linux-kernel - [PATCH v14 12/12] unwind deferred/x86: Do not defer stack tracing for compat tasks

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Message-ID: <20250717004958.432327787@kernel.org>
Date: Wed, 16 Jul 2025 20:49:22 -0400
From: Steven Rostedt <rostedt@...nel.org>
To: linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org,
 x86@...nel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>,
 Andrii Nakryiko <andrii@...nel.org>,
 Indu Bhagat <indu.bhagat@...cle.com>,
 "Jose E. Marchesi" <jemarch@....org>,
 Beau Belgrave <beaub@...ux.microsoft.com>,
 Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Jens Axboe <axboe@...nel.dk>,
 Florian Weimer <fweimer@...hat.com>,
 Sam James <sam@...too.org>
Subject: [PATCH v14 12/12] unwind deferred/x86: Do not defer stack tracing for compat tasks

From: Steven Rostedt <rostedt@...dmis.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@...dmis.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 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 53a75f8f9b7e..9972096e93e8 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;
-- 
2.47.2