Re: switching ARC to 64-bit time_t (Re: [RFC v6 07/23] RISC-V: Use 64-bit time_t and off_t for RV32 and RV64)

Lukasz Majewski
2020-02-19T23:11:46Z

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Hi Vineet,

> Hi Alistair, Arnd
>
> On 2/14/20 2:39 PM, Alistair Francis wrote:
> > On Tue, Feb 11, 2020 at 5:30 PM Joseph Myers
> > <joseph@codesourcery.com> wrote:
> >> On Tue, 11 Feb 2020, Alistair Francis wrote:
> >>
> >>>>> diff --git a/sysdeps/unix/sysv/linux/riscv/bits/typesizes.h
> >>>>> b/sysdeps/unix/sysv/linux/riscv/bits/typesizes.h new file mode
> >>>>> 100644 index 0000000000..0da3bdeb5d
> >>>>> --- /dev/null
> >>>>> +++ b/sysdeps/unix/sysv/linux/riscv/bits/typesizes.h
> >>>> I was hoping newer arches could simply use the asm-generic one ?
> >>>>
> >>> We need to specify that RV32 uses a 64-bit time_t. The generic
> >>> ones don't do that for 32-bit arches.
> >> Since it seems we'd like future 32-bit ports of glibc to use
> >> 64-bit time and offsets, we should make that as easy as possible.
> >>
> >> That is, you need an RISC-V-specific bits/timesize.h. But you
> >> shouldn't need an RISC-V-specific bits/typesizes.h - rather, make
> >> the linux/generic one do the right thing for __TIME_T_TYPE based
> >> on bits/timesize.h. And have some other header that 32-bit
> >> linux/generic ports can use to say whether they use the 64-bit
> >> offset/stat/statfs interface, that bits/typesizes.h can use
> >> together with its existing __LP64__ check, and make the
> >> definitions of __OFF_T_TYPE etc. check that as well, and then you
> >> shouldn't need an RISC-V-specific bits/typesizes.h - the
> >> RISC-V-specific headers should be strictly minimal. (No
> >> architecture-specific bits/time64.h headers should be needed in
> >> any case.)
> > Ok, I have updated this. I'll send the patch once my "Always use
> > 32-bit time_t for certain syscalls" series is in (the headers are
> > changed in that series).
>
> I guess you haven't pushed changes yet, which don't make full copy of
> typesizes.h ?
>
> Anyhow I have a version based on your prior next branch where I
> switched ARC to 64-bit time_t - things work fine in general but I see
> some additional failures with the testsuite.
>
> Consider io/test-stat2.c which calls stat() and stat64() and compares
> the results: it now fails for ctime mismatch
>
> | ...
> | st_atime: [72] 644245094405576070 vs [72] 644245094405576070  OK
> | st_mtime: [88] 1975684956160000000 vs [88] 1975684956160000000  OK
> | st_ctime: [104] 0 vs [104] 2306351876938924035  FAIL
>
>
> In kernel asm-generic stat64 has 32-bit ctime (secs)
>
>     struct stat64 {
>     ...
>         int                   st_atime;            <-- offset 72
>         unsigned int    st_atime_nsec;
>         int                   st_mtime;            <-- offset *80*
>         unsigned int    st_mtime_nsec;
>         int                   st_ctime;            <-- offset 92
>         unsigned int    st_ctime_nsec;
>     ...
>     };
>
> In glibc, we have 64-bit time_t based timestamps so the structure
> diverges with kernel counterpart from time fields onwards.
>
>     __extension__ typedef __int64_t __time_t;        <-- ARC switched
> to 64-bit time_t
>
>     struct timespec
>     {
>       __time_t tv_sec;    <-- 8
>       long int tv_nsec;    <-- 4
>       int: 32;                   <-- 4
>     };
>
>     struct stat64
>       {
>     ...
>         struct timespec st_atim;    <-- offset 72
>         struct timespec st_mtim;    <-- offset *88*
>         struct timespec st_ctim;    <-- offset 104
>         int __glibc_reserved[2];
>       };
>
> However glibc stat64()wrapper calls
> sysdeps/unix/sysv/linux/generic/wordsize-32/xstat64.c
>
> which for ARC is just doing a pass thru syscall because we do have
> __NR_fstatat64
> - hence the issues I see.
>
> It needs itemized copy
>
>     __xstat64 (int vers, const char *name, struct stat64 *buf)
>     {
>     #ifdef __NR_fstatat64
>           return INLINE_SYSCALL (fstatat64, 4, AT_FDCWD, name, buf,
> 0); #else
>     ....
>           int rc = INLINE_SYSCALL (statx, 5, AT_FDCWD, name,
> AT_NO_AUTOMOUNT, STATX_BASIC_STATS, &tmp);
>         __cp_stat64_statx (buf, &tmp);
>     #endif
>
> An the reason this all works on RISCV is that your kernel doesn't
> define __ARCH_WANT_STAT64 -> lacks __NR_statat64 and instead uses the
> statx call which does itemized copy and would work fine when copying
> from 32-bits time (in kernel) to 64-bits container in glibc. Is this
> is right understanding or am I missing something here.
>
> How do I build a latest RISCV 32-bit kernel + userland - do you have
> a buildroot branch somewhere that I can build / test with qemu ?

Maybe a bit off topic - there is such QEMU and Yocto/OE based test
sandbox for ARM32:

https://github.com/lmajewski/meta-y2038

(the README provides steps for setup).

>
> Thx,
> -Vineet
>
>
>

Best regards,

Lukasz Majewski

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