[RFC PATCH 12/15] sched/task_stack: Introduce ARCH_HAS_VARIABLE_STACK_SIZE

[Mostafa Saleh <smostafa@google.com>]

When CONFIG_ARCH_HAS_VARIABLE_STACK_SIZE is enabled, the kernel stack
may not be fully populated with the THREAD_SIZE virtual area.

Teach task_stack_size() to return the size of the vm area and not a
fixed size.

This propagates to end_of_stack() which makes functions like
stack_not_used() and set_task_stack_end_magic() to use the lowest
valid stack address instead of THREAD_SIZE which would cause data
aborts.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
---
 arch/Kconfig                     | 10 ++++++++++
 include/linux/sched/task_stack.h |  5 +++++
 2 files changed, 15 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 45c657772362..d6fa2ccb0599 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -431,6 +431,16 @@ config HAVE_ASM_MODVERSIONS
 	  <asm/asm-prototypes.h> to support the module versioning for symbols
 	  exported from assembly code.
 
+config ARCH_HAS_VARIABLE_STACK_SIZE
+	bool
+	depends on VMAP_STACK
+	depends on !STACK_GROWSUP
+	depends on THREAD_INFO_IN_TASK
+	help
+	  An arch should select this if it supports dynamically configuring the
+	  kernel stack size.
+	  For example the kernel stack size can be from the command line.
+
 config HAVE_REGS_AND_STACK_ACCESS_API
 	bool
 	help
diff --git a/include/linux/sched/task_stack.h b/include/linux/sched/task_stack.h
index 4a808f9c4631..07d73adaecd1 100644
--- a/include/linux/sched/task_stack.h
+++ b/include/linux/sched/task_stack.h
@@ -10,6 +10,7 @@
 #include <linux/magic.h>
 #include <linux/refcount.h>
 #include <linux/kasan.h>
+#include <linux/vmalloc.h>
 
 #ifdef CONFIG_THREAD_INFO_IN_TASK
 
@@ -96,6 +97,10 @@ static inline unsigned long *end_of_stack(const struct task_struct *p)
  */
 static __always_inline unsigned long task_stack_size(const struct task_struct *task)
 {
+#ifdef CONFIG_ARCH_HAS_VARIABLE_STACK_SIZE
+	if (task->stack_vm_area)
+		return task->stack_vm_area->nr_pages * PAGE_SIZE;
+#endif
 	return THREAD_SIZE;
 }
 
-- 
2.56.0.rc1.315.gc6ed9934b7-goog