On Fri, Jun 09, 2017 at 11:36:27AM -0400, Joe Lawrence wrote:
> On 06/08/2017 12:49 PM, Josh Poimboeuf wrote:
> > On Thu, Jun 01, 2017 at 02:25:24PM -0400, Joe Lawrence wrote:
> >> Add three exported API for livepatch modules:
> >>
> >> void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data);
> >> void klp_shadow_detach(void *obj, char *var);
> >> void *klp_shadow_get(void *obj, char *var);
> >>
> >> that implement "shadow" variables, which allow callers to associate new
> >> shadow fields to existing data structures.
> >>
> >> Signed-off-by: Joe Lawrence <joe.lawrence@xxxxxxxxxx>
> >
> > Overall the patch looks good to me. It's a simple API but we've found
> > it to be very useful for certain patches.
> >
> > One comment below:
> >
> >> +void *klp_shadow_attach(void *obj, char *var, gfp_t gfp, void *data)
> >> +{
> >> + unsigned long flags;
> >> + struct klp_shadow *shadow;
> >> +
> >> + shadow = kmalloc(sizeof(*shadow), gfp);
> >> + if (!shadow)
> >> + return NULL;
> >> +
> >> + shadow->obj = obj;
> >> +
> >> + shadow->var = kstrdup(var, gfp);
> >> + if (!shadow->var) {
> >> + kfree(shadow);
> >> + return NULL;
> >> + }
> >> +
> >> + shadow->data = data;
> >> +
> >> + spin_lock_irqsave(&klp_shadow_lock, flags);
> >> + hash_add_rcu(klp_shadow_hash, &shadow->node, (unsigned long)obj);
> >> + spin_unlock_irqrestore(&klp_shadow_lock, flags);
> >> +
> >> + return shadow->data;
> >> +}
> >> +EXPORT_SYMBOL_GPL(klp_shadow_attach);
> >
> > I wonder if we should worry about people misusing the API by calling
> > klp_shadow_attach() for a shadow variable that already exists. Maybe we
> > should add a check and return NULL if it already exists.
> >
>
> I don't think the API (the shadow or the underlying hash table calls)
> currently protects against double-adds... adding a check to do so would
> probably need to occur with the klp_shadow_lock to protect against
> concurrent detach calls.
>
> I could implement this protection in a v2, or leave it up to the caller.
> What do you think?
Yeah, I don't have a strong opinion either way. It's fine with me to
leave it as it is and trust the patch author not to mess it up.
--
Josh
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