Re: [Xen-devel] [PATCH] xen/x86: Remove unnecessary cast on void pointer

[Wei Liu <wl@xxxxxxx>]

On Sat, Mar 28, 2020 at 05:47:18PM +0530, Simran Singhal wrote:
[...]
> >
> > >      int offset;
> > >      u64 res;
> > >
> > > diff --git a/xen/arch/x86/mm/p2m-pt.c b/xen/arch/x86/mm/p2m-pt.c
> > > index eb66077496..058b9b8adf 100644
> > > --- a/xen/arch/x86/mm/p2m-pt.c
> > > +++ b/xen/arch/x86/mm/p2m-pt.c
> > > @@ -127,7 +127,7 @@ p2m_find_entry(void *table, unsigned long
> > *gfn_remainder,
> > >          return NULL;
> > >      }
> > >      *gfn_remainder &= (1 << shift) - 1;
> > > -    return (l1_pgentry_t *)table + index;
> > > +    return table + index;
> >
> > I don't think removing this cast is correct, as you would be doing a
> > plain addition to a pointer instead of fetching the next entry in the
> > array of l1_pgentry_t entries.
> >
> > If you want to get rid of the cast here you need to change the type of
> > the table parameter to l1_pgentry_t * instead of void *.
> >
> 
> Yes, you are correct. Since void* is a pointer to an unknown type we can't
> do pointer arithmetic on it, as the compiler wouldn't know how big the
> thing pointed to is. Thus, it is necessary to keep the cast on the "table".
> 
> Ah! I am sorry for this mistake. But, I am afraid why I didn't get warning
> during compilation.

Pointer arithmetic on void* is allowed. It is treated as if the size of
the object is 1. That's probably why you didn't get a warning.

Wei.