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

Pavel Begunkov
2021-05-29T12:30:46Z

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.

--
Pavel Begunkov