Date:   Wed, 13 Jan 2021 17:48:45 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     "Wangshaobo (bobo)" <bobo.shaobowang@...wei.com>,
        <naveen.n.rao@...ux.ibm.com>, <anil.s.keshavamurthy@...el.com>,
        <davem@...emloft.net>, <linux-kernel@...r.kernel.org>,
        <huawei.libin@...wei.com>, <cj.chengjian@...wei.com>
Subject: Re: [PATCH] kretprobe: avoid re-registration of the same kretprobe
 earlier


Anything more on this?

-- Steve


On Tue, 22 Dec 2020 20:03:56 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> On Mon, 21 Dec 2020 21:31:42 +0800
> "Wangshaobo (bobo)" <bobo.shaobowang@...wei.com> wrote:
> 
> > Hi steven, Masami,
> > We have encountered a problem, when we attempted to use steven's suggestion as following,
> >   
> > >>> If you call this here, you must make sure kprobe_addr() is called on rp->kp.
> > >>> But if kretprobe_blacklist_size == 0, kprobe_addr() is not called before
> > >>> this check. So it should be in between kprobe_on_func_entry() and
> > >>> kretprobe_blacklist_size check, like this
> > >>>
> > >>> 	if (!kprobe_on_func_entry(rp->kp.addr, rp->kp.symbol_name, rp->kp.offset))
> > >>> 		return -EINVAL;
> > >>>
> > >>> 	addr = kprobe_addr(&rp->kp);
> > >>> 	if (IS_ERR(addr))
> > >>> 		return PTR_ERR(addr);
> > >>> 	rp->kp.addr = addr;  
> > 
> > //there exists no-atomic operation risk, we should not modify any rp->kp's information, not all arch ensure atomic operation here.
> >   
> > >>>
> > >>> 	ret = check_kprobe_rereg(&rp->kp);
> > >>> 	if (WARN_ON(ret))
> > >>> 		return ret;
> > >>>
> > >>>           if (kretprobe_blacklist_size) {
> > >>> 		for (i = 0; > > +	ret = check_kprobe_rereg(&rp->kp);  
> > 
> > it returns failure from register_kprobe() end called by register_kretprobe() when
> > we registered a kretprobe through .symbol_name at first time(through .addr is OK),
> > kprobe_addr() called at the begaining of register_kprobe() will recheck and
> > failed at following place because at this time we symbol_name is not NULL and addr is also.  
> 
> Good catch! Yes, it will reject if both kp->addr and kp->symbol are set.
> 
> > 
> >    static kprobe_opcode_t *_kprobe_addr(const char *symbol_name,
> >                           unsigned int offset)
> >     {
> >           if ((symbol_name && addr) || (!symbol_name && !addr))  //we failed here
> > 
> > 
> > So we attempted to move this sentence rp->kp.addr = addr to __get_valid_kprobe() like this to
> > avoid explict usage of rp->kp.addr = addr in register_kretprobe().
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index dd5821f753e6..ea014779edfe 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -1502,10 +1502,15 @@ static kprobe_opcode_t *kprobe_addr(struct kprobe *p)
> >   static struct kprobe *__get_valid_kprobe(struct kprobe *p)
> >   {
> >          struct kprobe *ap, *list_p;
> > +       void *addr;
> > 
> >          lockdep_assert_held(&kprobe_mutex);
> > 
> > -       ap = get_kprobe(p->addr);
> > +       addr = kprobe_addr(p);
> > +       if (IS_ERR(addr))
> > +               return NULL;
> > +
> > +       ap = get_kprobe(addr);
> >          if (unlikely(!ap))
> >                  return NULL;
> > 
> > But it also failed when we second time attempted to register a same kretprobe, it is also
> > becasue symbol_name and addr is not NULL when we used __get_valid_kprobe().  
> 
> What the "second time" means? If you reuse the kretprobe (and kprobe) you must
> reset (cleanup) the kp->addr or kp->symbol_name. That is the initial state.
> I think the API should not allow users to enter inconsistent information.
> 
> > 
> > So it seems has no idea expect for modifying _kprobe_addr() like following this, the reason is that
> > the patch 0bd476e6c671 ("kallsyms: unexport kallsyms_lookup_name() and kallsyms_on_each_symbol()")
> > has telled us we'd better use symbol name to register but not address anymore.
> > 
> > -static kprobe_opcode_t *_kprobe_addr(kprobe_opcode_t *addr,
> > -                       const char *symbol_name, unsigned int offset)
> > +static kprobe_opcode_t *_kprobe_addr(const char *symbol_name,
> > +                       unsigned int offset)
> >   {
> > -       if ((symbol_name && addr) || (!symbol_name && !addr))
> > +       kprobe_opcode_t *addr;
> > +       if (!symbol_name)
> >                  goto invalid;  
> 
> No, there are cases that the user will set only kp->addr, but no kp->symbol_name.
> 
> > 
> > For us, this modification has not caused a big impact on other modules, only expects a little
> > influence on bpf from calling trace_kprobe_on_func_entry(), it can not use addr to fill in
> > rp.kp in struct trace_event_call anymore.
> > 
> > So i want to know your views, and i will resend this patch soon.  
> 
> OK, I think it is simpler to check the rp->kp.addr && rp->kp.symbol_name
> because it is not allowed (it can lead inconsistent setting).
> 
> How about this code? Is this work for you?
> 
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 41fdbb7953c6..73500be564be 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -2103,6 +2103,14 @@ int register_kretprobe(struct kretprobe *rp)
>         int i;
>         void *addr;
>  
> +       /* It is not allowed to specify addr and symbol_name at the same time */
> +       if (rp->kp.addr && rp->kp.symbol_name)
> +               return -EINVAL;
> +
> +       /* If only rp->kp.addr is specified, check reregistering kprobes */
> +       if (rp->kp.addr && check_kprobe_rereg(&rp->kp))
> +               return -EINVAL;
> +
>         if (!kprobe_on_func_entry(rp->kp.addr, rp->kp.symbol_name, rp->kp.offset))
>                 return -EINVAL;
>  
> 
> Thank you,
> 

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