linux-kernel - Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access

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Message-ID: <CAHk-=wgK1Pbj4DD4OLFuFg1Tgvup85h9W5ZroCOwAE1qCDWuBg@mail.gmail.com>
Date:   Wed, 22 Jan 2020 12:15:41 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access_begin()

On Wed, Jan 22, 2020 at 12:00 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> A bit more re-organization also allows us to do the unsafe_put_user()
> unconditionally.

I meant the "user_access_begin()", of course.

Code was right, explanation was wrong.

That said, with this model, we _could_ make the

        unsafe_put_user(offset, &prev->d_off, efault_end);

be unconditional too, since now 'prev' will actually be a valid
pointer - it will match 'dirent' if there was no prev.

But since we want to test whether we had a previous entry anyway (for
the signal handling latency issue), making the write to the previous
d_reclen unconditional (and then overwriting it the next iteration)
doesn't actually buy us anything.

It was the user_access_begin() I'd rather have unconditional, since
otherwise it gets duplicated in two (very slightly) different versions
and we have unnecessary code bloat.

           Linus