Re: [PATCH v2 2/2] unwind deferred/x86: Do not defer stack tracing for compat tasks

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Sep 23, 2025 at 12:45:15PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 27, 2025 at 03:36:46PM -0400, Steven Rostedt wrote:
> > 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.
> 
> Changelog seems to forget mentioning *why* we can't unwind compat.
> 
> I'm sure I've seen compat FP unwind support at some point in this
> series. Did that go missing somewhere?


I'm thinking something like the below ought to work. That's just about
as complicated as not supporting compat.

---

Subject: unwind: Implement compat fp unwind
From: Peter Zijlstra <peterz@infradead.org>
Date: Tue Sep 23 13:27:34 CEST 2025


Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/unwind_user_types.h |    1 +
 kernel/unwind/user.c              |   25 +++++++++++++++++++++----
 2 files changed, 22 insertions(+), 4 deletions(-)

--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -36,6 +36,7 @@ struct unwind_user_state {
 	unsigned long				ip;
 	unsigned long				sp;
 	unsigned long				fp;
+	unsigned int				ws;
 	enum unwind_user_type			current_type;
 	unsigned int				available_types;
 	bool					done;
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -15,6 +15,20 @@ static const struct unwind_user_frame fp
 #define for_each_user_frame(state) \
 	for (unwind_user_start(state); !(state)->done; unwind_user_next(state))
 
+static __always_inline int
+get_user_word(unsigned long *word, unsigned long __user *addr, int size)
+{
+#ifdef CONFIG_COMPAT
+	if (size == sizeof(int)) {
+		unsigned int data;
+		int ret = get_user(data, (unsigned int __user *)addr);
+		*word = data;
+		return ret;
+	}
+#endif
+	return get_user(*word, addr);
+}
+
 static int unwind_user_next_fp(struct unwind_user_state *state)
 {
 	const struct unwind_user_frame *frame = &fp_frame;
@@ -29,21 +43,23 @@ static int unwind_user_next_fp(struct un
 	}
 
 	/* Get the Canonical Frame Address (CFA) */
-	cfa += frame->cfa_off;
+	cfa += state->ws * frame->cfa_off;
 
 	/* stack going in wrong direction? */
 	if (cfa <= state->sp)
 		return -EINVAL;
 
 	/* Make sure that the address is word aligned */
-	if (cfa & (sizeof(long) - 1))
+	if (cfa & (state->ws - 1))
 		return -EINVAL;
 
 	/* Find the Return Address (RA) */
-	if (get_user(ra, (unsigned long *)(cfa + frame->ra_off)))
+	if (get_user_word(&ra, (void __user *)cfa + (state->ws * frame->ra_off),
+			  state->ws))
 		return -EINVAL;
 
-	if (frame->fp_off && get_user(fp, (unsigned long __user *)(cfa + frame->fp_off)))
+	if (frame->fp_off && get_user_word(&fp, (void __user *)cfa +
+					        (state->ws * frame->fp_off), state->ws))
 		return -EINVAL;
 
 	state->ip = ra;
@@ -100,6 +116,7 @@ static int unwind_user_start(struct unwi
 	state->ip = instruction_pointer(regs);
 	state->sp = user_stack_pointer(regs);
 	state->fp = frame_pointer(regs);
+	state->ws = compat_user_mode(regs) ? sizeof(int) : sizeof(long);
 
 	return 0;
 }

---


Subject: unwind_user/x86: Enable frame pointer unwinding on x86
From: Josh Poimboeuf <jpoimboe@kernel.org>
Date: Wed, 27 Aug 2025 15:36:45 -0400

From: Josh Poimboeuf <jpoimboe@kernel.org>

Use ARCH_INIT_USER_FP_FRAME to describe how frame pointers are unwound
on x86, and enable CONFIG_HAVE_UNWIND_USER_FP accordingly so the
unwind_user interfaces can be used.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 arch/x86/Kconfig                   |    1 +
 arch/x86/include/asm/ptrace.h      |    9 +++++++++
 arch/x86/include/asm/unwind_user.h |   11 +++++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 arch/x86/include/asm/unwind_user.h

--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -297,6 +297,7 @@ config X86
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UACCESS_VALIDATION		if HAVE_OBJTOOL
 	select HAVE_UNSTABLE_SCHED_CLOCK
+	select HAVE_UNWIND_USER_FP		if X86_64
 	select HAVE_USER_RETURN_NOTIFIER
 	select HAVE_GENERIC_VDSO
 	select VDSO_GETRANDOM			if X86_64
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -255,6 +255,15 @@ static inline bool any_64bit_mode(struct
 #endif
 }
 
+static inline bool compat_user_mode(struct pt_regs *regs)
+{
+#ifdef CONFIG_X86_64
+	return !user_64bit_mode(regs);
+#else
+	return false;
+#endif
+}
+
 #ifdef CONFIG_X86_64
 #define current_user_stack_pointer()	current_pt_regs()->sp
 #define compat_user_stack_pointer()	current_pt_regs()->sp
--- /dev/null
+++ b/arch/x86/include/asm/unwind_user.h
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_UNWIND_USER_H
+#define _ASM_X86_UNWIND_USER_H
+
+#define ARCH_INIT_USER_FP_FRAME				\
+	.cfa_off	=  2,				\
+	.ra_off		= -1,				\
+	.fp_off		= -2,				\
+	.use_fp		= true,
+
+#endif /* _ASM_X86_UNWIND_USER_H */