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

[Paolo Bonzini <pbonzini@redhat.com>]

On 13/09/2017 23:03, 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 addition of 0 to force proper arithmetic promotion).
> Unpatched, ROUND_UP(2ULL*1024*1024*1024*1024, 512) produces 0,
> instead of the intended 2TiB.
> 
> Broken since its introduction in commit 292c8e50 (v1.5.0).
> 
> CC: qemu-trivial@nongnu.org
> Signed-off-by: Eric Blake <eblake@redhat.com>
> 
> ---
> I did not audit to see how many potential users of ROUND_UP
> are actually passing in different sized types where the first
> argument can be larger than UINT32_MAX; I stumbled across the
> problem when iotests 190 started failing on a patch where I
> added a new use.  We can either be conservative and put this
> on qemu-stable no matter what, or go through the effort of an
> audit to see what might be broken (many callers in the block
> layer, but not just there).
> ---
>  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..7a3000efc5 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) & -((n) - (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
> 
>  /*
> 


Cc: qemu-stable@nongnu.org

Paolo