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Re: [libvirt] [PATCH] virsh: Avoid division by 0 in vshCalloc

On 07/04/12 11:43, Daniel P. Berrange wrote:
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:

# ifndef xalloc_oversized
#  define xalloc_oversized(n, s) \
     ((size_t) (sizeof(ptrdiff_t) <= sizeof(size_t) ? -1 : -2) / (s) < (n))
# endif

   (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.

I found the actual problem for this. I think that this macro is in order. S is the size of the array element, which doesn't make sense to be 0 and N is the count of elements. The floating point exception happened because the count and size arguments were interchanged in the problematic place. I'll send the patch anyways, but I don't think now, that the macro requires fixing.



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