linux-kernel - regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad commit 166afb64511

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Message-ID: <20150323165725.GA25008@pog.tecnopolis.ca>
Date:	Mon, 23 Mar 2015 11:57:25 -0500
From:	Trevor Cordes <trevor@...nopolis.ca>
To:	linux-kernel@...r.kernel.org
Subject: regression in ktime.h circa 3.16.0-rc5+ breaks lirc irsend, bad
 commit 166afb64511

Hello everyone, this is my first attempt at bisecting a kernel to solve a 
bug.  Please bear with me.

I have successfully bisected and located a commit that is causing my 
problem.  Look at commit 166afb64511.

ktime_to_us returns s64, but the commit changes it so ktime_to_us just 
returns what ktime_divns returns, and ktime_divns returns a u64!  If the 
u64 is big enough, wouldn't it wrap s64 around to a negative number?  Or, 
perhaps if some caller is passing in negative ktime_t to begin with it 
will trigger without having to hit big numbers.  With my limited knowledge 
of C, I am stabbing in the dark here.

That's just my guess as to why this commit causes my problem.  My bug 
symptom is my previously working MythTV lirc blaster no longer reliably 
sends IR signals.  Using irsend to test I can see irsend is just timing 
out (and only sometimes blasts, usually the first attempt).  On good 
kernels it returns immediately after blasting.

This little patch (at bottom of email) that puts the code back in place 
and gets rid of the function call fixes the problem for me.  I applied 
this patch to the very latest FC21 kernel-PAE-3.19.1-201.fc21.i686 src.rpm 
and rpmbuilded and the bug is gone!  I can once again MythTV.  Hooray.

I suspect no one else is seeing this because less people are running 
32-bit now, and perhaps in most code paths the value of the u64 never gets 
above 2^63.  I suspect something in drivers/media (possibly) is passing 
very high or negative values (possibly another bug) to these calls.

Obviously my patch isn't the real solution, the real solution is to make 
the new function calls use a consistent 64-bit type, or figure out what in 
my code path is calling these functions and check it for value sanity.

I've documented the whole process / details of this bug in RHBZ:
https://bugzilla.redhat.com/show_bug.cgi?id=1200353

Thanks!

diff -uNr a/include/linux/ktime.h b/include/linux/ktime.h
--- a/include/linux/ktime.h	2015-02-08 20:54:22.000000000 -0600
+++ b/include/linux/ktime.h	2015-03-23 01:09:43.000000000 -0500
@@ -173,12 +173,16 @@

 static inline s64 ktime_to_us(const ktime_t kt)
 {
-	return ktime_divns(kt, NSEC_PER_USEC);
+/*	return ktime_divns(kt, NSEC_PER_USEC); */
+	struct timeval tv = ktime_to_timeval(kt);
+	return (s64) tv.tv_sec * USEC_PER_SEC + tv.tv_usec;
 }

 static inline s64 ktime_to_ms(const ktime_t kt)
 {
-	return ktime_divns(kt, NSEC_PER_MSEC);
+/*	return ktime_divns(kt, NSEC_PER_MSEC); */
+	struct timeval tv = ktime_to_timeval(kt);
+	return (s64) tv.tv_sec * MSEC_PER_SEC + tv.tv_usec / USEC_PER_MSEC;
 }

 static inline s64 ktime_us_delta(const ktime_t later, const ktime_t earlier)
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