linux-kernel - Re: [PATCH] trace: Return ENOTCONN instead of EBADF

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Date:   Mon, 12 Oct 2020 14:26:48 +0000
From:   "Enderborg, Peter" <Peter.Enderborg@...y.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] trace: Return ENOTCONN instead of EBADF

On 10/12/20 3:53 PM, Steven Rostedt wrote:
> On Mon, 12 Oct 2020 10:26:42 +0200
> Peter Enderborg <peter.enderborg@...y.com> wrote:
>
>> When there is no clients listening on event the trace return
>> EBADF. The file is not a bad file descriptor and to get the
>> userspace able to do a proper error handling it need a different
>> error code that separate a bad file descriptor from a empty listening.
> I have no problem with this patch, but your description is incorrect. And
> before making this change, I want to make sure that what you think is
> happening is actually happening.
>
> This has nothing to do with "clients listening". This happens when the ring
> buffer is disabled for some reason. The most likely case of this happening
> is if someone sets /sys/kernel/tracing/tracing_on to zero.

I see that as no one is listening. You start to listen by setting this tracing on
some instance, but that is for trace_pipe. Is it the same flag for raw access and all ways you
can invoke a trace?

Would

"When there is no instances listening on events the trace return
EBADF, it is when the tracing_on is off globally. The file is not a bad file
descriptor and to get the userspace able to do a proper error handling it need a different
error code that separate a bad file descriptor from a empty listening."

be a ok?


> If this is still something you want applied, please update the change log
> to a more accurate scenario.
>
> Thanks,
>
> -- Steve
>
>
>> Signed-off-by: Peter Enderborg <peter.enderborg@...y.com>
>> ---
>>  kernel/trace/trace.c | 8 ++++----
>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index d3e5de717df2..6e592bf736df 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -6651,8 +6651,8 @@ tracing_mark_write(struct file *filp, const char __user *ubuf,
>>  	event = __trace_buffer_lock_reserve(buffer, TRACE_PRINT, size,
>>  					    irq_flags, preempt_count());
>>  	if (unlikely(!event))
>> -		/* Ring buffer disabled, return as if not open for write */
>> -		return -EBADF;
>> +		/* Ring buffer disabled, return as if not connected */
>> +		return -ENOTCONN;
>>  
>>  	entry = ring_buffer_event_data(event);
>>  	entry->ip = _THIS_IP_;
>> @@ -6731,8 +6731,8 @@ tracing_mark_raw_write(struct file *filp, const char __user *ubuf,
>>  	event = __trace_buffer_lock_reserve(buffer, TRACE_RAW_DATA, size,
>>  					    irq_flags, preempt_count());
>>  	if (!event)
>> -		/* Ring buffer disabled, return as if not open for write */
>> -		return -EBADF;
>> +		/* Ring buffer disabled, return not connected */
>> +		return -ENOTCONN;
>>  
>>  	entry = ring_buffer_event_data(event);
>>