RE: [PATCH] ntp: use memset and offsetof init

[David Laight <David.Laight@ACULAB.COM>]

From: Thomas Gleixner
> Sent: 05 February 2021 17:34
> 
> 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@gmail.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.

It is also, at best, a code size optimisation.
If memset() is actually called (not inlined) then you get a whole
lot of tests against the size and alignment before any writes
of zero happen - which will be the same ones as in the inline code.

It can be worth using memcpy to copy part of a structure
but usually for one with lots of small fields (especially bitfields).

	David

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