Multiboot v1 memory map offset wrong

I'm developping a multiboot kernel for multiboot v1
My multiboot header contains the V1 magic (0x1BADB002) and the flags 0x00010243  (with enabled memory detection, and boot loader name)


When booted in multiboot,
Qemu gives me two pointers:
unsigned long mmap_length;
unsigned long mmap_addr;

mmap_addr shall points to this structure:
struct multiboot_mmap_entry
{
multiboot_uint32_t size;
multiboot_uint64_t addr;
multiboot_uint64_t len;
multiboot_uint32_t type;
} 


According to the multiboot v1 specification, mmap_addr should not point  to the start of this structure, but instead, should point to the "addr "field.

Work-arround:
Detect if qemu is used using bootloader_name field.
If it is, do NOT apply a -4 offset to mmap_addr

http://git.savannah.gnu.org/cgit/grub.git/tree/doc/multiboot.texi?h=multiboot

I forgot to tell how i detected the issue:
I print all the fields, and in "type", i had the size of  the next entry (and in size, i have always zero, which corresponds to the high part of addr)

Hi,

Have you tested your code with GRUB (Legacy) itself? Looking at code from one of my own hobby kernels, and from a hobby kernel that has largely not been written by me, both are interpreting the fields as qemu is (i.e. mmap_addr points to a multiboot_mmap_entry, and not to multiboot_mmap_entry + 4), and I know both to work just fine with GRUB.

I considered the -4 offset to signify that the size field simply does not count itself, i.e. the size of one multiboot_mmap_entry is $size + 4.

Max

Tested today with GRUB 2.0. Indeed, mmap_entry points to the start of the structure, without a fancy offset. I guess we can close this ticket