linux-kernel - Re: MIPS: fcsr31 may be dirty after execve when kernel preempt is enabled (was: Re: [PATCH v2] LoongArch: Fix and simplify fcsr initialization on execve)

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Date: Tue, 2 Jan 2024 19:20:57 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xi Ruoyao <xry111@...111.site>
Cc: WANG Xuerui <kernel@...0n.name>, Jiaxun Yang <jiaxun.yang@...goat.com>, 
	Eric Biederman <ebiederm@...ssion.com>, Kees Cook <keescook@...omium.org>, 
	Tiezhu Yang <yangtiezhu@...ngson.cn>, Jinyang He <hejinyang@...ngson.cn>, 
	loongarch@...ts.linux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	stable@...r.kernel.org, linux-mips@...r.kernel.org
Subject: Re: MIPS: fcsr31 may be dirty after execve when kernel preempt is
 enabled (was: Re: [PATCH v2] LoongArch: Fix and simplify fcsr initialization
 on execve)

On Tue, Jan 2, 2024 at 6:48 PM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Tue, 2024-01-02 at 18:25 +0800, Xi Ruoyao wrote:
> > On Tue, 2024-01-02 at 18:17 +0800, Xi Ruoyao wrote:
> > > The only other architecture setting FCSR in SET_PERSONALITY2 is MIPS.
> > > They do this for supporting different FP flavors (NaN encodings etc).
> > > which do not exist on LoongArch.  I'm not sure how MIPS evades the issue
> > > (or maybe it's just buggy too) but I'll investigate it later.
> >
> > Phew.  I just managed to recommission my 3A4000 and I can reproduce the
> > issue as well with Linux 5.18.1 (the latest kernel release when I
> > decommissioned it) and CONFIG_PREEMPT=y.
> >
> > % cat measure.c
> > #include <fenv.h>
> > int main() { return fetestexcept(FE_INEXACT); }
> >
> > % echo $((1./3))
> > 0.33333333333333331
> >
> > % while ./a.out; do ; done
> > (stopped in seconds)
> >
> > I'm building the mainline kernel on the 3A4000 now, will see if the
> > issue still exists...
>
> Still happening with 6.7.0-rc8.  I'm not sure how to fix it for MIPS.
> Maybe lose_fpu in SET_PERSONALITY2? But to me doing so will be really
> nasty.  Anyway I'll leave this for MIPS maintainers.
Disable preemption in SET_PERSONALITY2 and enable in START_THREAD?

Huacai

>
> --
> Xi Ruoyao <xry111@...111.site>
> School of Aerospace Science and Technology, Xidian University