Re: [Qemu-devel] [PATCH v2] osdep: Fix ROUND_UP(64-bit, 32-bit)

[Laszlo Ersek <lersek@redhat.com>]

On 09/14/17 15:49, Eric Blake wrote:
> When using bit-wise operations that exploit the power-of-two
> nature of the second argument of ROUND_UP(), we still need to
> ensure that the mask is as wide as the first argument (done
> by using a ternary to force proper arithmetic promotion).
> Unpatched, ROUND_UP(2ULL*1024*1024*1024*1024, 512U) produces 0,
> instead of the intended 2TiB, because negation of an unsigned
> 32-bit quantity followed by widening to 64-bits does not
> sign-extend the mask.
> 
> Broken since its introduction in commit 292c8e50 (v1.5.0).
> Callers that passed the same width type to both macro parameters,
> or that had other code to ensure the first parameter's maximum
> runtime value did not exceed the second parameter's width, are
> unaffected, but I did not audit to see which (if any) existing
> clients of the macro could trigger incorrect behavior (I found
> the bug while adding a new use of the macro).
> 
> While preparing the patch, checkpatch complained about poor
> spacing, so I also fixed that here and in the nearby DIV_ROUND_UP.
> 
> CC: qemu-trivial@nongnu.org
> CC: qemu-stable@nongnu.org
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> v2: use ternary instead of addition of 0 [Laszlo], improve commit message
> ---
>  include/qemu/osdep.h | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 6855b94bbf..f4ff372d41 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -189,13 +189,13 @@ extern int daemon(int, int);
> 
>  /* Round number up to multiple. Requires that d be a power of 2 (see
>   * QEMU_ALIGN_UP for a safer but slower version on arbitrary
> - * numbers) */
> + * numbers); works even if d is a smaller type than n.  */
>  #ifndef ROUND_UP
> -#define ROUND_UP(n,d) (((n) + (d) - 1) & -(d))
> +#define ROUND_UP(n, d) (((n) + (d) - 1) & -(0 ? (n) : (d)))
>  #endif
> 
>  #ifndef DIV_ROUND_UP
> -#define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> +#define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
>  #endif
> 
>  /*
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>