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

[Steven Rostedt]

On Thu, 14 Sep 2023 15:11:02 +0200
Clément Léger <cle...@rivosinc.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