linux-kernel - Re: [PATCH v4] x86/mce: Avoid infinite loop for copy from user recovery

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Date:   Tue, 19 Jan 2021 11:56:32 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     x86@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, linux-edac@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v4] x86/mce: Avoid infinite loop for copy from user
 recovery

On Fri, Jan 15, 2021 at 03:23:46PM -0800, Luck, Tony wrote:
> On Fri, Jan 15, 2021 at 12:51:03PM -0800, Luck, Tony wrote:
> >  static void kill_me_now(struct callback_head *ch)
> >  {
> > +	p->mce_count = 0;
> >  	force_sig(SIGBUS);
> >  }
> 
> Brown paper bag time ... I just pasted that line from kill_me_maybe()
> and I thought I did a re-compile ... but obviously not since it gives
> 
> error: ‘p’ undeclared (first use in this function)
> 
> Option a) (just like kill_me_maybe)
> 
> struct task_struct *p = container_of(cb, struct task_struct, mce_kill_me);
> 
> Option b) (simpler ... not sure why PeterZ did the container_of thing
> 
> 	current->mce_count = 0;

Right, he says it is the canonical way to get it out of callback_head.
I don't think current will change while the #MC handler runs but we can
adhere to the design pattern here and do container_of() ...

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette