Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 14 Aug 2023 17:58:17 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mateusz Guzik <mjguzik@gmail.com>, linux-kernel@vger.kernel.org,
        torvalds@linux-foundation.org, brauner@kernel.org,
        ebiederm@xmission.com, akpm@linux-foundation.org,
        linux-mm@kvack.org, koct9i@gmail.com, dave@stgolabs.net
Subject: Re: [PATCH] kernel/fork: stop playing lockless games for exe_file
 replacement
Message-ID: <20230814155817.GC17738@redhat.com>
References: <20230813123333.1705833-1-mjguzik@gmail.com>
 <20230814150530.GB17738@redhat.com>
 <20230814152038.GA2367@redhat.com>
 <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <52192c2f-c7b1-9c07-7ca2-10fc6bd347b0@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Precedence: bulk

On 08/14, David Hildenbrand wrote:
>
> >OK, I seem to understand... without mmap_read_lock() it is possible that
> >
> >	- dup_mm_exe_file() sees mm->exe_file = old_exe_file
> >
> >	- replace_mm_exe_file() does allow_write_access(old_exe_file)
> >
> >	- another process does get_write_access(old_exe_file)
> >
> >	- dup_mm_exe_file()->deny_write_access() fails
> >
> >Right?
>
> From what I recall, yes.

Thanks! but then... David, this all is subjective, feel free to ignore, but
the current code doesn't look good to me, I mean the purpose of mmap_read_lock()
is very unclear. To me something like

	if (old_exe_file) {
		/*
		 * Ensure that if we race with dup_mm_exe_file() and it sees
		 * mm->exe_file == old_exe_file deny_write_access(old_exe_file)
		 * can't fail after we do allow_write_access() and another task
		 * does get_write_access(old_exe_file).
		 */
		mmap_read_lock(mm);
		mmap_read_unlock(mm);

		allow_write_access(old_exe_file);
		fput(old_exe_file);
	}

looks more understandable...


But this patch from Mateusz looks even better to me. So, FWIW,

Acked-by: Oleg Nesterov <oleg@redhat.com>