[RFC PATCH 05/15] sched/task_stack: Add helpers for stack high/low

[Mostafa Saleh <smostafa@google.com>]

Soon, memory mapped for a kernel stack can be less than THREAD_SIZE.
However, many parts of the kernel assume it is.
Add new helpers for the kernel stack that can be called regardless of
CONFIG_STACK_GROWSUP or the kernel stack size:
- task_stack_low(): Lowest usable address on the stack.
- task_stack_high(): Highest usable address on the stack.
- task_stack_size(): The size of a task kernel stack, it is still
  THREAD_SIZE at the moment but that will change soon.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
---
 include/linux/sched/task_stack.h | 56 ++++++++++++++++++++++++++++++--
 1 file changed, 53 insertions(+), 3 deletions(-)

diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
index 1fab7e9043a3..4a808f9c4631 100644
--- a/include/linux/sched/task_stack.h
+++ b/include/linux/sched/task_stack.h
@@ -13,6 +13,8 @@
 
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 
+static __always_inline void *task_stack_low(const struct task_struct *task);
+
 /*
  * When accessing the stack of a non-current task that might exit, use
  * try_get_task_stack() instead.  task_stack_page will return a pointer
@@ -30,7 +32,7 @@ static __always_inline unsigned long *end_of_stack(const struct task_struct *tas
 #ifdef CONFIG_STACK_GROWSUP
 	return (unsigned long *)((unsigned long)task->stack + THREAD_SIZE) - 1;
 #else
-	return task->stack;
+	return task_stack_low(task);
 #endif
 }
 
@@ -64,6 +66,53 @@ static inline unsigned long *end_of_stack(const struct task_struct *p)
 
 #endif
 
+/*
+ * Kernel stack layout, higher addresses at the top. task_stack_page() is
+ * the start of the THREAD_SIZE area, task_stack_low() => task_stack_high()
+ * is the usable stack.
+ *
+ * Case 1) Stack grows down: Only the top task_stack_size() bytes are mapped:
+ *
+ *   +-------------+ <- task_stack_high() = task_stack_page() + THREAD_SIZE
+ *   |    stack    |
+ *   |      |      |    task_stack_size() bytes
+ *   |      v      |
+ *   +-------------+ <- task_stack_low() = end_of_stack() (*)
+ *   |   may be    |
+ *   |  unmapped   |    THREAD_SIZE - task_stack_size() bytes
+ *   +-------------+ <- task_stack_page()
+ *
+ *   (*) Depending on CONFIG_THREAD_INFO_IN_TASK, end_of_stack() might be just
+ *       above it.
+ *
+ * Case 2) Stack grows up (CONFIG_STACK_GROWSUP): The whole area is used.
+ * No support for partially mapped stacks.
+ *
+ *   +-------------+ <- task_stack_high() = task_stack_page() + THREAD_SIZE
+ *   |      ^      |    end_of_stack() is the last unsigned long below it
+ *   |      |      |
+ *   |    stack    |    THREAD_SIZE bytes
+ *   +-------------+ <- task_stack_low() = task_stack_page()
+ */
+static __always_inline unsigned long task_stack_size(const struct task_struct *task)
+{
+	return THREAD_SIZE;
+}
+
+static __always_inline void *task_stack_low(const struct task_struct *task)
+{
+#if defined(CONFIG_STACK_GROWSUP)
+	return task_stack_page(task);
+#else
+	return task_stack_page(task) + THREAD_SIZE - task_stack_size(task);
+#endif
+}
+
+static __always_inline void *task_stack_high(const struct task_struct *task)
+{
+	return task_stack_low(task) + task_stack_size(task);
+}
+
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 static inline void *try_get_task_stack(struct task_struct *tsk)
 {
@@ -88,10 +137,11 @@ void exit_task_stack_account(struct task_struct *tsk);
 
 static inline int object_is_on_stack(const void *obj)
 {
-	void *stack = task_stack_page(current);
+	void *stack_base = task_stack_low(current);
+	void *stack_end = task_stack_high(current);
 
 	obj = kasan_reset_tag(obj);
-	return (obj >= stack) && (obj < (stack + THREAD_SIZE));
+	return (obj >= stack_base) && (obj < stack_end);
 }
 
 extern void thread_stack_cache_init(void);
-- 
2.56.0.rc1.315.gc6ed9934b7-goog