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

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Date:   Thu, 14 Sep 2023 13:17:00 -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 15:11:02 +0200
Clément Léger <cleger@...osinc.com> wrote:

> @@ -511,13 +511,19 @@ static int user_event_enabler_write(struct user_event_mm *mm,
>  	}
>  
>  	kaddr = kmap_local_page(page);
> +
> +	bit_offset = uaddr & (sizeof(unsigned long) - 1);
> +	if (bit_offset) {
> +		bit_offset *= 8;
> +		uaddr &= ~(sizeof(unsigned long) - 1);
> +	}
>  	ptr = kaddr + (uaddr & ~PAGE_MASK);
>  
>  	/* Update bit atomically, user tracers must be atomic as well */
>  	if (enabler->event && enabler->event->status)
> -		set_bit(ENABLE_BIT(enabler), ptr);
> +		set_bit(ENABLE_BIT(enabler) + bit_offset, ptr);
>  	else
> -		clear_bit(ENABLE_BIT(enabler), ptr);
> +		clear_bit(ENABLE_BIT(enabler) + bit_offset, ptr);
>  
>  	kunmap_local(kaddr);
>  	unpin_user_pages_dirty_lock(&page, 1, true);

Does this work for big endian machines too?

Little endian layout:

 uaddr = 0xbeef0004
 enabler = 1;

 memory at 0xbeef0000:  00 00 00 00 02 00 00 00
                                    ^
                                    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 02 00 00 00
                                     ^
				bit 33 of 0xbeef0000

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!

-- Steve