Re: [PATCH] arm64/traps: Avoid unnecessary kernel/user pointer conversion

[Amit Kachhap]

Hi,

On 9/14/21 9:30 PM, Mark Rutland wrote:
> On Tue, Sep 14, 2021 at 08:57:42PM +0530, Amit Daniel Kachhap wrote:
> > Annotating a pointer from kernel to __user and then back again might
> > confuse sparse. In call_undef_hook() it can be avoided by not using the
> > intermediate user pointer variable.
>
> When you say "might confuse sparse", does it complain today? If so, can
> you include an example of what goes wrong?

No it does not give warning. The __force option silences the warning. My
idea is to remove the unwanted __force annotations and not mix user and
kernel pointers.

> > Note: This patch adds no functional changes to code.
> >
> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > Cc: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Amit Daniel Kachhap <amit.kachhap@xxxxxxx>
> > ---
> >   arch/arm64/kernel/traps.c | 3 ++-
> >   1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
> > index b03e383d944a..357d10a8bbf5 100644
> > --- a/arch/arm64/kernel/traps.c
> > +++ b/arch/arm64/kernel/traps.c
> > @@ -404,7 +404,8 @@ static int call_undef_hook(struct pt_regs *regs)
> >   
> >   	if (!user_mode(regs)) {
> >   		__le32 instr_le;
> > -		if (get_kernel_nofault(instr_le, (__force __le32 *)pc))
> > +		if (get_kernel_nofault(instr_le,
> > +				       (__le32 *)instruction_pointer(regs)))
>
> Can we make `pc` an unsigned long, instead?

I think it can be done.

> It'd be nice to handle all three cases consistently, even if that means
> adding __force to the two user cases.

Agree with your suggestion. Even in the 2 user cases, __force may not be
needed as the typecast will be from from unsigned long to user pointer.

BR,
Amit
> Thanks,
> Mark.
>
> >   			goto exit;
> >   		instr = le32_to_cpu(instr_le);
> >   	} else if (compat_thumb_mode(regs)) {
> > --
> > 2.17.1