Re: [PATCH v2 3/7] y2038: Provide conversion helpers for struct __timespec64

[Lukasz Majewski <lukma at denx dot de>]

Hi Stepan,

> 29.04.2019 в 12:46:09 +0200 Lukasz Majewski написал:
> > +/* Set to zero the struct __timespec64's tv_pad.  */
> > +static inline void
> > +timespec64_clear_padding (const struct __timespec64 *tp)
> > +{
> > +  ((struct __timespec64*)tp)->tv_pad = 0;
> > +}  
> 
> Modification of const values isn't going to work.

This was added as a special precaution to avoid passing random data to
kernel syscalls for the Y2038 support use case on 32 bit systems (where
tv_nsec is 32 bit long) .

As we discussed in the other thread - kernel relies on "implementation
depended" behaviour from GCC (which is now to discard higher 32 bits
from tv_nsec [1]).

It is up to glibc community to decide if such precaution is necessary or
shall we only rely on kernel (and gcc) behaviour.


With this particular patch - the problem is with struct timespec's *tp
pointer being const for data being passed to the kernel (to avoid
creating explicit copy of struct timespec and pass it to kernel
syscall).


Note:

[1] - kernel/time/time.c -> get_timespec64: 
ts->tv_nsec = kts.tv_nsec;

(64 bit's kts.tv_nsec to 32 bit tv_nsec truncation).

Discussion:
https://patchwork.ozlabs.org/patch/1085397/
https://lkml.org/lkml/2019/4/26/600


Best regards,

Lukasz Majewski

--

DENX Software Engineering GmbH,      Managing Director: Wolfgang Denk
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-59 Fax: (+49)-8142-66989-80 Email: lukma@denx.de

Attachment: pgpMgbtZwmtzm.pgp
Description: OpenPGP digital signature