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SubjectRe: [PATCH] tracing/user_events: align uaddr on unsigned long alignment
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On 14/09/2023 19:29, Steven Rostedt wrote:
> On Thu, 14 Sep 2023 13:17:00 -0400
> Steven Rostedt <rostedt@goodmis.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

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