Re: [PATCH v3 4/4] RISC-V: Allow booting kernel from any 4KB aligned address

[Mike Rapoport]

On Wed, Mar 27, 2019 at 12:54:41AM -0700, Christoph Hellwig wrote:
> On Mon, Mar 25, 2019 at 09:46:59PM +0530, Anup Patel wrote:
> > > Why do you even care about kernel mappings for non-existant ram.
> > 
> > We care because there will always be some buggy kernel driver/code going
> > out-of-bound and accessing non-existent RAM. If we by default map all
> > possible kernel virtual address then behaviour of buggy accesses will be
> > unpredictable.
> > 
> > Further, I think we should also make .text and .rodata sections of kernel
> > as read-only. This will protect kernel code and rodata.
> 
> All of that is useful at the final_setup_vm() time - but none of it
> matters during early setup_vm where life is complicated.
> 
> Mike suggested on the previous iteration that you only do smaller
> mappings when setting up the final mapping to avoid the ops churn,
> and I fully agree with him. 
> 
> So I would suggest we avoid complicated the fiddly early boot changes
> that just add complxity, and you instead redirect your efforts to
> say implemented proper ro and non-executable sections using 4k mappings
> in the final VM setup only.  That should actuall lead to less code
> and complexity, and provide more benefits.

It might be worth keeping trampoline_pg_dir if we are to split setup_vm().
Then setup_vm() will only initialize the trampoline_pg_dir and
final_setup_vm() will setup the swapper_pg_dir and switch to it.
Otherwise final_setup_vm() would need to update live mappings which might
be fragile.

-- 
Sincerely yours,
Mike.