Re: [PATCH 2/2] xen/x86: Livepatch: support patching CET-enhanced functions

[David Vrabel <dvrabel@xxxxxxxxxx>]

On 07/03/2022 14:03, Jan Beulich wrote:
> On 07.03.2022 12:53, Bjoern Doebel wrote:
> > @@ -104,18 +122,36 @@ void noinline arch_livepatch_revive(void)
> >   
> >   int arch_livepatch_verify_func(const struct livepatch_func *func)
> >   {
> > +    BUILD_BUG_ON(sizeof(struct x86_livepatch_meta) != LIVEPATCH_OPAQUE_SIZE);
> > +
> >       /* If NOPing.. */
> >       if ( !func->new_addr )
> >       {
> > +        struct x86_livepatch_meta *lp;
> > +
> > +        lp = (struct x86_livepatch_meta *)func->opaque;
> >           /* Only do up to maximum amount we can put in the ->opaque. */
> > -        if ( func->new_size > sizeof(func->opaque) )
> > +        if ( func->new_size > sizeof(lp->instruction) )
> >               return -EOPNOTSUPP;
> >   
> >           if ( func->old_size < func->new_size )
> >               return -EINVAL;
> >       }
> I continue to be concerned of the new local variable causing compiler
> warnings. With the adjustment made compared to v1, the specific
> warning would have changed, and we're now liable to see set-but-never-
> used ones.
Linux has a sizeof_field() macro for this sort of use.

    /**
     * sizeof_field() - Report the size of a struct field in bytes
     *
     * @TYPE: The structure containing the field of interest
     * @MEMBER: The field to return the size of
     */
    #define sizeof_field(TYPE, MEMBER) sizeof((((TYPE *)0)->MEMBER))

David