From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Aurelien Jarno <aurelien@aurel32.net>,
Palmer Dabbelt <palmer@sifive.com>
Cc: linux-riscv@lists.infradead.org
Subject: Re: BUG: FP registers leak across execve
Date: Fri, 04 Jan 2019 14:12:06 +1100 [thread overview]
Message-ID: <18c5803467f84868790442a541f269318395afe7.camel@kernel.crashing.org> (raw)
In-Reply-To: <20190104013932.ksexuhjssygj4jml@aurel32.net>
On Fri, 2019-01-04 at 02:39 +0100, Aurelien Jarno wrote:
> On 2019-01-03 15:36, Palmer Dabbelt wrote:
> > On Mon, 10 Dec 2018 14:13:28 PST (-0800), aurelien@aurel32.net wrote:
> > > Hi all,
> > >
> > > Debugging some glibc testsuite math failures, I have found out that most
> > > of the time, the FP status register and the FP registers are not zeroed
> > > as they should. This can be tested with the attached code. The best way
> > > to reproduce it is to execute from Python (i guess Perl or another
> > > interpreted language that support FP computation should work). When
> > > running an FP computation before calling the program, the result of the
> > > computation can be seen in f10.
> > >
> > > The zeroing of the FP status happens in kernel/process.c in the
> > > flush_thread function. It seems that the kernel restore that state only
> > > if a context switch happens between flush_thread and the first FP
> > > instruction of the executed program.
> > >
> > > A possible workaround is to restore of the FP registers in flush_thread,
> > > but that's probably not the best way to do that:
> > >
> > >
> > > --- a/arch/riscv/kernel/process.c
> > > +++ b/arch/riscv/kernel/process.c
> > > @@ -93,6 +93,7 @@ void flush_thread(void)
> > > * fflags: accrued exceptions cleared
> > > */
> > > memset(¤t->thread.fstate, 0, sizeof(current->thread.fstate));
> > > + fstate_restore(current, task_pt_regs(current));
> > > #endif
> > > }
> >
> > Are you running this in QEMU? IIRC there was a bug here and we might not
> > have the right fix upstream yet.
>
> I can reproduce the issue in a QEMU 3.1 VM running a 4.20 kernel, but
> also on an HFU board running the original kernel.
You can do that or force a restore in start_thread I think. (powerpc
does all that stuff in start_thread rather than flush_thread but that's
probably not that great an idea).
Cheers,
Ben.
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next prev parent reply other threads:[~2019-01-04 3:14 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-10 22:13 BUG: FP registers leak across execve Aurelien Jarno
2019-01-03 23:36 ` Palmer Dabbelt
2019-01-04 1:39 ` Aurelien Jarno
2019-01-04 3:12 ` Benjamin Herrenschmidt [this message]
2019-01-22 20:31 ` Palmer Dabbelt
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