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

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Message-ID: <603c39d4-2764-4f85-b173-069ced2a3a13@rivosinc.com>
Date:   Mon, 27 Nov 2023 11:37:28 +0100
From:   Clément Léger <cleger@...osinc.com>
To:     David Laight <David.Laight@...LAB.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



On 27/11/2023 11:35, David Laight wrote:
> 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.

Ok, good to know ! As I said, I'll modify that in a subsequent patch.

Thanks,

Clément

> 
> 	David
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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