linux-kernel - Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs without ERMS

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Message-ID: <20230903232802.GO3390869@ZenIV>
Date:   Mon, 4 Sep 2023 00:28:02 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        bp@...en8.de
Subject: Re: [PATCH v2] x86: bring back rep movsq for user access on CPUs
 without ERMS

On Sun, Sep 03, 2023 at 11:18:44PM +0200, Mateusz Guzik wrote:
> On 9/3/23, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> > On Sun, 3 Sept 2023 at 14:06, Mateusz Guzik <mjguzik@...il.com> wrote:
> >>
> >> I don't think it is *that* bad. I did a quick sanity check on that
> >> front by rolling with bpftrace on cases which pass AT_EMPTY_PATH *and*
> >> provide a path.
> >
> > I guess you are right - nobody sane would use AT_EMPTY_PATH except if
> > they don't have a path.
> >
> > Of course, the only reason we're discussing this in the first place is
> > because people are doing insane things, which makes _that_ particular
> > argument very weak indeed...
> >
> 
> I put blame on whoever allowed non-NULL path and AT_EMPTY_PATH as a
> valid combination, forcing the user buf to be accessed no matter what.
> But I'm not going to go digging for names. ;)

ITYM s/allowed/mandated/ - AT_EMPTY_PATH with NULL is -EFAULT.