linux-kernel - Re: [PATCH v6 5/5] arm64: futex: support futex with FEAT

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Message-ID: <aKCcbQWioNfG1MMQ@e129823.arm.com>
Date: Sat, 16 Aug 2025 15:57:49 +0100
From: Yeoreum Yun <yeoreum.yun@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: will@...nel.org, broonie@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, shameerali.kolothum.thodi@...wei.com,
	joey.gouly@....com, james.morse@....com, ardb@...nel.org,
	scott@...amperecomputing.com, suzuki.poulose@....com,
	yuzenghui@...wei.com, mark.rutland@....com,
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 5/5] arm64: futex: support futex with FEAT_LSUI

Hi Catalin,

[...]
> > > +#ifdef CONFIG_AS_HAS_LSUI
> > > +
> > > +#define __LSUI_PREAMBLE	".arch_extension lsui\n"
> > > +
> > > +#define LSUI_FUTEX_ATOMIC_OP(op, asm_op, mb)				\
> > > +static __always_inline int						\
> > > +__lsui_futex_atomic_##op(int oparg, u32 __user *uaddr, int *oval)	\
> > > +{									\
> > > +	int ret = 0;							\
> > > +	int val;							\
> > > +									\
> > > +	mte_enable_tco();						\
> >
>
> > The reason uaccess_disable_privileged() sets the MTE TCO (tag check
> > override) is because the user and the kernel may have different settings
> > for tag checking. If we use the user instructions provided by FEAT_LSUI,
> > we leave the MTE checking as is.
> >
> > The same comment for all the other functions here.
>
> You're right. Thanks for catching this :)

But one bikeshedding question.
why we need to care about the different settings for tag checking when
we use uaccess_disable_privileged()?

IIUC, the reason we uses to uaccess_disaable_privileged() to access
user memory with copy_from/to_user() and etc.
But, although tag check fault happens in kernel side,
It seems to be handled by fixup code if user address is wrong.

Am I missing something?

> --
> Sincerely,
> Yeoreum Yun
>

--
Sincerely,
Yeoreum Yun