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?