linux-kernel - [PATCH v3 4/7] seccomp: mark first listener in the tree

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Message-ID: <20251211124614.161900-5-aleksandr.mikhalitsyn@canonical.com>
Date: Thu, 11 Dec 2025 13:46:08 +0100
From: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
To: kees@...nel.org
Cc: linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>,
	Jonathan Corbet <corbet@....net>,
	Shuah Khan <shuah@...nel.org>,
	Aleksa Sarai <cyphar@...har.com>,
	Tycho Andersen <tycho@...ho.pizza>,
	Andrei Vagin <avagin@...il.com>,
	Christian Brauner <brauner@...nel.org>,
	Stéphane Graber <stgraber@...raber.org>,
	Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
Subject: [PATCH v3 4/7] seccomp: mark first listener in the tree

Let's note if listener was a first one installed in the seccomp
filters tree. We will need this information to retain old
quirk behavior (as before seccomp nesting introduced).

Also, rename has_duplicate_listener() to check_duplicate_listener(),
cause now this function is not read-only, but also modifies a state
of a new_child seccomp_filter.

No functional change intended at this point.

Cc: linux-doc@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Cc: Kees Cook <kees@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Will Drewry <wad@...omium.org>
Cc: Jonathan Corbet <corbet@....net>
Cc: Shuah Khan <shuah@...nel.org>
Cc: Aleksa Sarai <cyphar@...har.com>
Cc: Tycho Andersen <tycho@...ho.pizza>
Cc: Andrei Vagin <avagin@...il.com>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Stéphane Graber <stgraber@...raber.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@...onical.com>
---
 kernel/seccomp.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 89ae81f06743..1a139f9ef39b 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -205,6 +205,7 @@ static inline void seccomp_cache_prepare(struct seccomp_filter *sfilter)
  * @log: true if all actions except for SECCOMP_RET_ALLOW should be logged
  * @wait_killable_recv: Put notifying process in killable state once the
  *			notification is received by the userspace listener.
+ * @first_listener: true if this is the first seccomp listener installed in the tree.
  * @prev: points to a previously installed, or inherited, filter
  * @prog: the BPF program to evaluate
  * @notif: the struct that holds all notification related information
@@ -226,6 +227,7 @@ struct seccomp_filter {
 	refcount_t users;
 	bool log : 1;
 	bool wait_killable_recv : 1;
+	bool first_listener : 1;
 	struct action_cache cache;
 	struct seccomp_filter *prev;
 	struct bpf_prog *prog;
@@ -1939,7 +1941,7 @@ static struct file *init_listener(struct seccomp_filter *filter)
  * Note that @new_child is not hooked up to its parent at this point yet, so
  * we use current->seccomp.filter.
  */
-static bool has_duplicate_listener(struct seccomp_filter *new_child)
+static bool check_duplicate_listener(struct seccomp_filter *new_child)
 {
 	struct seccomp_filter *cur;
 
@@ -1953,6 +1955,8 @@ static bool has_duplicate_listener(struct seccomp_filter *new_child)
 			return true;
 	}
 
+	/* Mark first listener in the tree. */
+	new_child->first_listener = true;
 	return false;
 }
 
@@ -2035,7 +2039,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
 	if (!seccomp_may_assign_mode(seccomp_mode))
 		goto out;
 
-	if (has_duplicate_listener(prepared)) {
+	if (check_duplicate_listener(prepared)) {
 		ret = -EBUSY;
 		goto out;
 	}
-- 
2.43.0