Libguestfs - Nov 2017 - [PATCH] diff: avoid potential null pointer dereference on error

If visit_guest() fails, then it returns a null pointer; later on,
free_tree() is called unconditionally on the variables, thus
dereferencing null pointers.

Thus guard the free_tree() invocations.
---
 diff/diff.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/diff/diff.c b/diff/diff.c
index 5851a1c9c..1f1ab6933 100644
--- a/diff/diff.c
+++ b/diff/diff.c
@@ -389,8 +389,10 @@ main (int argc, char *argv[])
       errors++;
   }
 
-  free_tree (tree1);
-  free_tree (tree2);
+  if (tree1)
+    free_tree (tree1);
+  if (tree2)
+    free_tree (tree2);
 
   free_drives (drvs);
   free_drives (drvs2);
-- 
2.13.6

On 11/03/2017 11:19 AM, Pino Toscano wrote:
> If visit_guest() fails, then it returns a null pointer; later on,
> free_tree() is called unconditionally on the variables, thus
> dereferencing null pointers.
> 
> Thus guard the free_tree() invocations.
Would it be any better to teach free_tree() to be more free()-like by
being a no-op on NULL?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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On Fri, Nov 03, 2017 at 12:48:37PM -0500, Eric Blake
wrote:
> On 11/03/2017 11:19 AM, Pino Toscano wrote:
> > If visit_guest() fails, then it returns a null pointer; later on,
> > free_tree() is called unconditionally on the variables, thus
> > dereferencing null pointers.
> > 
> > Thus guard the free_tree() invocations.
> 
> Would it be any better to teach free_tree() to be more free()-like by
> being a no-op on NULL?
Yes, I agree with Eric on this.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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