Re: [PATCH 2/5] x86, KASLR: Drop CONFIG_RANDOMIZE_BASE_MAX_OFFSET

[Ingo Molnar]

* Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> >> +       Since the kernel is built using 2GB addressing,
> >
> > Does that try to refer to the 1G kernel and 1G fixmap pagetable
> > mappings? I.e., level2_kernel_pgt and level2_fixmap_pgt in
> > arch/x86/kernel/head_64.S?
> 
> The "2GB addressing" part is in reference to:
> 
>        -mcmodel=kernel
>            Generate code for the kernel code model.  The kernel runs in the
>            negative 2 GB of the address space.  This model has to be used for
>            Linux kernel code.

On x86-64 this is a special GCC compiler small memory model, it is called the 
'kernel code model', which is rather generic and no 'real name' ever stuck.

Due to RIP-relative addressing and the sign-extension of 48 bit virtual addresses, 
this allows nearly as compact kernel code and (static) kernel data definitions as 
a 32-bit kernel would allow.

The (positive) 0-4GB virtual memory range has similar advantages, but is of course 
frequently used by user-space code. Negative addresses are reserved for the kernel 
only.

Thanks,

	Ingo