[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
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
iteration (MULTIBOOT_HEADER_ALIGN=8).
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?
Jacob
- Multiboot 2 Header Alignment: implementation contradicts specification,
Jacob Paul <=