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

[Jiri Olsa]

On Tue, Jul 06, 2010 at 04:11:40PM -0700, john stultz wrote:
> On Tue, 2010-07-06 at 16:03 -0700, john stultz wrote:
> > On Fri, Jul 2, 2010 at 12:41 AM, Jiri Olsa <jolsa@xxxxxxxxxx> 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@xxxxxxxxxx>
> 
> 
> 
> 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;
> 
> 

I think there's a typo in using "vsyscall_gtod_data" inside
the vtime call it should be "__vsyscall_gtod_data" intead.

attaching changed patch

wbr,
jirka

---
[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@xxxxxxxxxx>
Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
---
diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
index 1c0c6ab..dce0c3c 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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