linux-kernel - Re: [PATCH] kasan: Increase the number of bits to shift when recording extra timestamps

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Date: Fri, 16 Feb 2024 00:38:14 +0100
From: Andrey Konovalov <andreyknvl@...il.com>
To: Juntong Deng <juntong.deng@...look.com>
Cc: ryabinin.a.a@...il.com, glider@...gle.com, dvyukov@...gle.com, 
	vincenzo.frascino@....com, akpm@...ux-foundation.org, 
	kasan-dev@...glegroups.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kasan: Increase the number of bits to shift when
 recording extra timestamps

On Thu, Feb 15, 2024 at 7:41 PM Juntong Deng <juntong.deng@...look.com> wrote:
>
> Fix the mistake before, I thought printk only display 99999 seconds
> at max, but actually printk can display larger number of seconds.
>
> So increase the number of bits to shift when recording the extra
> timestamp (44 bits), without affecting the precision, shift it right by
> 9 bits, discarding all bits that do not affect the microsecond part
> (nanoseconds will not be shown).
>
> Currently the maximum time that can be displayed is 9007199.254740s,
> because
>
> 11111111111111111111111111111111111111111111 (44 bits) << 9
> = 11111111111111111111111111111111111111111111000000000
> = 9007199.254740
>
> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
> ---
>  mm/kasan/common.c | 2 +-
>  mm/kasan/report.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 6ca63e8dda74..e7c9a4dc89f8 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -55,7 +55,7 @@ void kasan_set_track(struct kasan_track *track, depot_stack_handle_t stack)
>         u64 ts_nsec = local_clock();
>
>         track->cpu = cpu;
> -       track->timestamp = ts_nsec >> 3;
> +       track->timestamp = ts_nsec >> 9;
>  #endif /* CONFIG_KASAN_EXTRA_INFO */
>         track->pid = current->pid;
>         track->stack = stack;
> diff --git a/mm/kasan/report.c b/mm/kasan/report.c
> index 7afa4feb03e1..b48c768acc84 100644
> --- a/mm/kasan/report.c
> +++ b/mm/kasan/report.c
> @@ -267,7 +267,7 @@ static void print_track(struct kasan_track *track, const char *prefix)
>         u64 ts_nsec = track->timestamp;
>         unsigned long rem_usec;
>
> -       ts_nsec <<= 3;
> +       ts_nsec <<= 9;
>         rem_usec = do_div(ts_nsec, NSEC_PER_SEC) / 1000;
>
>         pr_err("%s by task %u on cpu %d at %lu.%06lus:\n",
> --
> 2.39.2
>

Acked-by: Andrey Konovalov <andreyknvl@...il.com>