Re: [Xen-devel] [PATCH] tools: libxl: make it illegal to pass libxl__realloc(gc) a non-gc ptr

[Ian Campbell <ian.campbell@xxxxxxxxxx>]

On Wed, 2016-02-10 at 17:33 +0000, Andrew Cooper wrote:
> On 10/02/16 17:27, Ian Campbell wrote:
> > That is, if gc is not NOGC and ptr is not NULL then ptr must be
> > associated gc.
> >
> > Currently in this case the new_ptr would not be registered with any
> > gc, which Coverity rightly points out would be a memory leak.
> >
> > It would also be possible to fix this by adding a libxl__ptr_add()
> at
> > the same point, however semantically it seems like a programming
> error
> > to gc-realloc a pointer which is not associated with the gc in
> > question, so treat it as such.
> >
> > Compile tested only, this change could expose latent bugs.
> >
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > ---
> >  tools/libxl/libxl_internal.c | 7 +++++++
> >  tools/libxl/libxl_internal.h | 4 +++-
> >  2 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/tools/libxl/libxl_internal.c
> b/tools/libxl/libxl_internal.c
> > index 328046b..179d618 100644
> > --- a/tools/libxl/libxl_internal.c
> > +++ b/tools/libxl/libxl_internal.c
> > @@ -122,6 +122,13 @@ void *libxl__realloc(libxl__gc *gc, void *ptr,
> size_t new_size)
> >                  break;
> >              }
> >          }
> > +        /*
> > +         * If we get here then ptr was not previously registered
> with
> > +         * this gc.
> > +         */
> > +        LOG(CRITICAL,
> > +            "libxl__realloc on a pointer which tracked by the
> given gc");
> 
> "is not" ?

Yes.

More importantly I'd somehow convinced myself that the break in the for
loop went one scope further than it did. IOW this code _really_ needs
an i == gc->alloc_maxsize check before it makes a fuss!

At which point I may just turn it into an assert.

Ian.

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