linux-kernel - Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution

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Date:   Wed, 4 Sep 2019 15:38:20 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>, Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
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        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
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Subject: Re: [PATCH v12 10/12] namei: aggressively check for nd->root escape
 on ".." resolution

On Wed, Sep 4, 2019 at 3:31 PM David Howells <dhowells@...hat.com> wrote:
>
> It ought to be reasonably easy to make them per-sb at least, I think.  We
> don't allow cross-super rename, right?

Right now the sequence count handling very much depends on it being a
global entity on the reader side, at least.

And while the rename sequence count could (and probably should) be
per-sb, the same is very much not true of the mount one.

So the rename seqcount is likely easier to fix than the mount one, but
neither of them are entirely trivial, afaik.

               Linus