Re: [PATCH 1/2] time: x86: Fix race switching from vsyscall to non-vsyscall clock

[John Stultz <john.stultz@linaro.org>]

On 03/14/2012 05:34 PM, Thomas Gleixner wrote:
> On Wed, 14 Mar 2012, John Stultz wrote:
>>   notrace static noinline int do_realtime(struct timespec *ts)
>>   {
>>   	unsigned long seq, ns;
>> +	int mode;
> Please keep a newline between declarations and code.

Fixed below. Thanks!
(Let me know if you see whitespace damage, I switched mail clients today 
and am learning the quirks here.)
-john


When switching from a vsyscall capable to a non-vsyscall capable
clocksource, there was a small race, where the last vsyscall
gettimeofday before the switch might return a invalid time value
using the new non-vsyscall enabled clocksource values after the
switch is complete.

This is due to the vsyscall code checking the vclock_mode once
outside of the seqcount protected section. After it reads the
vclock mode, it doesn't re-check that the sampled clock data
that is obtained in the seqcount critical section still matches.

The fix is to sample vclock_mode inside the protected section,
and as long as it isn't VCLOCK_NONE, return the calculated
value. If it has changed and is now VCLOCK_NONE, fall back
to the syscall gettime calculation.

v2:
   * Cleanup checks as suggested by tglx
   * Also fix same issue present in gettimeofday path

CC: Andy Lutomirski<luto@amacapital.net>
CC: Thomas Gleixner<tglx@linutronix.de>
Signed-off-by: John Stultz<john.stultz@linaro.org>
---
  arch/x86/vdso/vclock_gettime.c |   68 +++++++++++++++++++++++++--------------
  1 files changed, 43 insertions(+), 25 deletions(-)

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index 6bc0e72..e5ba922 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -70,6 +70,16 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
  	return ret;
  }

+notrace static long vdso_fallback_gtod(struct timeval *tv, struct timezone *tz)
+{
+	long ret;
+
+	asm("syscall" : "=a" (ret) :
+	    "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
+	return ret;
+}
+
+
  notrace static inline long vgetns(void)
  {
  	long v;
@@ -85,21 +95,28 @@ notrace static inline long vgetns(void)
  notrace static noinline int do_realtime(struct timespec *ts)
  {
  	unsigned long seq, ns;
+	int mode;
+
  	do {
  		seq = read_seqbegin(&gtod->lock);
+		mode = gtod->clock.vclock_mode;
  		ts->tv_sec = gtod->wall_time_sec;
  		ts->tv_nsec = gtod->wall_time_nsec;
  		ns = vgetns();
  	} while (unlikely(read_seqretry(&gtod->lock, seq)));
+
  	timespec_add_ns(ts, ns);
-	return 0;
+	return mode;
  }

  notrace static noinline int do_monotonic(struct timespec *ts)
  {
  	unsigned long seq, ns, secs;
+	int mode;
+
  	do {
  		seq = read_seqbegin(&gtod->lock);
+		mode = gtod->clock.vclock_mode;
  		secs = gtod->wall_time_sec;
  		ns = gtod->wall_time_nsec + vgetns();
  		secs += gtod->wall_to_monotonic.tv_sec;
@@ -116,7 +133,7 @@ notrace static noinline int do_monotonic(struct timespec *ts)
  	ts->tv_sec = secs;
  	ts->tv_nsec = ns;

-	return 0;
+	return mode;
  }

  notrace static noinline int do_realtime_coarse(struct timespec *ts)
@@ -156,14 +173,14 @@ notrace static noinline int do_monotonic_coarse(struct timespec *ts)

  notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
  {
+	int ret = VCLOCK_NONE;
+
  	switch (clock) {
  	case CLOCK_REALTIME:
-		if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
-			return do_realtime(ts);
+		ret = do_realtime(ts);
  		break;
  	case CLOCK_MONOTONIC:
-		if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
-			return do_monotonic(ts);
+		ret = do_monotonic(ts);
  		break;
  	case CLOCK_REALTIME_COARSE:
  		return do_realtime_coarse(ts);
@@ -171,32 +188,33 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
  		return do_monotonic_coarse(ts);
  	}

-	return vdso_fallback_gettime(clock, ts);
+	if (ret == VCLOCK_NONE)
+		return vdso_fallback_gettime(clock, ts);
+	return 0;
  }
  int clock_gettime(clockid_t, struct timespec *)
  	__attribute__((weak, alias("__vdso_clock_gettime")));

  notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
  {
-	long ret;
-	if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
-		if (likely(tv != NULL)) {
-			BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
-				     offsetof(struct timespec, tv_nsec) ||
-				     sizeof(*tv) != sizeof(struct timespec));
-			do_realtime((struct timespec *)tv);
-			tv->tv_usec /= 1000;
-		}
-		if (unlikely(tz != NULL)) {
-			/* Avoid memcpy. Some old compilers fail to inline it */
-			tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
-			tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
-		}
-		return 0;
+	long ret = VCLOCK_NONE;
+
+	if (likely(tv != NULL)) {
+		BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
+			     offsetof(struct timespec, tv_nsec) ||
+			     sizeof(*tv) != sizeof(struct timespec));
+		ret = do_realtime((struct timespec *)tv);
+		tv->tv_usec /= 1000;
  	}
-	asm("syscall" : "=a" (ret) :
-	    "0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
-	return ret;
+	if (unlikely(tz != NULL)) {
+		/* Avoid memcpy. Some old compilers fail to inline it */
+		tz->tz_minuteswest = gtod->sys_tz.tz_minuteswest;
+		tz->tz_dsttime = gtod->sys_tz.tz_dsttime;
+	}
+
+	if (ret == VCLOCK_NONE)
+		return vdso_fallback_gtod(tv, tz);
+	return 0;
  }
  int gettimeofday(struct timeval *, struct timezone *)
  	__attribute__((weak, alias("__vdso_gettimeofday")));
-- 
1.7.3.2.146.gca209