kernel/sysctl.c: threads-max observe limits

Users can change the maximum number of threads by writing to
/proc/sys/kernel/threads-max.

With the patch the value entered is checked against the same limits that
apply when fork_init is called.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sysctl.h

@@ -212,4 +212,7 @@ static inline void setup_sysctl_set(struct ctl_table_set *p,
 
 #endif /* CONFIG_SYSCTL */
 
+int sysctl_max_threads(struct ctl_table *table, int write,
+		       void __user *buffer, size_t *lenp, loff_t *ppos);
+
 #endif /* _LINUX_SYSCTL_H */

kernel/fork.c

@@ -74,6 +74,7 @@
 #include <linux/uprobes.h>
 #include <linux/aio.h>
 #include <linux/compiler.h>
+#include <linux/sysctl.h>
 
 #include <asm/pgtable.h>
 #include <asm/pgalloc.h>
@@ -266,7 +267,7 @@ void __init __weak arch_task_cache_init(void) { }
 /*
  * set_max_threads
  */
-static void set_max_threads(void)
+static void set_max_threads(unsigned int max_threads_suggested)
 {
 	u64 threads;
 
@@ -280,6 +281,9 @@ static void set_max_threads(void)
 		threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
 				    (u64) THREAD_SIZE * 8UL);
 
+	if (threads > max_threads_suggested)
+		threads = max_threads_suggested;
+
 	max_threads = clamp_t(u64, threads, MIN_THREADS, MAX_THREADS);
 }
 
@@ -298,7 +302,7 @@ void __init fork_init(void)
 	/* do the arch specific task caches init */
 	arch_task_cache_init();
 
-	set_max_threads();
+	set_max_threads(MAX_THREADS);
 
 	init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
 	init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
@@ -2020,3 +2024,26 @@ int unshare_files(struct files_struct **displaced)
 	task_unlock(task);
 	return 0;
 }
+
+int sysctl_max_threads(struct ctl_table *table, int write,
+		       void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct ctl_table t;
+	int ret;
+	int threads = max_threads;
+	int min = MIN_THREADS;
+	int max = MAX_THREADS;
+
+	t = *table;
+	t.data = &threads;
+	t.extra1 = &min;
+	t.extra2 = &max;
+
+	ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
+	if (ret || !write)
+		return ret;
+
+	set_max_threads(threads);
+
+	return 0;
+}

kernel/sysctl.c

@@ -93,11 +93,9 @@
 #include <linux/nmi.h>
 #endif
 
-
 #if defined(CONFIG_SYSCTL)
 
 /* External variables not in a header file. */
-extern int max_threads;
 extern int suid_dumpable;
 #ifdef CONFIG_COREDUMP
 extern int core_uses_pid;
@@ -710,10 +708,10 @@ static struct ctl_table kern_table[] = {
 #endif
 	{
 		.procname	= "threads-max",
-		.data		= &max_threads,
+		.data		= NULL,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
+		.proc_handler	= sysctl_max_threads,
 	},
 	{
 		.procname	= "random",