Re: [RFC PATCH -next V3 4/6] arm64: add copy_{to, from}_user to machine check safe

[Robin Murphy]

On 12/04/2022 6:08 pm, Robin Murphy wrote:
[...]
> > @@ -62,7 +63,11 @@ SYM_FUNC_START(__arch_copy_from_user)
> >       ret
> >       // Exception fixups
> > -9997:    cmp    dst, dstin
> > +9997:    mrs esr, esr_el1            // Check exception first
> > +    and esr, esr, #ESR_ELx_FSC
> > +    cmp esr, #ESR_ELx_FSC_EXTABT
>
> Should we be checking EC to make sure it's a data abort - and thus FSC 
> is valid - in the first place? I'm a little fuzzy on all the possible 
> paths into fixup_exception(), and it's not entirely obvious whether this 
> is actually safe or not.

In fact, thinking some more about that, I don't think there should be 
any need for this sort of logic in these handlers at all. The 
fixup_exception() machinery should already know enough about the 
exception that's happened and the extable entry to figure this out and 
not bother calling the handler at all.

Thanks,
Robin.