Re: [PATCH -next v4 4/7] arm64: add copy_{to, from}_user to machine check safe

[Tong Tiangen]

在 2022/5/4 18:26, Catalin Marinas 写道:
> On Wed, Apr 20, 2022 at 03:04:15AM +0000, Tong Tiangen wrote:
> > Add copy_{to, from}_user() to machine check safe.
> >
> > If copy fail due to hardware memory error, only the relevant processes are
> > affected, so killing the user process and isolate the user page with
> > hardware memory errors is a more reasonable choice than kernel panic.
>
> Just to make sure I understand - we can only recover if the fault is in
> a user page. That is, for a copy_from_user(), we can only handle the
> faults in the source address, not the destination.

At the beginning, I also thought we can only recover if the fault is in 
a user page.
After discussion with a Mark[1], I think no matter user page or kernel 
page, as long as it is triggered by the user process, only related 
processes will be affected. According to this
understanding, it seems that all uaccess can be recovered.

[1]https://patchwork.kernel.org/project/linux-arm-kernel/patch/20220406091311.3354723-6-tongtiangen@xxxxxxxxxx/

Thanks,
Tong.

> > diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> > index 34e317907524..480cc5ac0a8d 100644
> > --- a/arch/arm64/lib/copy_from_user.S
> > +++ b/arch/arm64/lib/copy_from_user.S
> > @@ -25,7 +25,7 @@
> >   	.endm
> >   
> >   	.macro strb1 reg, ptr, val
> > -	strb \reg, [\ptr], \val
> > +	USER_MC(9998f, strb \reg, [\ptr], \val)
> >   	.endm
>
> So if I got the above correctly, why do we need an exception table entry
> for the store to the kernel address?