linux-kernel - Re: [PATCH v2 2/5] exec: Factor unshare_sighand out of de

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Message-ID: <20200310212124.l6rajcjkitt65fxr@wittgenstein>
Date:   Tue, 10 Mar 2020 22:21:24 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Bernd Edlinger <bernd.edlinger@...mail.de>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Andrei Vagin <avagin@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Yuyang Du <duyuyang@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Anshuman Khandual <anshuman.khandual@....com>,
        David Howells <dhowells@...hat.com>,
        James Morris <jamorris@...ux.microsoft.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Christian Kellner <christian@...lner.me>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>
Subject: Re: [PATCH v2 2/5] exec: Factor unshare_sighand out of de_thread and
 call it separately

On Sun, Mar 08, 2020 at 04:36:17PM -0500, Eric W. Biederman wrote:
> 
> This makes the code clearer and makes it easier to implement a mutex
> that is not taken over any locations that may block indefinitely waiting
> for userspace.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  fs/exec.c | 39 ++++++++++++++++++++++++++-------------
>  1 file changed, 26 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index c3f34791f2f0..ff74b9a74d34 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1194,6 +1194,23 @@ static int de_thread(struct task_struct *tsk)
>  	flush_itimer_signals();
>  #endif
>  
> +	BUG_ON(!thread_group_leader(tsk));
> +	return 0;
> +
> +killed:
> +	/* protects against exit_notify() and __exit_signal() */
> +	read_lock(&tasklist_lock);
> +	sig->group_exit_task = NULL;
> +	sig->notify_count = 0;
> +	read_unlock(&tasklist_lock);
> +	return -EAGAIN;
> +}
> +
> +
> +static int unshare_sighand(struct task_struct *me)
> +{
> +	struct sighand_struct *oldsighand = me->sighand;
> +
>  	if (refcount_read(&oldsighand->count) != 1) {
>  		struct sighand_struct *newsighand;
>  		/*
> @@ -1210,23 +1227,13 @@ static int de_thread(struct task_struct *tsk)
>  
>  		write_lock_irq(&tasklist_lock);
>  		spin_lock(&oldsighand->siglock);
> -		rcu_assign_pointer(tsk->sighand, newsighand);
> +		rcu_assign_pointer(me->sighand, newsighand);
>  		spin_unlock(&oldsighand->siglock);
>  		write_unlock_irq(&tasklist_lock);
>  
>  		__cleanup_sighand(oldsighand);
>  	}

This is fine for now but we share an aweful lot of code with
copy_sighand(). We should earmark this to look into consolidating the
core operations into a common helper called from both copy_sighand() and
unshare_sighand() maybe even dumbing it down to one helper. But not
needed for now.

Otherwise:
Acked-by: Christian Brauner <christian.brauner@...ntu.com>