[RFC][PATCH 04/24] kernel/fork: Allow arch-specific stack initialization

[Dave Hansen]

Right now, the entire thread_union has uniform permissions. Shadow
stacks obviously do not have the same page permissions as the rest
of the stacks. Add a new arch-specific hook so shadow stack page
permissions can be established.

Note: this is slow. It requires TLB invalidation in the arch code,
which is abhorrently stupid. It will be fixed up in due time.

---

 b/include/linux/thread_info.h |    5 +++++
 b/kernel/fork.c               |   14 +++++++++++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff -puN include/linux/thread_info.h~generic-arch_init_thread_stacks include/linux/thread_info.h
--- a/include/linux/thread_info.h~generic-arch_init_thread_stacks	2026-10-21 12:36:34.114873761 -0700
+++ b/include/linux/thread_info.h	2026-10-21 12:36:34.177873882 -0700
@@ -228,6 +228,11 @@ static inline int arch_within_stack_fram
 static inline void arch_setup_new_exec(void) { }
 #endif
 
+struct thread_stacks;
+#ifndef arch_init_thread_stacks
+static inline void arch_init_thread_stacks(struct thread_stacks *ts) { }
+#endif
+
 void arch_task_cache_init(void); /* for CONFIG_SH */
 void arch_release_task_struct(struct task_struct *tsk);
 int arch_dup_task_struct(struct task_struct *dst,
diff -puN kernel/fork.c~generic-arch_init_thread_stacks kernel/fork.c
--- a/kernel/fork.c~generic-arch_init_thread_stacks	2026-10-21 12:36:34.175873878 -0700
+++ b/kernel/fork.c	2026-10-21 12:36:34.178873884 -0700
@@ -332,9 +332,17 @@ err:
 
 static void *__vmalloc_thread_stack_node(int node)
 {
-	return __vmalloc_node(sizeof(union thread_union), THREAD_ALIGN,
-			      GFP_VMAP_STACK,
-			      node, __builtin_return_address(0));
+	union thread_union *tu;
+
+	tu = __vmalloc_node(sizeof(union thread_union), THREAD_ALIGN,
+			    GFP_VMAP_STACK,
+			    node, __builtin_return_address(0));
+	if (!tu)
+		return NULL;
+
+	arch_init_thread_stacks(&tu->stacks);
+
+	return tu;
 }
 
 static int alloc_thread_stack_node(struct task_struct *tsk, int node)
_