Re: [Qemu-devel] [PATCH v3] exec: Fix non-power-of-2 sized accesses

[Laszlo Ersek]

On 08/16/13 18:00, Alex Williamson wrote:
> Since commit 23326164 we align access sizes to match the alignment of
> the address, but we don't align the access size itself.  This means we
> let illegal access sizes (ex. 3) slip through if the address is
> sufficiently aligned (ex. 4).  This results in an abort which would be
> easy for a guest to trigger.  Account for aligning the access size.
> 
> Signed-off-by: Alex Williamson <address@hidden>
> Cc: address@hidden
> ---
> 
> v3: Highest power of 2, not lowest
> v2: Remove unnecessary loop condition
> 
>  exec.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/exec.c b/exec.c
> index 3ca9381..8c90cef 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1924,6 +1924,13 @@ static int memory_access_size(MemoryRegion *mr, 
> unsigned l, hwaddr addr)
>          }
>      }
>  
> +    /* Size must be a power of 2 */
> +    if (l & (l - 1)) {
> +        while (!(l & access_size_max) && l & (access_size_max - 1)) {
> +            access_size_max >>= 1;
> +        }
> +    }
> +
>      /* Don't attempt accesses larger than the maximum.  */
>      if (l > access_size_max) {
>          l = access_size_max;
> 

Apologies, but I'm now totally confused.

Suppose that the new code is reached with (access_size_max == 4).

Now, l==9 and l==3 will enter the loop just the same, both shifting
"access_size_max" right at least once, even though 9 is greater than 4,
and 3 is less than 4.

Is that OK? What's the goal here?

Sorry for being dense...

Thanks,
Laszlo