linux-kernel - Re: [PATCH v3 1/3] exec: separate thread_count for files

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Date:   Mon, 17 Sep 2018 17:24:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Jeff Layton <jlayton@...nel.org>, viro@...iv.linux.org.uk,
        berrange@...hat.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/3] exec: separate thread_count for files_struct

On 09/16, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > As for binder.c, in this case we probably actually want to unshare ->files
> > on exec so we can ignore it?
>
> Looking at the binder case it only captures ->files on mmap.  Exec
> ditches the mmap.  So if the order of operations are correct than
> the dropping of the old mm will also drop the count on files_struct
> held by binder.
>
> So semantically binder should not effect locks on exec,

Agreed, but it does.

Before your "[PATCH 0/3] exec: Moving unshare_files_struct" unshare_files()
is called before exec_mmap().

And even with this series we can have another CLONE_VM process.

Howver, I think this doesn't really matter. binder does __fd_install(files),
so if it actually has a reference to execing_task->files, I think it should
be unshared anyway.

> In short as long as we get the oder of operations correct we should be
> able to safely ignore binder, and not have binder affect the results of
> this code.

Agreed.

Oleg.