Re: [PATCH 17/18] tracing: Add indirect to indirect access for function based events

[Namhyung Kim]

On Fri, Feb 09, 2018 at 10:47:58AM -0500, Steven Rostedt wrote:
> On Fri, 9 Feb 2018 14:13:01 +0900
> Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> 
> > On Fri, Feb 02, 2018 at 06:05:15PM -0500, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > > 
> > > Allow the function based events to retrieve not only the parameters offsets,
> > > but also get data from a pointer within a parameter structure. Something
> > > like:
> > > 
> > >  # echo 'ip_rcv(string skdev+16[0][0] | x8[6] skperm+16[0]+558)' > function_events
> > > 
> > >  # echo 1 > events/functions/ip_rcv/enable
> > >  # cat trace
> > >     <idle>-0     [003] ..s3   310.626391: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   310.626400: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.183775: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.184329: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.303895: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.304610: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.471980: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   312.472908: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > >     <idle>-0     [003] ..s3   313.135804: __netif_receive_skb_core->ip_rcv(skdev=em1, skperm=b4,b5,2f,ce,18,65)
> > > 
> > > That is, we retrieved the net_device of the sk_buff and displayed its name
> > > and perm_addr info.
> > > 
> > >   sk->dev->name, sk->dev->perm_addr
> > > 
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> > > ---  
> > 
> > [SNIP]
> > > +static unsigned long process_redirects(struct func_arg *arg, unsigned long val,
> > > +				       char *buf)
> > > +{
> > > +	struct func_arg_redirect *redirect;
> > > +	int ret;
> > > +
> > > +	if (arg->indirect) {
> > > +		ret = probe_kernel_read(buf, (void *)val, sizeof(long));
> > > +		if (ret)
> > > +			return 0;
> > > +		val = *(unsigned long *)buf;
> > > +	}
> > > +
> > > +	list_for_each_entry(redirect, &arg->redirects, list) {
> > > +		val += redirect->index;
> > > +		if (redirect->indirect) {
> > > +			val += (redirect->indirect ^ INDIRECT_FLAG);
> > > +			ret = probe_kernel_read(buf, (void *)val, sizeof(long));
> > > +			if (ret)
> > > +				return 0;
> > > +		}
> > > +	}
> > > +	return val;
> > > +}
> > > +
> > > +static long long __get_arg(struct func_arg *arg, unsigned long long val)
> > >  {
> > >  	char buf[8];
> > >  	int ret;
> > >  
> > >  	val += arg->index;
> > >  
> > > -	if (!arg->indirect)
> > > -		return val;
> > > +	if (arg->indirect)
> > > +		val += (arg->indirect ^ INDIRECT_FLAG);
> > >  
> > > -	val = val + (arg->indirect ^ INDIRECT_FLAG);
> > > +	if (!list_empty(&arg->redirects))
> > > +		val = process_redirects(arg, val, buf);
> > > +
> > > +	if (!val)
> > > +		return 0;
> > >  
> > >  	/* Arrays and strings do their own indirect reads */
> > > -	if (arg->array || arg->func_type == FUNC_TYPE_string)
> > > +	if (!arg->indirect || arg->array || arg->func_type == FUNC_TYPE_string)
> > >  		return val;  
> > 
> > It seems the indirect is processed twice with redirects.  Consider
> > "x64 foo[0]+4", the process_redirects() will call probe_kernel_read()
> > and then here again.
> > 
> 
> 
> Good catch!
> 
> It should have been:
> 
> 		return process_redirects(arg, val, buf);

But I think you need to consider data type of the arg when
dereferencing the last redirect.

Thanks,
Namhyung