From: Steven Rostedt <rostedt@goodmis.org>

Currently compat tasks are not supported. If a deferred user space stack
trace is requested on a compat task, it should fail and return an error so
that the profiler can use an alternative approach (whatever it uses
today).

Add a arch_unwind_can_defer() macro that is called in
unwind_deferred_request(). Have x86 define it to a function that makes
sure that the current task is running in 64bit mode, and if it is not, it
returns false. This will cause unwind_deferred_request() to error out and
the caller can use the current method of user space stack tracing.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v1: https://lore.kernel.org/20250820190639.843442474@kernel.org

- Added #include <linux/compat.h> to use user_64bit_mode() in
  unwind_user.h. When adding this to the ftrace code, it failed to build
  due to the missing header.

 arch/x86/include/asm/unwind_user.h | 12 ++++++++++++
 include/linux/unwind_deferred.h    |  5 +++++
 kernel/unwind/deferred.c           |  3 +++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index 8597857bf896..0cef2e3b08c8 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -2,6 +2,18 @@
 #ifndef _ASM_X86_UNWIND_USER_H
 #define _ASM_X86_UNWIND_USER_H
 
+#ifdef CONFIG_IA32_EMULATION
+#include <linux/compat.h>
+/* Currently compat mode is not supported for deferred stack trace */
+static inline bool arch_unwind_can_defer(void)
+{
+	struct pt_regs *regs = task_pt_regs(current);
+
+	return user_64bit_mode(regs);
+}
+# define arch_unwind_can_defer	arch_unwind_can_defer
+#endif /* CONFIG_IA32_EMULATION */
+
 #define ARCH_INIT_USER_FP_FRAME							\
 	.cfa_off	= (s32)sizeof(long) *  2,				\
 	.ra_off		= (s32)sizeof(long) * -1,				\
diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h
index 26122d00708a..0124865aaab4 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -16,6 +16,11 @@ struct unwind_work {
 	int				bit;
 };
 
+/* Architectures can add a test to not defer unwinding */
+#ifndef arch_unwind_can_defer
+# define arch_unwind_can_defer()	(true)
+#endif
+
 #ifdef CONFIG_UNWIND_USER
 
 enum {
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index dc6040aae3ee..3601b2efe48d 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -237,6 +237,9 @@ int unwind_deferred_request(struct unwind_work *work, u64 *cookie)
 
 	*cookie = 0;
 
+	if (!arch_unwind_can_defer())
+		return -EINVAL;
+
 	if ((current->flags & (PF_KTHREAD | PF_EXITING)) ||
 	    !user_mode(task_pt_regs(current)))
 		return -EINVAL;
-- 
2.50.1