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

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Message-ID: <20180209104758.093d31ad@gandalf.local.home>
Date:   Fri, 9 Feb 2018 10:47:58 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>,
        linux-rt-users@...r.kernel.org, linux-trace-users@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Clark Williams <williams@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kernel-team@....com
Subject: Re: [PATCH 17/18] tracing: Add indirect to indirect access for
 function based events

On Fri, 9 Feb 2018 14:13:01 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> On Fri, Feb 02, 2018 at 06:05:15PM -0500, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > 
> > 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@...dmis.org>
> > ---  
> 
> [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);

Thanks!

-- Steve