linux-kernel - Re: [PATCH 2/2] futex, x86/mce: Avoid double machine checks

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Message-ID: <20210108231459.GC2453@worktop.programming.kicks-ass.net>
Date:   Sat, 9 Jan 2021 00:14:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Darren Hart <dvhart@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/2] futex, x86/mce: Avoid double machine checks

On Fri, Jan 08, 2021 at 11:08:58PM +0000, Luck, Tony wrote:
> > I think this is horrid; why can't we have it return something different
> > then -EFAULT instead?
> 
> I did consider this ... but it appears that architectures aren't unified in the
> return value from get_user()

But surely none are currently returning -EMEMERR or whatever name we
come up with.

> Here's another function involved in the futex call chain leading to this:
> 
> static int get_futex_value_locked(u32 *dest, u32 __user *from)
> {
>         int ret;
> 
>         pagefault_disable();
>         ret = __get_user(*dest, from);
>         pagefault_enable();
> 
>         return ret ? -EFAULT : 0;
> }
> 
> It seems like the expectation here is just "zero or not" and we
> don't care what the "not" value is ... just turn it into -EFAULT.

Yeah, saw that, but that should be trivially fixable I'm thinking.