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Date:   Wed, 22 Jan 2020 17:41:29 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Christophe Leroy <christophe.leroy@c-s.fr>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
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Subject: Re: [PATCH v1 1/6] fs/readdir: Fix filldir() and filldir64() use of
 user_access_begin()
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On Wed, Jan 22, 2020 at 08:13:12AM -0800, Linus Torvalds wrote:
> 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().

I would suggest simply covering the range from dirent->d_off to
buf->current_dir->d_name[namelen]; they are going to be close to
each other and we need those addresses anyway...