linux-kernel - Re: [PATCH] time/fs - file's time race with vgettimeofday

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Date:	Tue, 06 Jul 2010 16:11:40 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	eric.dumazet@...il.com, oleg@...hat.com
Subject: Re: [PATCH] time/fs - file's time race with vgettimeofday

On Tue, 2010-07-06 at 16:03 -0700, john stultz wrote:
> On Fri, Jul 2, 2010 at 12:41 AM, Jiri Olsa <jolsa@...hat.com> wrote:
> > This issue was described in the BZ 244697
> >
> >        Time goes backward - gettimeofday() vs. rename()
> >        https://bugzilla.redhat.com/show_bug.cgi?id=244697
> >
> >
> > It's not just issue of the creat but few others like rename.
> >
> >
> > The following patch will prevent the race by adding the
> > CURRENT_TIME_SEC_REAL macro, which will return seconds from
> > the getnstimeofday call, ensuring it's computed on current tick.
> > It fixes the 'creat' case for ext4.
> >
> >
> > I'm not sure how much trouble is having this race unfixed compared
> > to the performance impact the fix might have. Maybe there're
> > better ways to fix this.
> 
> I do worry that your patch will have too much of a performance hit. I
> think the right fix would be in vtime().
> 
> Test patch to follow shortly.

So the following (untested) patch _should_ resolve this in mainline on
x86. Please let me know if it works for you.

However, for older kernels, this patch won't be sufficient, as it
depends on update_vsyscall getting the time at the last tick in its
wall_time, and kernels that don't have the logarithmic accumulation code
& remove xtime_cache patches can't be fixed so easily. 

Once we get this upstream, let me know if you have any questions about
how to backport this to older kernels.

thanks
-john



[PATCH] x86: Fix vtime/file timestamp inconsistencies

Due to vtime calling vgettimeofday(), its possible that an application
could call  time();create("stuff",O_RDRW);  only to see the file's
creation timestamp to be before the value returned by time.

The proper fix is to make vtime use the same time value as
current_kernel_time() (which is exported via update_vsyscall) instead of
vgettime().


Signed-off-by: John Stultz <johnstul@...ibm.com>



diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 1c0c6ab..ce9a4fa 100644
--- a/arch/x86/kernel/vsyscall_64.c
+++ b/arch/x86/kernel/vsyscall_64.c
@@ -169,13 +169,18 @@ int __vsyscall(0) vgettimeofday(struct timeval * tv, struct timezone * tz)
  * unlikely */
 time_t __vsyscall(1) vtime(time_t *t)
 {
-	struct timeval tv;
+	unsigned seq;
 	time_t result;
 	if (unlikely(!__vsyscall_gtod_data.sysctl_enabled))
 		return time_syscall(t);
 
-	vgettimeofday(&tv, NULL);
-	result = tv.tv_sec;
+	do {
+		seq = read_seqbegin(&__vsyscall_gtod_data.lock);
+
+		result = vsyscall_gtod_data.wall_time_sec;
+
+	} while (read_seqretry(&__vsyscall_gtod_data.lock, seq));
+
 	if (t)
 		*t = result;
 	return result;


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