linux-kernel - [PATCH 1/3 v3] kernel/fork.c: avoid division by zero

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Date:	Sat, 21 Feb 2015 23:19:04 +0100
From:	Heinrich Schuchardt <xypron.glpk@....de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joe Perches <joe@...ches.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Kees Cook <keescook@...omium.org>,
	David Rientjes <rientjes@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andy Lutomirski <luto@...capital.net>,
	Michael Marineau <mike@...ineau.org>,
	Aaron Tomlin <atomlin@...hat.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jens Axboe <axboe@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Guenter Roeck <linux@...ck-us.net>,
	linux-kernel@...r.kernel.org,
	Heinrich Schuchardt <xypron.glpk@....de>
Subject: [PATCH 1/3 v3] kernel/fork.c: avoid division by zero

PAGE_SIZE is not guaranteed to be equal to or less than 8 times the
THREAD_SIZE.

E.g. architecture hexagon may have page size 1M and thread size 4096.
This would lead to a division by zero in the calculation of max_threads.

With 32-bit calculus there is no solution which delivers valid results
for all possible combinations of the parameters.
The code is only called once.
Hence a 64-bit calculation can be used as solution.

The calculation of max_threads is moved to a separate function.
This allows future patches to use the same logic, e.g. when
 - max_threads is set by writing to /proc/sys/kernel/threads-max
 - when adding or removing memory.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@....de>
---
 kernel/fork.c | 59 ++++++++++++++++++++++++++++++++++++++++++-----------------
 1 file changed, 42 insertions(+), 17 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 4dc2dda..69c30cd 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -88,6 +88,16 @@
 #include <trace/events/task.h>
 
 /*
+ * Minimum number of threads to boot the kernel
+ */
+#define MIN_THREADS 20
+
+/*
+ * Maximum number of threads
+ */
+#define MAX_THREADS FUTEX_TID_MASK
+
+/*
  * Protected counters by write_lock_irq(&tasklist_lock)
  */
 unsigned long total_forks;	/* Handle normal Linux uptimes. */
@@ -253,6 +263,37 @@ EXPORT_SYMBOL_GPL(__put_task_struct);
 
 void __init __weak arch_task_cache_init(void) { }
 
+/*
+ * set_max_threads tries to set default limit to the suggested value.
+ */
+static void set_max_threads(unsigned int max_threads_suggested)
+{
+	u64 threads;
+
+	/*
+	 * The number of threads shall be limited such that the thread
+	 * structures may only consume a small part of the available memory.
+	 */
+	threads = div64_u64((u64) totalram_pages * (u64) PAGE_SIZE,
+			    (u64) THREAD_SIZE * 8UL);
+
+	if (threads > max_threads_suggested)
+		threads = max_threads_suggested;
+
+	if (threads > MAX_THREADS)
+		threads = MAX_THREADS;
+
+	if (threads < MIN_THREADS)
+		threads = MIN_THREADS;
+
+	max_threads = (int) threads;
+
+	init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = (int) threads / 2;
+	init_task.signal->rlim[RLIMIT_NPROC].rlim_max = (int) threads / 2;
+	init_task.signal->rlim[RLIMIT_SIGPENDING] =
+		init_task.signal->rlim[RLIMIT_NPROC];
+}
+
 void __init fork_init(unsigned long mempages)
 {
 #ifndef CONFIG_ARCH_TASK_STRUCT_ALLOCATOR
@@ -268,23 +309,7 @@ void __init fork_init(unsigned long mempages)
 	/* do the arch specific task caches init */
 	arch_task_cache_init();
 
-	/*
-	 * The default maximum number of threads is set to a safe
-	 * value: the thread structures can take up at most half
-	 * of memory.
-	 */
-	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
-
-	/*
-	 * we need to allow at least 20 threads to boot a system
-	 */
-	if (max_threads < 20)
-		max_threads = 20;
-
-	init_task.signal->rlim[RLIMIT_NPROC].rlim_cur = max_threads/2;
-	init_task.signal->rlim[RLIMIT_NPROC].rlim_max = max_threads/2;
-	init_task.signal->rlim[RLIMIT_SIGPENDING] =
-		init_task.signal->rlim[RLIMIT_NPROC];
+	set_max_threads(UINT_MAX);
 }
 
 int __weak arch_dup_task_struct(struct task_struct *dst,
-- 
2.1.4

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