[PATCH 2/2] pid: Optional first-fit pid allocation

["Michal Koutný" <mkoutny@suse.com>]

Noone would need to use this allocation strategy (it's slower, pid
numbers collide sooner). Its primary purpose are pid namespaces in
conjunction with pids.max cgroup limit which keeps (virtual) pid numbers
below the given limit. This is for 32-bit userspace programs that may
not work well with pid numbers above 65536.

Link: https://lore.kernel.org/r/20241122132459.135120-1-aleksandr.mikhalitsyn@canonical.com/
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 Documentation/admin-guide/sysctl/kernel.rst |  2 ++
 include/linux/pid_namespace.h               |  3 +++
 kernel/pid.c                                | 12 +++++++--
 kernel/pid_namespace.c                      | 28 +++++++++++++++------
 4 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index a43b78b4b6464..f5e68d1c8849f 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -1043,6 +1043,8 @@ The last pid allocated in the current (the one task using this sysctl
 lives in) pid namespace. When selecting a pid for a next task on fork
 kernel tries to allocate a number starting from this one.
 
+When set to -1, first-fit pid numbering is used instead of the next-fit.
+
 
 powersave-nap (PPC only)
 ========================
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index f9f9931e02d6a..10bf66ca78590 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -41,6 +41,9 @@ struct pid_namespace {
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
 	int memfd_noexec_scope;
 #endif
+#ifdef CONFIG_IA32_EMULATION
+	bool pid_noncyclic;
+#endif
 } __randomize_layout;
 
 extern struct pid_namespace init_pid_ns;
diff --git a/kernel/pid.c b/kernel/pid.c
index aa2a7d4da4555..e9da1662b8821 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -191,6 +191,10 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 
 	for (i = ns->level; i >= 0; i--) {
 		int tid = 0;
+		bool pid_noncyclic = 0;
+#ifdef CONFIG_IA32_EMULATION
+		pid_noncyclic = READ_ONCE(tmp->pid_noncyclic);
+#endif
 
 		if (set_tid_size) {
 			tid = set_tid[ns->level - i];
@@ -235,8 +239,12 @@ struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
 			 * Store a null pointer so find_pid_ns does not find
 			 * a partially initialized PID (see below).
 			 */
-			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
-					      pid_max, GFP_ATOMIC);
+			if (likely(!pid_noncyclic))
+				nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+						      pid_max, GFP_ATOMIC);
+			else
+				nr = idr_alloc(&tmp->idr, NULL, pid_min,
+						      pid_max, GFP_ATOMIC);
 		}
 		spin_unlock_irq(&pidmap_lock);
 		idr_preload_end();
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 0f23285be4f92..ceda94a064294 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -113,6 +113,9 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	ns->pid_allocated = PIDNS_ADDING;
 #if defined(CONFIG_SYSCTL) && defined(CONFIG_MEMFD_CREATE)
 	ns->memfd_noexec_scope = pidns_memfd_noexec_scope(parent_pid_ns);
+#endif
+#ifdef CONFIG_IA32_EMULATION
+	ns->pid_noncyclic = READ_ONCE(parent_pid_ns->pid_noncyclic);
 #endif
 	return ns;
 
@@ -260,7 +263,7 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
 	return;
 }
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(CONFIG_IA32_EMULATION)
 static int pid_ns_ctl_handler(const struct ctl_table *table, int write,
 		void *buffer, size_t *lenp, loff_t *ppos)
 {
@@ -271,12 +274,23 @@ static int pid_ns_ctl_handler(const struct ctl_table *table, int write,
 	if (write && !checkpoint_restore_ns_capable(pid_ns->user_ns))
 		return -EPERM;
 
-	next = idr_get_cursor(&pid_ns->idr) - 1;
+	next = -1;
+#ifdef CONFIG_IA32_EMULATION
+	if (!pid_ns->pid_noncyclic)
+#endif
+		next += idr_get_cursor(&pid_ns->idr);
 
 	tmp.data = &next;
 	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
-	if (!ret && write)
-		idr_set_cursor(&pid_ns->idr, next + 1);
+	if (!ret && write) {
+		if (next > -1)
+			idr_set_cursor(&pid_ns->idr, next + 1);
+		else if (!IS_ENABLED(CONFIG_IA32_EMULATION))
+			ret = -EINVAL;
+#ifdef CONFIG_IA32_EMULATION
+		WRITE_ONCE(pid_ns->pid_noncyclic, next == -1);
+#endif
+	}
 
 	return ret;
 }
@@ -288,11 +302,11 @@ static const struct ctl_table pid_ns_ctl_table[] = {
 		.maxlen = sizeof(int),
 		.mode = 0666, /* permissions are checked in the handler */
 		.proc_handler = pid_ns_ctl_handler,
-		.extra1 = SYSCTL_ZERO,
+		.extra1 = SYSCTL_NEG_ONE,
 		.extra2 = &pid_max,
 	},
 };
-#endif	/* CONFIG_CHECKPOINT_RESTORE */
+#endif	/* CONFIG_CHECKPOINT_RESTORE || CONFIG_IA32_EMULATION */
 
 int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 {
@@ -449,7 +463,7 @@ static __init int pid_namespaces_init(void)
 {
 	pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC | SLAB_ACCOUNT);
 
-#ifdef CONFIG_CHECKPOINT_RESTORE
+#if defined(CONFIG_CHECKPOINT_RESTORE) || defined(CONFIG_IA32_EMULATION)
 	register_sysctl_init("kernel", pid_ns_ctl_table);
 #endif
 
-- 
2.48.1