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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 22 Jan 2020 08:13:12 -0800
Message-ID: <CAHk-=whTzEu5=sMEVLzuf7uOnoCyUs8wbfw87njes9FyE=mj1w@mail.gmail.com>
Subject: Re: [PATCH v1 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access_begin()
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>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
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On Wed, Jan 22, 2020 at 5:00 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
>
> Modify filldir() and filldir64() to request the real area they need
> to get access to.

Not like this.

This makes the situation for architectures like x86 much worse, since
you now use "put_user()" for the previous dirent filling. Which does
that expensive user access setup/teardown twice again.

So either you need to cover both the dirent's with one call, or you
just need to cover the whole (original) user buffer passed in. But not
this unholy mixing of both unsafe_put_user() and regular put_user().

              Linus