linux-kernel - [PATCH] cred: add get_cred_many and put_cred

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Message-Id: <20230807163014.1425738-1-mjguzik@gmail.com>
Date:   Mon,  7 Aug 2023 18:30:14 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     dhowells@...hat.com, keescook@...omium.org
Cc:     linux-kernel@...r.kernel.org, Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH] cred: add get_cred_many and put_cred_many

Shaves back-to-back atomics in a few places.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---
 include/linux/cred.h | 27 +++++++++++++++++++++------
 kernel/cred.c        | 29 +++++++++++++++++------------
 2 files changed, 38 insertions(+), 18 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 9ed9232af934..b2b570ba204a 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -226,12 +226,17 @@ static inline bool cap_ambient_invariant_ok(const struct cred *cred)
  * Get a reference on the specified set of new credentials.  The caller must
  * release the reference.
  */
-static inline struct cred *get_new_cred(struct cred *cred)
+static inline struct cred *get_new_cred_many(struct cred *cred, int nr)
 {
-	atomic_inc(&cred->usage);
+	atomic_add(nr, &cred->usage);
 	return cred;
 }
 
+static inline struct cred *get_new_cred(struct cred *cred)
+{
+	return get_new_cred_many(cred, 1);
+}
+
 /**
  * get_cred - Get a reference on a set of credentials
  * @cred: The credentials to reference
@@ -245,14 +250,19 @@ static inline struct cred *get_new_cred(struct cred *cred)
  * accidental alteration of a set of credentials that should be considered
  * immutable.
  */
-static inline const struct cred *get_cred(const struct cred *cred)
+static inline const struct cred *get_cred_many(const struct cred *cred, int nr)
 {
 	struct cred *nonconst_cred = (struct cred *) cred;
 	if (!cred)
 		return cred;
 	validate_creds(cred);
 	nonconst_cred->non_rcu = 0;
-	return get_new_cred(nonconst_cred);
+	return get_new_cred_many(nonconst_cred, nr);
+}
+
+static inline const struct cred *get_cred(const struct cred *cred)
+{
+	return get_cred_many(cred, 1);
 }
 
 static inline const struct cred *get_cred_rcu(const struct cred *cred)
@@ -278,17 +288,22 @@ static inline const struct cred *get_cred_rcu(const struct cred *cred)
  * on task_struct are attached by const pointers to prevent accidental
  * alteration of otherwise immutable credential sets.
  */
-static inline void put_cred(const struct cred *_cred)
+static inline void put_cred_many(const struct cred *_cred, int nr)
 {
 	struct cred *cred = (struct cred *) _cred;
 
 	if (cred) {
 		validate_creds(cred);
-		if (atomic_dec_and_test(&(cred)->usage))
+		if (atomic_sub_and_test(nr, &cred->usage))
 			__put_cred(cred);
 	}
 }
 
+static inline void put_cred(const struct cred *cred)
+{
+	put_cred_many(cred, 1);
+}
+
 /**
  * current_cred - Access the current task's subjective credentials
  *
diff --git a/kernel/cred.c b/kernel/cred.c
index 811ad654abd1..8a506bc7c1b8 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -159,23 +159,30 @@ EXPORT_SYMBOL(__put_cred);
  */
 void exit_creds(struct task_struct *tsk)
 {
-	struct cred *cred;
+	struct cred *real_cred, *cred;
 
 	kdebug("exit_creds(%u,%p,%p,{%d,%d})", tsk->pid, tsk->real_cred, tsk->cred,
 	       atomic_read(&tsk->cred->usage),
 	       read_cred_subscribers(tsk->cred));
 
-	cred = (struct cred *) tsk->real_cred;
+	real_cred = (struct cred *) tsk->real_cred;
 	tsk->real_cred = NULL;
-	validate_creds(cred);
-	alter_cred_subscribers(cred, -1);
-	put_cred(cred);
 
 	cred = (struct cred *) tsk->cred;
 	tsk->cred = NULL;
-	validate_creds(cred);
-	alter_cred_subscribers(cred, -1);
-	put_cred(cred);
+
+	if (real_cred == cred) {
+		validate_creds(cred);
+		alter_cred_subscribers(cred, -2);
+		put_cred_many(cred, 2);
+	} else {
+		validate_creds(real_cred);
+		validate_creds(cred);
+		alter_cred_subscribers(real_cred, -1);
+		put_cred(real_cred);
+		alter_cred_subscribers(cred, -1);
+		put_cred(cred);
+	}
 
 #ifdef CONFIG_KEYS_REQUEST_CACHE
 	key_put(tsk->cached_requested_key);
@@ -352,8 +359,7 @@ int copy_creds(struct task_struct *p, unsigned long clone_flags)
 #endif
 		clone_flags & CLONE_THREAD
 	    ) {
-		p->real_cred = get_cred(p->cred);
-		get_cred(p->cred);
+		p->real_cred = get_cred_many(p->cred, 2);
 		alter_cred_subscribers(p->cred, 2);
 		kdebug("share_creds(%p{%d,%d})",
 		       p->cred, atomic_read(&p->cred->usage),
@@ -517,8 +523,7 @@ int commit_creds(struct cred *new)
 		proc_id_connector(task, PROC_EVENT_GID);
 
 	/* release the old obj and subj refs both */
-	put_cred(old);
-	put_cred(old);
+	put_cred_many(old, 2);
 	return 0;
 }
 EXPORT_SYMBOL(commit_creds);
-- 
2.39.2