Re: [PATCH v2 2/7] y2038: Introduce __ASSUME_64BIT_TIME define

[Lukasz Majewski <lukma at denx dot de>]

Hi Stepan,

> 02.05.2019 в 10:51:08 +0200 Lukasz Majewski написал:
> > Hi Stepan,
> >   
> > > 30.04.2019 в 11:05:05 +0200 Lukasz Majewski написал:  
> > > > IMHO, the abstraction would be:
> > > > 
> > > > 1. The __ASSUME_64BIT_TIME is _never_ defined for 64 bit native
> > > > systems
> > > > 
> > > > 2. It is defined by default in:
> > > > sysdeps/unix/sysv/linux/kernel-features.h for 32 bit systems
> > > > (and the actual presence of the syscall is decided upon
> > > > definitions of __NR_xxx* (i.e. # ifdef
> > > > __NR_clock_settime64).    
> > > 
> > > I think that __NR_clock_settime64 should be used unconditionally
> > > when __ASSUME_64BIT_TIME is defined.  
> > 
> > Could you clarify it a bit?  
> 
> I meant something like this:
> 
> int
> __clock_settime64 (clockid_t clock_id, const struct __timespec64 *tp)
> {
> #ifdef __ASSUME_64BIT_TIME
>   return INLINE_SYSCALL_CALL (clock_settime64, clock_id, tp);
> #else
> …
> 
> But I see now that most of the existing code would just miscompile in
> cases where __ASSUME_* is defined while corresponding __NR_* is not.
> 
> > In the code as proposed in:
> > https://patchwork.ozlabs.org/patch/1092583/
> > 
> > The call to clock_settime64 is protected with # ifdef
> > __NR_clock_settime64 - otherwise we do a fallback to (32 bit)
> > clock_settime.
> > 
> > Moreover, the # ifdef __ASSUME_64BIT_TIME provides a fallback path
> > if kernel version is older than 5.1.  
> 
> The fallback would be wrong in cases where __NR_clock_settime is not
> defined or is not 32-bit.
> 
> > > > As those syscalls are provided on almost every 32 bit system now
> > > > (5.1-rc6):
> > > > git grep -n "clock_settime64"
> > > > 
> > > > gives support for: arm, arm64 (compat mode), m68k, microblaze,
> > > > mips, parisc, powerpc, s390, sh, sparc, x86, xtensa
> > > > 
> > > > So it would be reasonable to just add this __ASSUME definition
> > > > code to sysdeps/unix/sysv/linux/kernel-features.h and #undef it
> > > > for architectures not supporting it (i.e. c-sky and riscv).    
> > > 
> > > I believe that the only 32-bit architecture without
> > > __NR_clock_settime64 is x32.   
> > 
> > Ok, I see. 
> > 
> > Please correct me - would it be feasible to just #undef
> > __ASSYME_TIME64_SYSCALLS for x32 ?  
> 
> You'll need to know whether to use __NR_clock_settime64 or
> __NR_clock_settime in cases where __TIMESIZE == 64 and
> __WORDSIZE == 32.
> 
> One way would be by defining __ASSUME_TIME64_SYSCALLS unconditionally
> on x32 and then defining __NR_clock_settime64 to __NR_clock_settime
> when __ASSUME_TIME64_SYSCALLS is defined while __NR_clock_settime64
> isn't.
> 

I see. Thanks for the hint.

> > > While newer 32-bit architectures like
> > > riscv do not have __NR_clock_settime:
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=d4c08b9776b392e20efc6198ebe1bc8ec1911d9b  
> > 
> > Then it shall use clock_settime64 from the outset if support
> > added.  
> 
> It probably should have __TIMESIZE == 64 though.


Best regards,

Lukasz Majewski

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