linux-kernel - Re: [PATCH 24/25] arm64:ilp32: add vdso-ilp32 and use for signal return

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Message-ID: <3846428.6xx69KGEja@wuerfel>
Date:	Tue, 03 May 2016 11:05:02 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	linux-arch@...r.kernel.org, linux-s390@...r.kernel.org,
	pinskia@...il.com, Prasun.Kapoor@...iumnetworks.com,
	schwab@...e.de, broonie@...nel.org, linux-doc@...r.kernel.org,
	heiko.carstens@...ibm.com, linux-kernel@...r.kernel.org,
	agraf@...e.de, klimov.linux@...il.com,
	Yury Norov <ynorov@...iumnetworks.com>,
	bamvor.zhangjian@...wei.com, joseph@...esourcery.com,
	schwidefsky@...ibm.com, Nathan_Lynch@...tor.com,
	Philipp Tomsich <philipp.tomsich@...obroma-systems.com>,
	linux-arm-kernel@...ts.infradead.org,
	christoph.muellner@...obroma-systems.com
Subject: Re: [PATCH 24/25] arm64:ilp32: add vdso-ilp32 and use for signal return

On Tuesday 03 May 2016 10:00:45 Catalin Marinas wrote:
> On Fri, Apr 29, 2016 at 07:30:19PM +0200, Arnd Bergmann wrote:
> > On Friday 29 April 2016 17:01:55 Catalin Marinas wrote:
> > > On Wed, Apr 06, 2016 at 01:08:46AM +0300, Yury Norov wrote:
> > > > ILP32 VDSO exports next symbols:
> > > >  __kernel_rt_sigreturn;
> > > >  __kernel_gettimeofday;
> > > >  __kernel_clock_gettime;
> > > >  __kernel_clock_getres;
> > > 
> > > [...]
> > > 
> > > > +$(obj)/gettimeofday-ilp32.o: $(src)/../vdso/gettimeofday.S
> > > > + $(call if_changed_dep,vdso-ilp32as)
> > > 
> > > Are struct timeval and timespec the same between ILP32 and LP64? For
> > > example, __kernel_gettimeofday() assumes TVAL_TV_SEC offset defined in
> > > asm-offsets.c based on the LP64 timeval.
> > 
> > No, ilp32 uses the generic 32-bit data structures, which have a 32-bit
> > time_t. I guess that means it can work for little-endian but not
> > big-endian, right?
> 
> I don't think it works for little-endian either. The LP64 struct timeval
> is 16 bytes while the ILP32 one is 8 bytes. The VDSO gettimeofday is
> storing 16 bytes (stp x10, x11, [x0, #TVAL_TV_SEC])

You are right. Yury asked pointed out the same thing on IRC as well.
Using the 64-bit gettimeofday() will put the right number in the
.tv_sec member on little-endian, but will write zeroes to tv_nsec
and corrupt the memory following it.

Yury also tried it out and noticed that for a (so far) unknown reason,
the vdso gets never used by his glibc build, so it has not triggered
any test case failures.

	Arnd