linux-kernel - RE: [PATCH v2] riscv: fix incorrect use of _

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Date:   Mon, 27 Nov 2023 10:35:42 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Clément Léger' <cleger@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Christoph Hellwig <hch@...radead.org>,
        Ben Dooks <ben.dooks@...ethink.co.uk>,
        kernel test robot <lkp@...el.com>
Subject: RE: [PATCH v2] riscv: fix incorrect use of __user pointer

From: Clément Léger
> Sent: 27 November 2023 10:24
> 
> On 25/11/2023 16:37, David Laight wrote:
> > ...
> >> @@ -491,7 +486,7 @@ int handle_misaligned_load(struct pt_regs *regs)
> >>
> >>  	val.data_u64 = 0;
> >>  	for (i = 0; i < len; i++) {
> >> -		if (load_u8(regs, (void *)(addr + i), &val.data_bytes[i]))
> >> +		if (load_u8(regs, addr + i, &val.data_bytes[i]))
> >>  			return -1;
> >>  	}
> >
> > I'd really have thought that you'd want to pull the kernel/user
> > check way outside the loop?
> 
> Hi David,
> 
> I hope the compiler is able to extract that 'if' out of the loop since
> regs isn't modified in the loop. Nevertheless, that could be more
> "clear" if put outside indeed.

If has access regs->xxx then the compiler can't do so because it
will must assume that the assignment might alias into 'regs'.
That is even true for byte writes if 'strict-aliasing' is enabled
- which it isn't for linux kernel builds.

It might do so if 'regs' were 'const'; it tends to assume that if
it can't change  something nothing can - although that isn't true.

	David

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