linux-kernel - Re: [PATCH] arm64/traps: Avoid unnecessary kernel/user pointer conversion

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Message-ID: <6662988b-b891-b6b0-cd7a-bd7f661fc737@arm.com>
Date:   Wed, 15 Sep 2021 19:26:55 +0530
From:   Amit Kachhap <amit.kachhap@....com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH] arm64/traps: Avoid unnecessary kernel/user pointer
 conversion

Hi,

On 9/14/21 9:30 PM, Mark Rutland wrote:
> On Tue, Sep 14, 2021 at 08:57:42PM +0530, Amit Daniel Kachhap wrote:
>> Annotating a pointer from kernel to __user and then back again might
>> confuse sparse. In call_undef_hook() it can be avoided by not using the
>> intermediate user pointer variable.
> 
> When you say "might confuse sparse", does it complain today? If so, can
> you include an example of what goes wrong?

No it does not give warning. The __force option silences the warning. My
idea is to remove the unwanted __force annotations and not mix user and
kernel pointers.

> 
>> Note: This patch adds no functional changes to code.
>>
>> Cc: Catalin Marinas <catalin.marinas@....com>
>> Cc: Will Deacon <will@...nel.org>
>> Signed-off-by: Amit Daniel Kachhap <amit.kachhap@....com>
>> ---
>>   arch/arm64/kernel/traps.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
>> index b03e383d944a..357d10a8bbf5 100644
>> --- a/arch/arm64/kernel/traps.c
>> +++ b/arch/arm64/kernel/traps.c
>> @@ -404,7 +404,8 @@ static int call_undef_hook(struct pt_regs *regs)
>>   
>>   	if (!user_mode(regs)) {
>>   		__le32 instr_le;
>> -		if (get_kernel_nofault(instr_le, (__force __le32 *)pc))
>> +		if (get_kernel_nofault(instr_le,
>> +				       (__le32 *)instruction_pointer(regs)))
> 
> Can we make `pc` an unsigned long, instead?

I think it can be done.

> 
> It'd be nice to handle all three cases consistently, even if that means
> adding __force to the two user cases.

Agree with your suggestion. Even in the 2 user cases, __force may not be
needed as the typecast will be from from unsigned long to user pointer.

BR,
Amit
> 
> Thanks,
> Mark.
> 
>>   			goto exit;
>>   		instr = le32_to_cpu(instr_le);
>>   	} else if (compat_thumb_mode(regs)) {
>> -- 
>> 2.17.1
>>