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by grahamedgecombe 4444 days ago
> Another thing that is missing is clearing the .BSS section before passing control to the C code. It's not used at the moment, though.

The Multiboot standard says that the boot loader will clear the .bss section for you - in section 3.1.3: "bss_end_addr Contains the physical address of the end of the bss segment. The boot loader initializes this area to zero"

I don't know what GRUB does if you rely on that fact it can parse ELF files and don't specify the fields like bss_end_addr though. I'm fairly sure it clears it in this case too, but I'm using Multiboot 2 for my OS so the behaviour could be different.
1 comments
exDM69 4444 days ago
> The Multiboot standard says that the boot loader will clear the .bss section for you - in section 3.1.3: "bss_end_addr Contains the physical address of the end of the bss segment. The boot loader initializes this area to zero"

Ok, good to know.

It certainly doesn't do that unless you tell it to (using the address tag), and this example (nor my hobby kernel) use that.

So the BSS must be cleared or the bootloader told to do so.

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grahamedgecombe 4444 days ago
I've just checked the GRUB source code and I think it will clear the .bss section even if it's loading an ELF file.

grub-core/loader/multiboot_elfxx.c has a function named grub_multiboot_load_elf32/64 which actually loads the segments of the ELF file. A segment has two fields defining its size: filesz (which is the amount of bytes to copy from the file) and memsz (which is its actual size once loaded). If memsz is greater than filesz, it zeroes the trailing bytes:

  if (phdr(i)->p_filesz < phdr(i)->p_memsz)
    grub_memset ((grub_uint8_t *) source + phdr(i)->p_filesz, 0,
      phdr(i)->p_memsz - phdr(i)->p_filesz);
The .bss section is placed by the linker at the end of a segment and increases memsz by the size of it (but not filesz, to avoid having to place lots of pointless zeroes in the ELF file) - for example this is one of the segments from my kernel's ELF file, which contains the .bss section at the end:

  LOAD off    0x0000000000020000 vaddr 0xffffffff8011f000 paddr 0x000000000011f000 align 2**12
       filesz 0x0000000000004be0 memsz 0x0000000000017678 flags rw-
Here you can see memsz is 0x17678 bytes and filesz is smaller at 0x4be0 bytes. The difference between them is the size of the .bss section.

grub_multiboot_load_elf32/64 is called in the case when the address tag is not present, so the .bss section will be cleared by GRUB in this case as well.

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