Re: [PATCHv3 1/7] uprobe: Add support for session consumer

[Jiri Olsa]

On Thu, Sep 12, 2024 at 06:20:29PM +0200, Oleg Nesterov wrote:
> On 09/09, Jiri Olsa wrote:
> >
> >  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
> >  {
> >  	struct uprobe_consumer *uc;
> >  	int remove = UPROBE_HANDLER_REMOVE;
> > -	bool need_prep = false; /* prepare return uprobe, when needed */
> > +	struct return_consumer *ric = NULL;
> > +	struct return_instance *ri = NULL;
> >  	bool has_consumers = false;
> >
> >  	current->utask->auprobe = &uprobe->arch;
> >
> >  	list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> >  				 srcu_read_lock_held(&uprobes_srcu)) {
> > +		__u64 cookie = 0;
> >  		int rc = 0;
> >
> >  		if (uc->handler) {
> > -			rc = uc->handler(uc, regs);
> > -			WARN(rc & ~UPROBE_HANDLER_MASK,
> > +			rc = uc->handler(uc, regs, &cookie);
> > +			WARN(rc < 0 || rc > 2,
> >  				"bad rc=0x%x from %ps()\n", rc, uc->handler);
> >  		}
> >
> > -		if (uc->ret_handler)
> > -			need_prep = true;
> > -
> > +		/*
> > +		 * The handler can return following values:
> > +		 * 0 - execute ret_handler (if it's defined)
> > +		 * 1 - remove uprobe
> > +		 * 2 - do nothing (ignore ret_handler)
> > +		 */
> >  		remove &= rc;
> >  		has_consumers = true;
> > +
> > +		if (rc == 0 && uc->ret_handler) {
> 
> should we enter this block if uc->handler == NULL?

yes, consumer can have just ret_handler defined

> 
> > +			/*
> > +			 * Preallocate return_instance object optimistically with
> > +			 * all possible consumers, so we allocate just once.
> > +			 */
> > +			if (!ri) {
> > +				ri = alloc_return_instance(uprobe->consumers_cnt);
> 
> This doesn't look right...
> 
> Suppose we have a single consumer C1, so uprobe->consumers_cnt == 1 and
> alloc_return_instance() allocates return_instance with for a single consumer,
> so that only ri->consumers[0] is valid.
> 
> Right after that uprobe_register()->consumer_add() adds another consumer
> C2 with ->ret_handler != NULL.
> 
> On the next iteration return_consumer_next() will return the invalid addr
> == &ri->consumers[1].
> 
> perhaps this needs krealloc() ?

damn.. there used to be a lock ;-) ok, for some reason I thought we are safe
in that list iteration and we are not.. I just made selftest that triggers that

I'm not sure the realloc will help, I feel like we need to allocate return
consumer for each called handler separately to be safe

> 
> > +				if (!ri)
> > +					return;
> 
> Not sure we should simply return if kzalloc fails... at least it would be better
> to clear current->utask->auprobe.
> 
> > +	if (ri && !remove)
> > +		prepare_uretprobe(uprobe, regs, ri); /* put bp at return */
> > +	else
> > +		kfree(ri);
> 
> Well, if ri != NULL then remove is not possible, afaics... ri != NULL means
> that at least one ->handler() returned rc = 0, thus "remove" must be zero.
> 
> So it seems you can just do
> 
> 	if (ri)
> 		prepare_uretprobe(...);

true, I think that should be enough

thanks,
jirka

> 
> 
> Didn't read other parts of your patch yet ;)
> 
> Oleg.
>