linux-kernel - Re: [PATCH] riscv: fix __user annotation in traps

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Date:   Fri, 24 Nov 2023 02:45:00 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Clément Léger <cleger@...osinc.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Ben Dooks <ben.dooks@...ethink.co.uk>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu
Subject: Re: [PATCH] riscv: fix __user annotation in traps_misaligned.c

On Fri, Nov 24, 2023 at 11:28:08AM +0100, Clément Léger wrote:
> I sent a similar patch two days ago with the same modification. I'm not
> sure to get it. Why is it better to pass the "unsigned long" type from
> the caller ? I mean, the resulting code would look like this right ?

Because you're legimitizing casting between address_space, which is a
horrible idea.  By casting either from the unsigned long you make it
very clear that deep magic is coming in and you make an informed
decisions based on the user_mode() predicate.  Witht a blind cast
to add/remove a __user you don't.

I'm actually surprised sparse even allows __user casts without __force.