Re: [PATCH 2/3] Keys: Use RCU to manage session keyring pointer

Paul E. McKenney
2005-04-11T22:45:36Z

On Wed, Mar 23, 2005 at 08:55:46PM +0000, David Howells wrote:
>
> The attached patch uses RCU to manage the session keyring pointer in struct
> signal_struct. This means that searching need not disable interrupts and get a
> the sighand spinlock to access this pointer. Furthermore, by judicious use of
> rcu_read_(un)lock(), this patch also avoids the need to take and put refcounts
> on the session keyring itself, thus saving on even more atomic ops.
>
> Signed-Off-By: David Howells <dhowells@redhat.com>

This looks quite good! A few questions interspersed below.

Thanx, Paul

PS. Sorry to be so slow in getting to this one!

> ---
> warthog>diffstat -p1 keys-rcu-session-2612rc1mm1.diff
> security/keys/process_keys.c | 42 +++++++++++++++++++++---------------------
> security/keys/request_key.c | 7 +++----
> 2 files changed, 24 insertions(+), 25 deletions(-)
>
> diff -uNrp linux-2.6.12-rc1-mm1-keys-umhelper/security/keys/process_keys.c linux-2.6.12-rc1-mm1-keys-rcu-session/security/keys/process_keys.c
> --- linux-2.6.12-rc1-mm1-keys-umhelper/security/keys/process_keys.c 2005-03-23 17:22:46.000000000 +0000
> +++ linux-2.6.12-rc1-mm1-keys-rcu-session/security/keys/process_keys.c 2005-03-23 18:27:12.055768099 +0000
> @@ -1,6 +1,6 @@
> /* process_keys.c: management of a process's keyrings
> *
> - * Copyright (C) 2004 Red Hat, Inc. All Rights Reserved.
> + * Copyright (C) 2004-5 Red Hat, Inc. All Rights Reserved.
> * Written by David Howells (dhowells@redhat.com)
> *
> * This program is free software; you can redistribute it and/or
> @@ -181,7 +181,7 @@ static int install_process_keyring(struc
> goto error;
> }
>
> - /* attach or swap keyrings */
> + /* attach keyring */
> spin_lock_irqsave(&tsk->sighand->siglock, flags);
> if (!tsk->signal->process_keyring) {
> tsk->signal->process_keyring = keyring;
> @@ -227,12 +227,14 @@ static int install_session_keyring(struc
>
> /* install the keyring */
> spin_lock_irqsave(&tsk->sighand->siglock, flags);
> - old = tsk->signal->session_keyring;
> - tsk->signal->session_keyring = keyring;
> + old = rcu_dereference(tsk->signal->session_keyring);

I don't understand why rcu_dereference() is needed in this case.
Since we are holding the lock, it should not be possible for
this to change, right? Or am I missing something? (Quite possible,
am not all that familiar with this code.)

> + rcu_assign_pointer(tsk->signal->session_keyring, keyring);
> spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
>
> ret = 0;
>
> + /* we're using RCU on the pointer */
> + synchronize_kernel();

This would want to become synchronize_rcu().

> key_put(old);
> error:
> return ret;
> @@ -245,8 +247,6 @@ static int install_session_keyring(struc
> */
> int copy_thread_group_keys(struct task_struct *tsk)
> {
> - unsigned long flags;
> -
> key_check(current->thread_group->session_keyring);
> key_check(current->thread_group->process_keyring);
>
> @@ -254,10 +254,10 @@ int copy_thread_group_keys(struct task_s
> tsk->signal->process_keyring = NULL;
>
> /* same session keyring */
> - spin_lock_irqsave(&current->sighand->siglock, flags);
> + rcu_read_lock();
> tsk->signal->session_keyring =
> - key_get(current->signal->session_keyring);
> - spin_unlock_irqrestore(&current->sighand->siglock, flags);
> + key_get(rcu_dereference(current->signal->session_keyring));
> + rcu_read_unlock();
>
> return 0;
>
> @@ -381,8 +381,7 @@ struct key *search_process_keyrings_aux(
> key_match_func_t match)
> {
> struct task_struct *tsk = current;
> - unsigned long flags;
> - struct key *key, *ret, *err, *tmp;
> + struct key *key, *ret, *err;
>
> /* we want to return -EAGAIN or -ENOKEY if any of the keyrings were
> * searchable, but we failed to find a key or we found a negative key;
> @@ -436,17 +435,18 @@ struct key *search_process_keyrings_aux(
> }
>
> /* search the session keyring last */
> - spin_lock_irqsave(&tsk->sighand->siglock, flags);
> -
> - tmp = tsk->signal->session_keyring;
> - if (!tmp)
> - tmp = tsk->user->session_keyring;
> - atomic_inc(&tmp->usage);
> -
> - spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
> + if (tsk->signal->session_keyring) {
> + rcu_read_lock();
> + key = keyring_search_aux(
> + rcu_dereference(tsk->signal->session_keyring),
> + type, description, match);
> + rcu_read_unlock();
> + }
> + else {
> + key = keyring_search_aux(tsk->user->session_keyring,
> + type, description, match);

This one is constant, right? If not, I don't understand the locking design.

> + }
>
> - key = keyring_search_aux(tmp, type, description, match);
> - key_put(tmp);
> if (!IS_ERR(key))
> goto found;
>
> diff -uNrp linux-2.6.12-rc1-mm1-keys-umhelper/security/keys/request_key.c linux-2.6.12-rc1-mm1-keys-rcu-session/security/keys/request_key.c
> --- linux-2.6.12-rc1-mm1-keys-umhelper/security/keys/request_key.c 2005-03-23 17:35:16.000000000 +0000
> +++ linux-2.6.12-rc1-mm1-keys-rcu-session/security/keys/request_key.c 2005-03-23 18:14:13.908029567 +0000
> @@ -175,13 +175,12 @@ static struct key *__request_key_constru
> key->expiry = now.tv_sec + key_negative_timeout;
>
> if (current->signal->session_keyring) {
> - unsigned long flags;
> struct key *keyring;
>
> - spin_lock_irqsave(&current->sighand->siglock, flags);
> - keyring = current->signal->session_keyring;
> + rcu_read_lock();
> + keyring = rcu_dereference(current->signal->session_keyring);
> atomic_inc(&keyring->usage);
> - spin_unlock_irqrestore(&current->sighand->siglock, flags);
> + rcu_read_unlock();
>
> key_link(keyring, key);
> key_put(keyring);
> -
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