On Thu, 10 Jul 2025 11:30:39 -0400
Steven Rostedt [off-list ref] wrote:
I'm not too comfortable with the compat patches at this stage. I'm
thinking of separating out the compat patches, and just reject the
deferred unwind if the task is in compat mode (forcing perf or other
tracers to use whatever it uses today).
I'll take Mathieu's patches and merge them with Josh's, but make them a
separate series.
So I'm removing the two compat patches from the series for now and plan
to replace it with this:
-- Steve
From: Steven Rostedt <rostedt@goodmis.org>
Subject: [PATCH] unwind deferred/x86: Do not defer stack tracing for compat
tasks
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>
---
arch/x86/include/asm/unwind_user.h | 11 +++++++++++
include/linux/unwind_deferred.h | 5 +++++
kernel/unwind/deferred.c | 3 +++
3 files changed, 19 insertions(+)
diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index 8597857bf896..220fd0a6e175 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -2,6 +2,17 @@
#ifndef _ASM_X86_UNWIND_USER_H
#define _ASM_X86_UNWIND_USER_H
+#ifdef CONFIG_IA32_EMULATION
+/* 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 a9d5b100d6b2..6ba4fff066dd 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
#define UNWIND_PENDING_BIT (BITS_PER_LONG - 1)diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 039e12700d49..745144e4717c 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -236,6 +236,9 @@ 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;
--
2.47.2