linux-kernel - Re: [PATCH 3/4] mmap locking API: Don't check locking if the mm isn't live yet

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Message-ID: <CAG48ez2z1R8MLS0_pjhBSGnJC8RwaHMpiGdv8GhJUCrwtsLOPg@mail.gmail.com>
Date:   Wed, 30 Sep 2020 14:50:28 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Michel Lespinasse <walken@...gle.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>
Subject: Re: [PATCH 3/4] mmap locking API: Don't check locking if the mm isn't
 live yet

On Wed, Sep 30, 2020 at 2:30 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> On Tue, Sep 29, 2020 at 06:20:00PM -0700, Jann Horn wrote:
> > In preparation for adding a mmap_assert_locked() check in
> > __get_user_pages(), teach the mmap_assert_*locked() helpers that it's fine
> > to operate on an mm without locking in the middle of execve() as long as
> > it hasn't been installed on a process yet.
>
> I'm happy to see lockdep being added here, but can you elaborate on
> why add this mmap_locked_required instead of obtaining the lock in the
> execv path?

My thinking was: At that point, we're logically still in the
single-owner initialization phase of the mm_struct. Almost any object
has initialization and teardown steps that occur in a context where
the object only has a single owner, and therefore no locking is
required. It seems to me that adding locking in places like
get_arg_page() would be confusing because it would suggest the
existence of concurrency where there is no actual concurrency, and it
might be annoying in terms of lockdep if someone tries to use
something like get_arg_page() while holding the mmap_sem of the
calling process. It would also mean that we'd be doing extra locking
in normal kernel builds that isn't actually logically required.

Hmm, on the other hand, dup_mmap() already locks the child mm (with
mmap_write_lock_nested()), so I guess it wouldn't be too bad to also
do it in get_arg_page() and tomoyo_dump_page(), with comments that
note that we're doing this for lockdep consistency... I guess I can go
change this in v2.