linux-kernel - Re: [PATCH v4 43/45] namei: initialize parameters passed to step

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Date:   Mon, 4 Jul 2022 13:47:50 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Vitaly Buka <vitalybuka@...gle.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into()

On Mon, Jul 4, 2022 at 1:24 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The mount point check should go around the "check dentry mount point",
> but it's a separate issue from the whole "we are now jumping to a
> different dentry, we should check that the previous dentry hasn't
> changed".

Maybe it doesn't really matter, because we never actually end up
dereferencing the previous dentry (exactly since we're following the
mount point on it).

It feels like the sequence point checks are basically tied to the
"we're looking at the inode that the dentry pointed to", and because
the mount-point traversal doesn't need to look at the inode, the
sequence point check also isn't done.

But it feels wrong to traverse a dentry under RCU - even if we don't
then look at the inode itself - without having verified that the
dentry is still valid.

Yes, the d_seq lock protects against the inode going away (aka
"unlink()") and that cannot happen when it's a mount-point.

But it _also_ ends up changing for __d_move() when the name of the
dentry changes.

And I think that name change is relevant even to "look up a mount
point", exactly because we used that name to look up the dentry in the
first place, so if the name is changing, we shouldn't traverse that
mount point.

But I may have just confused myself terminally here.

             Linus