Re: Trying Debian/armhf rebootstrap with time64
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On Wed, Mar 11, 2020 at 3:37 PM Lukasz Majewski <lukma@denx.de> wrote:
> > - stat()/fstat()/lstat(), nanosleep(), wait3()/wait4(), ppoll_chk()
> > are some of the other interfaces that take a time_t based
> > argument and need to grow a time64 version to avoid an ABI
> > mismatch.
>
> The stat() and friends will use statx internally, which supports 64 bit
> time from the outset.
> Unfortunately, it hasn't been yet converted.
>
> As statx was added in 4.1 (IIRC) - after the minimal supported Linux
> kernel version is bumped to this version (from 3.2 as now) it all will
> be fixed.
The problem I had with these was not on the kernel API side (I
still have CONFIG_COMPAT_32BIT_TIME enabled for now) but
on the application side. In particular, the 'struct stat' definition
(when __USE_XOPEN2K8 is defined) contains
struct timespec st_atim;
and similar fields that are interpreted using 64-bit time_t in the
application including the header, but with 32-bit time_t inside of
the ___fxstat64() implementation in glibc. The problem is caused
by the mismatched ABI, not by the time_t overflow, in the same
way that happens in the nptl library callers and in the other interfaces
I mentioned.
This is also what seems to cause most of the testcase failures
when the tests are built with __TIME_BITS=64.
> > - The timeval prototype appears to be broken, as it's missing
> > padding on architectures without native alignment of __time64
> > (e.g. i386) and on all big-endian architectures.
> >
>
> You mean the one "exported" to the system or one, which is internal to
> glibc (from ./include/time.h)?
I mean in the installed headers, where I get (after preprocessing)
typedef long long __time64_t;
typedef long __suseconds_t;
struct timeval
{
__time64_t tv_sec;
__suseconds_t tv_usec;
};
On i386 and m68k, this leads to a 12 byte structure when the kernel
interfaces expect a 16 byte structure. All other 32-bit architectures add
four byte padding at the end, so the size is correct, but on big-endian
systems, kernel also expects padding *before* tv_usec, in the same
way as the timespec definition does. IIRC the best way to handle this
is with a 64-bit suseconds_t, e.g. by adding a __suseconds64_t
defined the same way as __time64_t.
> > I have spent more time on this now than I had planned, and don't
> > expect to do further work on it anytime soon, but I hope my summary
> > is useful to others that are going to need this later. I can
> > obviously share my patches and build artifacts if anyone needs them.
>
> Could you upload them to any server? (kernel.org or github)?
I have uploaded the modified debian-glibc and rebootstrap
sources to https://git.linaro.org/people/arnd/ now, this should
be all that's needed to recreate the build, using these steps:
- build an x86-64 debian glibc-2.31 package (binary plus source)
based on the glibc package data
- create a pbuilder instance with that available as a source to apt
- log into the pbuilder and run the modified bootstrap.sh according
to information on the pbuilder web page.
The binary packages I created are not as useful, as they would
not work with any build of glibc, neither the version I built, nor
any fixed one. I could find a way to send that to you in private,
but it's hundreds of megabytes.
Unfortunately I lost the build logs during a crash yesterday.
> > There are two additional approaches that would likely get a Debian
> > bootstrap further, but that I have not tried as they were previously
> > dismissed:
> >
> > * Adding a time64 armhf as a separate (incompatible) target in glibc
> > that defines __TIMESIZE==64 and a 64-bit __time_t would avoid
> > most of the remaining ABI issues and put armhf-time64 in the same
> > category as riscv32 and arc, but this idea was so far rejected by
> > the glibc maintainers.
>
> As fair as I know riscv32 and arc will use generic syscall interface.
> The arm32 bit doesn't support it - so the code from those two
> aforementioned ports will not be used.
The differences between the generic syscall interface and the arm
version are much smaller than ABI differences between the ABIs
for 32-bit __time_t and the __time_t == __time64_t version.
In particular, as such a new port could mandate a minimum kernel
of v5.1, it could just use all the time64 syscalls, the split sysvipc
and statx as a baseline.
Arnd
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