On Thu, 10 Jul 2025 11:30:39 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> 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@xxxxxxxxxxx>
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@xxxxxxxxxxx>
---
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
