Re: [PATCH v6 5/5] arm64: futex: support futex with FEAT_LSUI

[Yeoreum Yun <yeoreum.yun@arm.com>]

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