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

[Namhyung Kim <namhyung@kernel.org>]

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

Thanks,
Namhyung


>  
>  	ret = probe_kernel_read(buf, (void *)val, arg->size);
> @@ -1162,6 +1246,7 @@ static void func_event_seq_stop(struct seq_file *m, void *v)
>  static int func_event_seq_show(struct seq_file *m, void *v)
>  {
>  	struct func_event *func_event = v;
> +	struct func_arg_redirect *redirect;
>  	struct func_arg *arg;
>  	bool comma = false;
>  	int last_arg = 0;
> @@ -1190,6 +1275,13 @@ static int func_event_seq_show(struct seq_file *m, void *v)
>  				seq_printf(m, "[%ld]",
>  					   (arg->indirect ^ INDIRECT_FLAG) / arg->size);
>  		}
> +		list_for_each_entry(redirect, &arg->redirects, list) {
> +			if (redirect->index)
> +				seq_printf(m, "+%ld", redirect->index);
> +			if (redirect->indirect)
> +				seq_printf(m, "[%d]",
> +					   (redirect->indirect ^ INDIRECT_FLAG) / arg->size);
> +		}
>  	}
>  	seq_puts(m, ")\n");
>  
> -- 
> 2.15.1
> 
>