Re: [PATCH] io_uring: Add to traces the req pointer when available

[Pavel Begunkov <asml.silence@gmail.com>]

On 5/29/21 1:30 PM, Pavel Begunkov wrote:
> On 5/28/21 11:42 PM, Steven Rostedt wrote:
>> On Wed, 26 May 2021 12:18:37 -0400
>> Olivier Langlois <olivier@trillion01.com> wrote:
>>
>>>> If that gets changed, could be also include the personality id and
>>>> flags here,
>>>> and maybe also translated the opcode and flags to human readable
>>>> strings?
>>>>   
>>> If Jens and Pavel agrees that they would like to see this info in the
>>> traces, I have no objection adding it.
>>>
>>> Still waiting input from Steven Rostedt which I believe is the trace
>>> system maintainer concerning the hash-ptr situation.
>>>
>>> I did receive an auto-respond from him saying that he was in vacation
>>> until May 28th...
>>
>> Yep, I'm back now.
>>
>> Here's how it works using your patch as an example:
>>
>>>  	TP_fast_assign(
>>>  		__entry->ctx		= ctx;
>>> +		__entry->req		= req;
>>
>> The "__entry" is a structure defined by TP_STRUCT__entry() that is located
>> on the ring buffer that can be read directly by user space (aka trace-cmd).
>> So yes, that value is never hashed, and one of the reasons that tracefs
>> requires root privilege to read it.
>>
>>>  		__entry->opcode		= opcode;
>>>  		__entry->user_data	= user_data;
>>>  		__entry->force_nonblock	= force_nonblock;
>>>  		__entry->sq_thread	= sq_thread;
>>>  	),
>>>  
>>> -	TP_printk("ring %p, op %d, data 0x%llx, non block %d, sq_thread %d",
>>> -			  __entry->ctx, __entry->opcode,
>>> -			  (unsigned long long) __entry->user_data,
>>> -			  __entry->force_nonblock, __entry->sq_thread)
>>> +	TP_printk("ring %p, req %p, op %d, data 0x%llx, non block %d, "
>>> +		  "sq_thread %d",  __entry->ctx, __entry->req,
>>> +		  __entry->opcode, (unsigned long long)__entry->user_data,
>>> +		  __entry->force_nonblock, __entry->sq_thread)
>>>  );
>>
>> The TP_printk() macro *is* used when reading the "trace" or "trace_pipe"
>> file, and that uses vsnprintf() to process it. Which will hash the values
>> for %p (by default, because that's what it always did when vsnprintf()
>> started hashing values).
>>
>> Masami Hiramatsu added the hash-ptr option (which I told him to be the
>> default as that was the behavior before that option was created), where the
>> use could turn off the hashing.
>>
>> There's lots of trace events that expose the raw pointers when hash-ptr is
>> off or if the ring buffers are read via the trace_pip_raw interface.
>>
>> What's special about these pointers to hash them before they are recorded?
> 
> io_uring offers all different operations and has internal request/memory
> recycling, so it may be an easy vector of attack in case of some
> vulnerabilities found, but nothing special. As that's the status quo,
> I wouldn't care, let's put aside my concerns and print them raw.

edit: not print obviously, have but have them raw in __entry

-- 
Pavel Begunkov