linux-kernel - Re: [PATCH] tracing/user_events: align uaddr on unsigned long alignment

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Date:   Thu, 14 Sep 2023 19:32:35 +0200
From:   Clément Léger <cleger@...osinc.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        Beau Belgrave <beaub@...ux.microsoft.com>
Subject: Re: [PATCH] tracing/user_events: align uaddr on unsigned long
 alignment



On 14/09/2023 19:29, Steven Rostedt wrote:
> On Thu, 14 Sep 2023 13:17:00 -0400
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
>> Now lets look at big endian layout:
>>
>>  uaddr = 0xbeef0004
>>  enabler = 1;
>>
>>  memory at 0xbeef0000:  00 00 00 00 00 00 00 02
>>                                     ^
>>                                     addr: 0xbeef0004
>>
>> 				(enabler is set )
>>
>> 	bitoffset = uaddr & (sizeof(unsigned long) - 1); bitoffset = 4
>> 	bit_offset *= 8;				 bitoffset = 32
>> 	uaddr &= ~(sizeof(unsigned long) - 1);		 uaddr = 0xbeef0000
>>
>> 	ptr = kaddr + (uaddr & ~PAGE_MASK);
>>
>> 	clear_bit(1 + 32, ptr);
>>
>>  memory at 0xbeef0000:  00 00 00 00 00 00 00 02
>>                                   ^
>> 				bit 33 of 0xbeef0000
>>
>> I don't think that's what you expected!
> 
> I believe the above can be fixed with:
> 
> 	bit_offset = uaddr & (sizeof(unsigned long) - 1);
> 	if (bit_offset) {
> #ifdef CONFIG_CPU_BIG_ENDIAN
> 		bit_offest = 0;
> #else
> 		bit_offset *= BITS_PER_BYTE;
> #endif
> 		uaddr &= ~(sizeof(unsigned long) - 1);
> 	}

Hi Steven,

Nice catch ! I don't have big endian at end but I'll check that.

Thanks,

> 
> -- Steve