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Date:   Tue, 22 Dec 2020 20:04:31 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     "Maciej W. Rozycki" <macro@linux-mips.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        linux-mips@vger.kernel.org, Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCHSET] saner elf compat
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On Wed, Dec 16, 2020 at 09:44:53AM +0000, Maciej W. Rozycki wrote:
> On Wed, 16 Dec 2020, Al Viro wrote:
> 
> > >  It may be worth pushing through GDB's gdb.threads/tls-core.exp test case, 
> > > making sure no UNSUPPORTED results have been produced due to resource 
> > > limits preventing a core from being dumped (and no FAILs, of course), with 
> > > o32/n32 native GDB.  This should guarantee our output is still as expected 
> > > by an interpreter.  Sadly I'm currently not set up for such testing though 
> > > eventually I mean to.
> > 
> > Umm...  What triple does one use for n32 gdb?
> 
>  I don't think there's a standardised one, just configure with CC/CXX set 
> for n32 compilation, e.g.:
> 
> $ /path/to/configure CC="gcc -mabi=n32" CXX="g++ -mabi=n32"
> 
> (and any other options set as usually).  This has to be with CC/CXX rather 
> than CFLAGS/CXXFLAGS so that it is guaranteed to be never overridden with 
> any logic that might do any fiddling with compilation options.  This will 
> set up the test suite accordingly.
> 
>  NB this may already be the compiler's default, depending on how it was 
> configured, i.e. if `--with-abi=n32' was used, in which case no extra 
> options will be required.  I don't know if any standard MIPS distribution 
> does it though; 64-bit MIPS/Debian might.  This will be reported with `gcc 
> --help -v', somewhere along the way.
> 
>  Let me know if there are issues with this approach.

FWIW, on debian/mips64el (both stretch and buster) the test fails with the
distro kernels (4.9- and 4.19-based) as well as with 5.10-rc1 and
5.10-rc1+that series, all in the same way:
[Current thread is 1 (LWP 4154)]
(gdb) p/x foo
Cannot find thread-local storage for LWP 4154, executable file <pathname>
Cannot find thread-local variables on this target

buster has libc6-2.28, so that should be fine for the test in question
(libthread_db definitely recent enough).  That was n32 gdb; considering
how much time it had taken to build that sucker I hadn't tried o32
yet.

Note that it's not just with native coredumps - gcore-produced ones give
the same result.  That was gdb from binutils-gdb.git; I'm not familiar
with gdb guts to start debugging it, so if you have any suggestions
in that direction that do not include a full rebuild...  In any case,
I won't get around to that until the next week.

Incidentally, build time is bloody awful - 3 days, with qemu-3.1 on
3.5GHz amd64 host, all spent pretty much entirely in userland (both
from guest and host POV).  g++-8 is atrociously slow...

That said, I don't see what in that series could possibly mess the
things up for tls, while leaving the registers working; the only
thing that realistically might've been fucked up is prstatus layout
(and possibly size), and that would've screwed the registers as
well.