Re: [PATCH 01/11] x86: Fix vtime/file timestamp inconsistencies

From: KOSAKI Motohiro
Date: Tue Jul 13 2010 - 22:40:47 EST


Hi

> 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.

Just dumb question.

Almost application are using gettimeofday() instead time(). It mean
your fix don't solve almost application.

So, Why can't we fix vgettimeofday() vs create() inconsistency?
This is just question, I don't intend to disagree you.


>
> A similar way to reproduce the issue is to compare the vsyscall time()
> with the syscall time(), and observe ordering issues.
>
> The modified test case from Oleg Nesterov below can illustrate this:
>
> int main(void)
> {
> time_t sec1,sec2;
> do {
> sec1 = time(&sec2);
> sec2 = syscall(__NR_time, NULL);
> } while (sec1 <= sec2);
>
> printf("vtime: %d.000000\n", sec1);
> printf("time: %d.000000\n", sec2);
> return 0;
> }
>
> 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().
>
> Thanks to Jiri Olsa for bringing up the issue and catching bugs in
> earlier verisons of this fix.
>
> Signed-off-by: John Stultz <johnstul@xxxxxxxxxx>
>
> CC: Jiri Olsa <jolsa@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Oleg Nesterov <oleg@xxxxxxxxxx>
> ---
> arch/x86/kernel/vsyscall_64.c | 11 ++++++++---
> 1 files changed, 8 insertions(+), 3 deletions(-)
>
> 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;
> --
> 1.6.0.4
>
> --
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