Re: [libvirt] [PATCH] virsh: Avoid division by 0 in vshCalloc
On 07/04/12 11:09, Daniel P. Berrange wrote:
On Wed, Jul 04, 2012 at 11:05:40AM +0200, Peter Krempa wrote:
> vshCalloc function uses xalloc_oversized macro that can't take 0 as it's
> second argument. If vshCalloc is called with size 0, virsh ends with a
> floating point exception.
>
> This patch changes vshCalloc to return NULL if no memory is requested.
> ---
> tools/virsh.c | 3 +++
> 1 files changed, 3 insertions(+), 0 deletions(-)
>
> diff --git a/tools/virsh.c b/tools/virsh.c
> index 53d1825..d3d5c6a 100644
> --- a/tools/virsh.c
> +++ b/tools/virsh.c
> @@ -460,6 +460,9 @@ _vshCalloc(vshControl *ctl, size_t nmemb, size_t size, const char
*filename, int
> {
> char *x;
>
> + if (!size)
> + return NULL;
> +
> if (!xalloc_oversized(nmemb, size) &&
IMHO this div-by-zero problem is a bug in the xalloc_oversized
macro & we should fix it there. The scenario seen here in virsh
is a fairly common and so div-by-zero could affect any other
usage of that macro
Yes it could. But the docs for the macro state that it shouldn't be called with 0 as
the second argument:
/* Return 1 if an array of N objects, each of size S, cannot exist due
to size arithmetic overflow. S must be positive and N must be
nonnegative. This is a macro, not an inline function, so that it
works correctly even when SIZE_MAX < N.
But assuming that 0 elements of something will never overflow we could change the macro
to:
from:
# ifndef xalloc_oversized
# define xalloc_oversized(n, s) \
((size_t) (sizeof(ptrdiff_t) <= sizeof(size_t) ? -1 : -2) / (s) < (n))
# endif
to:
(s?((size_t) (sizeof(ptrdiff_t) <= sizeof(size_t) ? -1 : -2) / (s) < (n)):0)
which would take care of the 0 argument.
Is this what you had in mind?
Peter
Daniel