linux-kernel - Re: [PATCH v8 06/12] unwind_user/sframe: Wire up unwind

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Message-ID: <20250710165157.6e0936e9@batman.local.home>
Date: Thu, 10 Jul 2025 16:51:57 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Steven Rostedt
 <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org,
 Masami Hiramatsu <mhiramat@...nel.org>, 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>,
 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>, Heiko Carstens
 <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v8 06/12] unwind_user/sframe: Wire up unwind_user to
 sframe

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