linux-kernel - Re: [PATCH 3/5] perf_counter: rework ioctl()s

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Date:	Tue, 12 May 2009 09:10:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...e.hu>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/5] perf_counter: rework ioctl()s

On Tue, 2009-05-12 at 08:27 +0200, Peter Zijlstra wrote:
> On Tue, 2009-05-12 at 16:22 +1000, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > Hmm, are you saying that the 3rd argument to unlocked_ioctl is actually
> > > (void __user *) instead of unsigned long?
> > 
> > He's saying (correctly) that using _IOR or _IOW implies that the ioctl
> > is going to read or write the memory location pointed to by the 3rd
> > argument to unlocked_ioctl.  If the 3rd argument is just a number, not
> > an address, I believe you should use _IO.
> 
> Oh, somewhat confusing all this. Would be good to spell out these things
> somewhere. Documentation/ioctl/ seems less than helpful.

Ah, so _IO() gets an unsigned long 3rd argument.
_IOW() treats the 3rd arg as a (type __user *) and copies the bits over
_IOR() copies the bits back out to userspace
_IORW() does both

In which case the below should fix things up, no?

---
diff --git a/include/linux/perf_counter.h b/include/linux/perf_counter.h
index 614f921..6a9cebc 100644
--- a/include/linux/perf_counter.h
+++ b/include/linux/perf_counter.h
@@ -159,10 +159,10 @@ struct perf_counter_hw_event {
 /*
  * Ioctls that can be done on a perf counter fd:
  */
-#define PERF_COUNTER_IOC_ENABLE		_IOW('$', 0, u32)
-#define PERF_COUNTER_IOC_DISABLE	_IOW('$', 1, u32)
-#define PERF_COUNTER_IOC_REFRESH	_IOW('$', 2, u32)
-#define PERF_COUNTER_IOC_RESET		_IOW('$', 3, u32)
+#define PERF_COUNTER_IOC_ENABLE		_IO('$', 0)
+#define PERF_COUNTER_IOC_DISABLE	_IO('$', 1)
+#define PERF_COUNTER_IOC_REFRESH	_IO('$', 2)
+#define PERF_COUNTER_IOC_RESET		_IO('$', 3)
 
 enum perf_counter_ioc_flags {
 	PERF_IOC_FLAG_GROUP		= 1U << 0,

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