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

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Message-ID: <9cb1fadf-bb83-4e9f-9c29-bff53e30b0c6@rivosinc.com>
Date:   Mon, 27 Nov 2023 11:24:25 +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 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.

> In any case, for a misaligned read why not just read (addr & ~7)[0]
> and (if needed) (addr & ~7)[1] and then ahift and or together?

Makes sense, the original code was like that but probably copy/pasted
from openSBI I guess. (?)

Let's keep that for the moment (this patch is about fixing wrong __user
address space). I will try to submit another series using what you proposed.

Regards,

Clément

> 
> clang will do it for misaligned structure members with known
> misalignment, but it is almost certainly also better for reads
> with unknown misalignment.
> 
> 	David
> 
> -
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