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Multiboot 2 Header Alignment: implementation contradicts specification

From: Jacob Paul
Subject: Multiboot 2 Header Alignment: implementation contradicts specification
Date: Sat, 23 May 2020 18:50:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0

The Multiboot2 specification specifies that the Multiboot2 header should
be 8-byte (64-bit) aligned:
>An OS image must contain an additional header called Multiboot2 header,
>besides the headers of the format used by the OS image. The Multiboot2
>header must be contained completely within the first 32768 bytes of the
>OS image, and must be 64-bit aligned. In general, it should come as
>early as possible, and may be embedded in the beginning of the text
>segment after the real executable header.

However, the implementation of find_header() in multiboot_mbi2.c looks
like this:
>static struct multiboot_header *
>find_header (grub_properly_aligned_t *buffer, grub_ssize_t len)
>  struct multiboot_header *header;
>  /* Look for the multiboot header in the buffer.  The header should
>     be at least 12 bytes and aligned on a 4-byte boundary.  */
>  for (header = (struct multiboot_header *) buffer;
>       ((char *) header <= (char *) buffer + len - 12);
> header = (struct multiboot_header *) ((grub_uint32_t *) header + >MULTIBOOT_HEADER_ALIGN / 4))
>    {
>      if (header->magic == MULTIBOOT2_HEADER_MAGIC
>      && !(header->magic + header->architecture
>           + header->header_length + header->checksum)
>      && header->architecture == MULTIBOOT2_ARCHITECTURE_CURRENT)
>    return header;
>    }
>  return NULL;

There are multiple things that doesn't look right to me.
The comment says that the header should be 4-byte aligned while at the
same time, the actual loop only increments header 2 bytes for every
It seems like this was just copied over from multiboot_mbi.c since they
basically are identical with even the same comment.

Is there a genuine problem here, or am I missing something?
If it actually is just lazy copy-pasting; should it be changed, as some
people might actually rely on grub finding the Multiboot2 header even
though it isn't 8-byte aligned?


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