On Wed, Jul 04, 2012 at 11:38:10AM +0200, Peter Krempa wrote:
> 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?
Yes, I think it is wrong to expect that 'S' must be non-zero,
so we should change the docs too.
Daniel
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