Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 9/25/24 08:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> As an intermediate step towards enabling PIE linking for the 64-bit x86
> kernel, enable PIE codegen for all objects that are linked into the
> kernel proper.
>
> This substantially reduces the number of relocations that need to be
> processed when booting a relocatable KASLR kernel.
This really seems like going completely backwards to me.

You are imposing a more restrictive code model on the kernel, optimizing 
for boot time in a way that will exert a permanent cost on the running 
kernel.
There is a *huge* difference between the kernel and user space here:

KERNEL MEMORY IS PERMANENTLY ALLOCATED, AND IS NEVER SHARED.

Dirtying user pages requires them to be unshared and dirty, which is 
undesirable. Kernel pages are *always* unshared and dirty.
> It also brings us much closer to the ordinary PIE relocation model used
> for most of user space, which is therefore much better supported and
> less likely to create problems as we increase the range of compilers and
> linkers that need to be supported.
We have been resisting *for ages* making the kernel worse to accomodate 
broken compilers. We don't "need" to support more compilers -- we need 
the compilers to support us. We have working compilers; any new compiler 
that wants to play should be expected to work correctly.
        -hpa