[RFC PATCH v2 09/10] powerpc: align stack to 2 * THREAD_SIZE with VMAP_STACK

[Christophe Leroy]

In order to ease stack overflow detection, align
stack to 2 * THREAD_SIZE when using VMAP_STACK.
This allow overflow detection using a single bit check.

Signed-off-by: Christophe Leroy <christophe.leroy at c-s.fr>
---
 arch/powerpc/include/asm/thread_info.h | 13 +++++++++++++
 arch/powerpc/kernel/setup_32.c         |  2 +-
 arch/powerpc/kernel/setup_64.c         |  2 +-
 arch/powerpc/kernel/vmlinux.lds.S      |  2 +-
 4 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/include/asm/thread_info.h b/arch/powerpc/include/asm/thread_info.h
index 488d5c4670ff..a2270749b282 100644
--- a/arch/powerpc/include/asm/thread_info.h
+++ b/arch/powerpc/include/asm/thread_info.h
@@ -22,6 +22,19 @@
 
 #define THREAD_SIZE		(1 << THREAD_SHIFT)
 
+/*
+ * By aligning VMAP'd stacks to 2 * THREAD_SIZE, we can detect overflow by
+ * checking sp & (1 << THREAD_SHIFT), which we can do cheaply in the entry
+ * assembly.
+ */
+#ifdef CONFIG_VMAP_STACK
+#define THREAD_ALIGN_SHIFT	(THREAD_SHIFT + 1)
+#else
+#define THREAD_ALIGN_SHIFT	THREAD_SHIFT
+#endif
+
+#define THREAD_ALIGN		(1 << THREAD_ALIGN_SHIFT)
+
 #ifndef __ASSEMBLY__
 #include <linux/cache.h>
 #include <asm/processor.h>
diff --git a/arch/powerpc/kernel/setup_32.c b/arch/powerpc/kernel/setup_32.c
index 94517e4a2723..ab0a9d6e3745 100644
--- a/arch/powerpc/kernel/setup_32.c
+++ b/arch/powerpc/kernel/setup_32.c
@@ -137,7 +137,7 @@ arch_initcall(ppc_init);
 
 static void *__init alloc_stack(void)
 {
-	void *ptr = memblock_alloc(THREAD_SIZE, THREAD_SIZE);
+	void *ptr = memblock_alloc(THREAD_SIZE, THREAD_ALIGN);
 
 	if (!ptr)
 		panic("cannot allocate %d bytes for stack at %pS\n",
diff --git a/arch/powerpc/kernel/setup_64.c b/arch/powerpc/kernel/setup_64.c
index 44b4c432a273..f630fe4d36a8 100644
--- a/arch/powerpc/kernel/setup_64.c
+++ b/arch/powerpc/kernel/setup_64.c
@@ -644,7 +644,7 @@ static void *__init alloc_stack(unsigned long limit, int cpu)
 
 	BUILD_BUG_ON(STACK_INT_FRAME_SIZE % 16);
 
-	ptr = memblock_alloc_try_nid(THREAD_SIZE, THREAD_SIZE,
+	ptr = memblock_alloc_try_nid(THREAD_SIZE, THREAD_ALIGN,
 				     MEMBLOCK_LOW_LIMIT, limit,
 				     early_cpu_to_node(cpu));
 	if (!ptr)
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 060a1acd7c6d..d38335129c06 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -346,7 +346,7 @@ SECTIONS
 #endif
 
 	/* The initial task and kernel stack */
-	INIT_TASK_DATA_SECTION(THREAD_SIZE)
+	INIT_TASK_DATA_SECTION(THREAD_ALIGN)
 
 	.data..page_aligned : AT(ADDR(.data..page_aligned) - LOAD_OFFSET) {
 		PAGE_ALIGNED_DATA(PAGE_SIZE)
-- 
2.13.3