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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 23 Jan 2020 10:02:33 -0800
Message-ID: <CAHk-=wg4HEABOZdjxMzbembNmxs1zYfrNAEc2L+JS9FBSnM8JA@mail.gmail.com>
Subject: Re: [PATCH v3 2/7] uaccess: Tell user_access_begin() if it's for a
 write or not
To:     Christophe Leroy <christophe.leroy@c-s.fr>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
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        "the arch/x86 maintainers" <x86@kernel.org>
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On Thu, Jan 23, 2020 at 4:59 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
>
> On 32 bits powerPC (book3s/32), only write accesses to user are
> protected and there is no point spending time on unlocking for reads.

Honestly, I'm starting to think that 32-bit ppc just needs to look
more like everybody else, than make these changes.

We used to have a read/write argument to the old "verify_area()" and
"access_ok()" model, and it was a mistake. It was due to odd i386 user
access issues. We got rid of it. I'm not convinced this is any better
- it looks very similar and for odd ppc access issues.

But if we really do want to do this, then:

> Add an argument to user_access_begin() to tell when it's for write and
> return an opaque key that will be used by user_access_end() to know
> what was done by user_access_begin().

You should make it more opaque than "unsigned long".

Also, it shouldn't be a "is this a write". What if it's a read _and_ a
write? Only a write? Only a read?

                    Linus