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>

2 files changed, 31 insertions, 6 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index c7f2e1a4187a..8807a129711b 100644
--- a/kernel/fork.c
+++ b/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;
+}
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 42b7fc2860c1..3c0998426b57 100644
--- a/kernel/sysctl.c
+++ b/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",