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

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Message-ID: <20190327075441.GA29894@infradead.org>
Date:   Wed, 27 Mar 2019 00:54:41 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Anup Patel <anup@...infault.org>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Anup Patel <Anup.Patel@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Atish Patra <Atish.Patra@....com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v3 4/4] RISC-V: Allow booting kernel from any 4KB aligned
 address

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.