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

David Howells
2005-03-23T21:07:48Z

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>
---
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);
+ 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();
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);
+ }

- 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);