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

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Date:   Thu, 14 Sep 2023 13:29:56 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Clément Léger <cleger@...osinc.com>
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 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);
	}

-- Steve