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From:   Nico Pache <npache@redhat.com>
To:     linux-kernel@vger.kernel.org, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Darren Hart <dvhart@infradead.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        =?UTF-8?q?Andr=C3=A9=20Almeida?= <andrealmeid@collabora.com>,
        Arjan van de Ven <arjan@infradead.org>,
        Ulrich Drepper <drepper@redhat.com>
Subject: [RFC 2/3] futex: exit: Print a warning when futex_cleanup fails
Date:   Thu, 21 Apr 2022 15:05:32 -0400
Message-Id: <20220421190533.1601879-3-npache@redhat.com>
In-Reply-To: <20220421190533.1601879-1-npache@redhat.com>
References: <20220421190533.1601879-1-npache@redhat.com>
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The futex_cleanup routines currently fails silently.

Allow the futex_cleanup routines to fail more verbosely so we can
leave a message behind for easier debugging/error detecting.

Fixes: 0771dfefc9e5 ("[PATCH] lightweight robust futexes: core")
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: "André Almeida" <andrealmeid@collabora.com>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Ulrich Drepper <drepper@redhat.com>
Signed-off-by: Nico Pache <npache@redhat.com>
---
 kernel/futex/core.c | 44 ++++++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index b22ef1efe751..8a62eb1b5d77 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -760,9 +760,9 @@ static inline int fetch_robust_entry(struct robust_list __user **entry,
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
  *
- * We silently return on any sign of list-walking problem.
+ * We return false on any sign of list-walking problem.
  */
-static void exit_robust_list(struct task_struct *curr)
+static bool exit_robust_list(struct task_struct *curr)
 {
 	struct robust_list_head __user *head = curr->robust_list;
 	struct robust_list __user *entry, *next_entry, *pending;
@@ -771,23 +771,25 @@ static void exit_robust_list(struct task_struct *curr)
 	unsigned long futex_offset;
 	int rc;
 
+	if (!futex_cmpxchg_enabled)
+		return false;
 	/*
 	 * Fetch the list head (which was registered earlier, via
 	 * sys_set_robust_list()):
 	 */
 	if (fetch_robust_entry(&entry, &head->list.next, &pi))
-		return;
+		return false;
 	/*
 	 * Fetch the relative futex offset:
 	 */
 	if (get_user(futex_offset, &head->futex_offset))
-		return;
+		return false;
 	/*
 	 * Fetch any possibly pending lock-add first, and handle it
 	 * if it exists:
 	 */
 	if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
-		return;
+		return false;
 
 	next_entry = NULL;	/* avoid warning with gcc */
 	while (entry != &head->list) {
@@ -803,10 +805,10 @@ static void exit_robust_list(struct task_struct *curr)
 		if (entry != pending) {
 			if (handle_futex_death((void __user *)entry + futex_offset,
 						curr, pi, HANDLE_DEATH_LIST))
-				return;
+				return false;
 		}
 		if (rc)
-			return;
+			return false;
 		entry = next_entry;
 		pi = next_pi;
 		/*
@@ -819,9 +821,10 @@ static void exit_robust_list(struct task_struct *curr)
 	}
 
 	if (pending) {
-		handle_futex_death((void __user *)pending + futex_offset,
+		return handle_futex_death((void __user *)pending + futex_offset,
 				   curr, pip, HANDLE_DEATH_PENDING);
 	}
+	return true;
 }
 
 #ifdef CONFIG_COMPAT
@@ -854,9 +857,9 @@ compat_fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **ent
  * Walk curr->robust_list (very carefully, it's a userspace list!)
  * and mark any locks found there dead, and notify any waiters.
  *
- * We silently return on any sign of list-walking problem.
+ * We return false on any sign of list-walking problem.
  */
-static void compat_exit_robust_list(struct task_struct *curr)
+static bool compat_exit_robust_list(struct task_struct *curr)
 {
 	struct compat_robust_list_head __user *head = curr->compat_robust_list;
 	struct robust_list __user *entry, *next_entry, *pending;
@@ -866,24 +869,26 @@ static void compat_exit_robust_list(struct task_struct *curr)
 	compat_long_t futex_offset;
 	int rc;
 
+	if (!futex_cmpxchg_enabled)
+		return false;
 	/*
 	 * Fetch the list head (which was registered earlier, via
 	 * sys_set_robust_list()):
 	 */
 	if (compat_fetch_robust_entry(&uentry, &entry, &head->list.next, &pi))
-		return;
+		return false;
 	/*
 	 * Fetch the relative futex offset:
 	 */
 	if (get_user(futex_offset, &head->futex_offset))
-		return;
+		return false;
 	/*
 	 * Fetch any possibly pending lock-add first, and handle it
 	 * if it exists:
 	 */
 	if (compat_fetch_robust_entry(&upending, &pending,
 			       &head->list_op_pending, &pip))
-		return;
+		return false;
 
 	next_entry = NULL;	/* avoid warning with gcc */
 	while (entry != (struct robust_list __user *) &head->list) {
@@ -902,10 +907,10 @@ static void compat_exit_robust_list(struct task_struct *curr)
 
 			if (handle_futex_death(uaddr, curr, pi,
 					       HANDLE_DEATH_LIST))
-				return;
+				return false;
 		}
 		if (rc)
-			return;
+			return false;
 		uentry = next_uentry;
 		entry = next_entry;
 		pi = next_pi;
@@ -920,8 +925,9 @@ static void compat_exit_robust_list(struct task_struct *curr)
 	if (pending) {
 		void __user *uaddr = futex_uaddr(pending, futex_offset);
 
-		handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
+		return handle_futex_death(uaddr, curr, pip, HANDLE_DEATH_PENDING);
 	}
+	return true;
 }
 #endif
 
@@ -1007,13 +1013,15 @@ static inline void exit_pi_state_list(struct task_struct *curr) { }
 static void futex_cleanup(struct task_struct *tsk)
 {
 	if (unlikely(tsk->robust_list)) {
-		exit_robust_list(tsk);
+		if (!exit_robust_list(tsk))
+			pr_info("futex: exit_robust_list failed");
 		tsk->robust_list = NULL;
 	}
 
 #ifdef CONFIG_COMPAT
 	if (unlikely(tsk->compat_robust_list)) {
-		compat_exit_robust_list(tsk);
+		if (!compat_exit_robust_list(tsk))
+			pr_info("futex: compat_exit_robust_list failed");
 		tsk->compat_robust_list = NULL;
 	}
 #endif
-- 
2.35.1