linux-kernel - Re: [PATCH] blktrace: reword comment about time overflow

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Message-ID: <x49porbr40l.fsf@segfault.boston.devel.redhat.com>
Date:	Mon, 20 Jun 2016 16:36:58 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Jens Axboe <axboe@...nel.dk>, y2038@...ts.linaro.org,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>,
	Hannes Reinecke <hare@...e.com>,
	Mike Christie <mchristi@...hat.com>, Shaohua Li <shli@...com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blktrace: reword comment about time overflow

Arnd Bergmann <arnd@...db.de> writes:

> Jeff Moyer looked up the blktrace source to see if an overflow might
> happen. The situation is as follows:
>
> - The time stamp is not used by the program itself, only for
>   printing human-readable output.
> - We normally don't print the timestamp at all, except when an
>   undocumented format option is given to blkparse.
> - The assumption is that no other program besides blktrace
>   even looks at this data, but of course cannot be sure.
> - On 64-bit systems, the time gets read from the unsigned
>   32-bit kernel structure into a timespec in a way that will
>   work correctly until 2106, so there is no 2038 problem.
> - On 32-bit systems that have a new (future) libc build with
>   a 64-bit time_t type, it will work the same way.
> - On current 32-bit systems, the time is passed into localtime(),
>   at which point the overflow happens, but those systems are
>   already broken.
>
> In short, it's good enough for now, so update the comment.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 59a37f8baeb2 ("blktrace: avoid using timespec")
> Cc: Jeff Moyer <jmoyer@...hat.com>

Acked-by: Jeff Moyer <jmoyer@...hat.com>

> ---
>  kernel/trace/blktrace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index b0816e4a61a5..4a3666779589 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -131,7 +131,8 @@ static void trace_note_time(struct blk_trace *bt)
>  	unsigned long flags;
>  	u32 words[2];
>  
> -	/* need to check user space to see if this breaks in y2038 or y2106 */
> +	/* blktrace converts this to a time_t and will overflow in
> +	   2106, not in 2038 */
>  	ktime_get_real_ts64(&now);
>  	words[0] = (u32)now.tv_sec;
>  	words[1] = now.tv_nsec;