Re: [PATCH -next v5 6/8] arm64: add support for machine check error safe

[Tong Tiangen]

在 2022/6/18 20:52, Mark Rutland 写道:
> On Sat, Jun 18, 2022 at 05:18:55PM +0800, Tong Tiangen wrote:
> > 在 2022/6/17 16:55, Mark Rutland 写道:
> > > On Sat, May 28, 2022 at 06:50:54AM +0000, Tong Tiangen wrote:
> > > > +static bool arm64_do_kernel_sea(unsigned long addr, unsigned int esr,
> > > > +				     struct pt_regs *regs, int sig, int code)
> > > > +{
> > > > +	if (!IS_ENABLED(CONFIG_ARCH_HAS_COPY_MC))
> > > > +		return false;
> > > > +
> > > > +	if (user_mode(regs) || !current->mm)
> > > > +		return false;
> > >
> > > What's the `!current->mm` check for? >>
> > At first, I considered that only user processes have the opportunity to
> > recover when they trigger memory error.
> >
> > But it seems that this restriction is unreasonable. When the kernel thread
> > triggers memory error, it can also be recovered. for instance:
> >
> > https://lore.kernel.org/linux-mm/20220527190731.322722-1-jiaqiyan@xxxxxxxxxx/
> >
> > And i think if(!current->mm) shoud be added below:
> >
> > if(!current->mm) {
> > 	set_thread_esr(0, esr);
> > 	arm64_force_sig_fault(...);
> > }
> > return true;
>
> Why does 'current->mm' have anything to do with this, though?

Sorry, typo, my original logic was:
if(current->mm) {
	[...]
}

> There can be kernel threads with `current->mm` set in unusual circumstances
> (and there's a lot of kernel code out there which handles that wrong), so if
> you want to treat user tasks differently, we should be doing something like
> checking PF_KTHREAD, or adding something like an is_user_task() helper.

OK, i do want to treat user tasks differently here and didn't take into 
account what you said. will be fixed next version according to your 
suggestiong.

As follows:
if (!(current->flags & PF_KTHREAD)) {
  set_thread_esr(0, esr);
  arm64_force_sig_fault(...);
}
return true;


> [...]
>
> > > > +
> > > > +	if (apei_claim_sea(regs) < 0)
> > > > +		return false;
> > > > +
> > > > +	if (!fixup_exception_mc(regs))
> > > > +		return false;
> > >
> > > I thought we still wanted to signal the task in this case? Or do you expect to
> > > add that into `fixup_exception_mc()` ?
> >
> > Yeah, here return false and will signal to task in do_sea() ->
> > arm64_notify_die().
>
> I mean when we do the fixup.
>
> I thought the idea was to apply the fixup (to stop the kernel from crashing),
> but still to deliver a fatal signal to the user task since we can't do what the
> user task asked us to.

Yes, that's what i mean. :)

> > > > +
> > > > +	set_thread_esr(0, esr);
> > >
> > > Why are we not setting the address? Is that deliberate, or an oversight?
> >
> > Here set fault_address to 0, i refer to the logic of arm64_notify_die().
> >
> > void arm64_notify_die(...)
> > {
> >           if (user_mode(regs)) {
> >                   WARN_ON(regs != current_pt_regs());
> >                   current->thread.fault_address = 0;
> >                   current->thread.fault_code = err;
> >
> >                   arm64_force_sig_fault(signo, sicode, far, str);
> >           } else {
> >                   die(str, regs, err);
> >           }
> > }
> >
> > I don't know exactly why and do you know why arm64_notify_die() did this? :)
>
> To be honest, I don't know, and that looks equally suspicious to me.
>
> Looking at the git history, that was added in commit:
>
>    9141300a5884b57c ("arm64: Provide read/write fault information in compat signal handlers")
>
> ... so maybe Catalin recalls why.
>
> Perhaps the assumption is just that this will be fatal and so unimportant? ...
> but in that case the same logic would apply to the ESR value, so it's not clear
> to me.

OK, let's proceed as set to 0, if there is any change later, the two 
positions shall be changed together.

Thanks,
Tong.

> Mark.
>
> .