilson maran.wilson@oracle.com

shim.c

@@ -1640,9 +1640,11 @@ static EFI_STATUS generate_path_from_image_path(EFI_LOADED_IMAGE *li,
 		bootpath[j] = '\0';
 	}
 
-	while (*ImagePath == '\\')
-		ImagePath++;
+	for (i = 0, last = 0; i < StrLen(ImagePath); i++)
+		if (ImagePath[i] == '\\')
+			last = i + 1;
 
+	ImagePath = ImagePath + last;
 	*PathName = AllocatePool(StrSize(bootpath) + StrSize(ImagePath));
 
 	if (!*PathName) {