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Subject[PATCH 1/3 v3] kernel/fork.c: avoid division by zero
Date
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@gmx.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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