linux-kernel - Re: [patch] trace: Add user-space event tracing/injection

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Date:	Wed, 17 Nov 2010 13:37:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch] trace: Add user-space event tracing/injection

On Wed, 2010-11-17 at 13:29 +0100, Peter Zijlstra wrote:
> > +             case PR_TASK_PERF_USER_TRACE:
> > +             {
> > +                     void __user *uevent_ptr = (void *)arg2;
> > +                     char kstring[MAX_USER_TRACE_SIZE+1];
> > +                     unsigned long uevent_len = arg3;
> > +
> > +                     if (uevent_len > MAX_USER_TRACE_SIZE)
> > +                             return -EINVAL;
> > +                     if (copy_from_user(kstring, uevent_ptr, uevent_len))
> > +                             return -EFAULT;
> > +                     kstring[uevent_len] = 0;
> > +
> > +                     trace_user(kstring);
> > +                     return 0;
> > +             } 

That implementation can be improved upon, no need to actually do the
copy_from_user() when the tracepoint isn't enabled.

Also, do we really need a max size here? I know ftrace (and the perf
tracepoint interface) are currently limited to 1 page of trace data but
that can be fixed (although not trivially).

Also, -EINVAL isn't really a helpful error code in that case, -EMSGSIZE
seems like a good candidate.

And the uevent name is already taken for something else, re-using it
here might cause confusion.
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