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Message-ID: <17369fe8574440ef8550cbe1bdfb08e515416fe5.camel@xry111.site>
Subject: Re: [PATCH] mips: Call lose_fpu(0) before initializing fcr31 in
 mips_set_personality_nan
From: Xi Ruoyao <xry111@xry111.site>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>, Jiaxun Yang
	 <jiaxun.yang@flygoat.com>, linux-mips@vger.kernel.org
Cc: Ralf Baechle <ralf@linux-mips.org>, "Maciej W. Rozycki"
 <macro@orcam.me.uk>,  YunQiang Su <wzssyqa@gmail.com>, Huacai Chen
 <chenhuacai@kernel.org>, WANG Xuerui <kernel@xen0n.name>, 
 linux-kernel@vger.kernel.org
Date: Sat, 27 Jan 2024 05:03:50 +0800
In-Reply-To: <20240126205920.11487-3-xry111@xry111.site>
References: <20240126205920.11487-3-xry111@xry111.site>
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Please ignore this for wrong address of stable@.  Will send v2.

On Sat, 2024-01-27 at 04:59 +0800, Xi Ruoyao wrote:
> If we still own the FPU after initializing fcr31, when we are preempted
> the dirty value in the FPU will be read out and stored into fcr31,
> clobbering our setting.=C2=A0 This can cause an improper floating-point
> environment after execve().=C2=A0 For example:
>=20
> =C2=A0=C2=A0=C2=A0 zsh% cat measure.c
> =C2=A0=C2=A0=C2=A0 #include <fenv.h>
> =C2=A0=C2=A0=C2=A0 int main() { return fetestexcept(FE_INEXACT); }
> =C2=A0=C2=A0=C2=A0 zsh% cc measure.c -o measure -lm
> =C2=A0=C2=A0=C2=A0 zsh% echo $((1.0/3)) # raising FE_INEXACT
> =C2=A0=C2=A0=C2=A0 0.33333333333333331
> =C2=A0=C2=A0=C2=A0 zsh% while ./measure; do ; done
> =C2=A0=C2=A0=C2=A0 (stopped in seconds)
>=20
> Call lose_fpu(0) before setting fcr31 to prevent this.
>=20
> Closes: https://lore.kernel.org/linux-mips/7a6aa1bbdbbe2e63ae96ff163fab03=
49f58f1b9e.camel@xry111.site/
> Fixes: 9b26616c8d9d ("MIPS: Respect the ISA level in FCSR handling")
> Cc: stable@vger.linux.org
> Signed-off-by: Xi Ruoyao <xry111@xry111.site>
> ---
> =C2=A0arch/mips/kernel/elf.c | 6 ++++++
> =C2=A01 file changed, 6 insertions(+)
>=20
> diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
> index 5582a4ca1e9e..7aa2c2360ff6 100644
> --- a/arch/mips/kernel/elf.c
> +++ b/arch/mips/kernel/elf.c
> @@ -11,6 +11,7 @@
> =C2=A0
> =C2=A0#include <asm/cpu-features.h>
> =C2=A0#include <asm/cpu-info.h>
> +#include <asm/fpu.h>
> =C2=A0
> =C2=A0#ifdef CONFIG_MIPS_FP_SUPPORT
> =C2=A0
> @@ -309,6 +310,11 @@ void mips_set_personality_nan(struct arch_elf_state =
*state)
> =C2=A0	struct cpuinfo_mips *c =3D &boot_cpu_data;
> =C2=A0	struct task_struct *t =3D current;
> =C2=A0
> +	/* Do this early so t->thread.fpu.fcr31 won't be clobbered in case
> +	 * we are preempted before the lose_fpu(0) in start_thread.
> +	 */
> +	lose_fpu(0);
> +
> =C2=A0	t->thread.fpu.fcr31 =3D c->fpu_csr31;
> =C2=A0	switch (state->nan_2008) {
> =C2=A0	case 0:

--=20
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University