linux-kernel - Re: [PATCH] ntp: use memset and offsetof init

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Date:   Fri, 05 Feb 2021 18:33:58 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yejune Deng <yejune.deng@...il.com>, john.stultz@...aro.org,
        sboyd@...nel.org
Cc:     linux-kernel@...r.kernel.org, yejune.deng@...il.com
Subject: Re: [PATCH] ntp: use memset and offsetof init

On Wed, Jan 20 2021 at 10:51, Yejune Deng wrote:
> In pps_fill_timex(), use memset and offsetof instead of '= 0'.
>
> Signed-off-by: Yejune Deng <yejune.deng@...il.com>
> ---
>  kernel/time/ntp.c | 13 +++++--------
>  1 file changed, 5 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
> index 87389b9e21ab..3416c0381104 100644
> --- a/kernel/time/ntp.c
> +++ b/kernel/time/ntp.c
> @@ -225,14 +225,11 @@ static inline int is_error_status(int status)
>  static inline void pps_fill_timex(struct __kernel_timex *txc)
>  {
>  	/* PPS is not implemented, so these are zero */
> -	txc->ppsfreq	   = 0;
> -	txc->jitter	   = 0;
> -	txc->shift	   = 0;
> -	txc->stabil	   = 0;
> -	txc->jitcnt	   = 0;
> -	txc->calcnt	   = 0;
> -	txc->errcnt	   = 0;
> -	txc->stbcnt	   = 0;
> +	int offset, len;
> +
> +	offset = offsetof(struct __kernel_timex, ppsfreq);
> +	len    = offsetof(struct __kernel_timex, tai) - offset;
> +	memset(txc + offset, 0, len);

That zeros bytes at a memory location which is

     (offset) * sizeof(struct __kernel_timex)

bytes away from txc. How did this every boot?

And no, even if you fix that pointer math problem then this kind of
calculation from the middle of a struct is error prone.

Thanks,

        tglx