[PATCH v2 0/2] x86: Enable the deferred unwinding infrastructure

[PATCH v2 0/2] x86: Enable the deferred unwinding infrastructure

[Steven Rostedt]

This enables deferred unwinding for x86.

Changes since v1: https://lore.kernel.org/linux-trace-kernel/20250820190546.172023727@kernel.org

- Added #include <linux/compat.h> to use user_64bit_mode() in
  unwind_user.h. When adding this to the ftrace code, it failed to build
  due to the missing header.

Josh Poimboeuf (1):
      unwind_user/x86: Enable frame pointer unwinding on x86

Steven Rostedt (1):
      unwind deferred/x86: Do not defer stack tracing for compat tasks

----
 arch/x86/Kconfig                   |  1 +
 arch/x86/include/asm/unwind_user.h | 23 +++++++++++++++++++++++
 include/linux/unwind_deferred.h    |  5 +++++
 kernel/unwind/deferred.c           |  3 +++
 4 files changed, 32 insertions(+)
 create mode 100644 arch/x86/include/asm/unwind_user.h

[PATCH v2 1/2] unwind_user/x86: Enable frame pointer unwinding on x86

[Steven Rostedt]

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>
---
 arch/x86/Kconfig                   |  1 +
 arch/x86/include/asm/unwind_user.h | 11 +++++++++++
 2 files changed, 12 insertions(+)
 create mode 100644 arch/x86/include/asm/unwind_user.h

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 58d890fe2100..8f94c58d4de8 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -299,6 +299,7 @@ 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
diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
new file mode 100644
index 000000000000..8597857bf896
--- /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	= (s32)sizeof(long) *  2,				\
+	.ra_off		= (s32)sizeof(long) * -1,				\
+	.fp_off		= (s32)sizeof(long) * -2,				\
+	.use_fp		= true,
+
+#endif /* _ASM_X86_UNWIND_USER_H */
-- 
2.50.1

Re: [PATCH v2 1/2] unwind_user/x86: Enable frame pointer unwinding on x86

[Peter Zijlstra]

On Wed, Aug 27, 2025 at 03:36:45PM -0400, Steven Rostedt wrote:
> 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>
> ---
>  arch/x86/Kconfig                   |  1 +
>  arch/x86/include/asm/unwind_user.h | 11 +++++++++++
>  2 files changed, 12 insertions(+)
>  create mode 100644 arch/x86/include/asm/unwind_user.h
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 58d890fe2100..8f94c58d4de8 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -299,6 +299,7 @@ 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
> diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
> new file mode 100644
> index 000000000000..8597857bf896
> --- /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	= (s32)sizeof(long) *  2,				\
> +	.ra_off		= (s32)sizeof(long) * -1,				\
> +	.fp_off		= (s32)sizeof(long) * -2,				\
> +	.use_fp		= true,
> +
> +#endif /* _ASM_X86_UNWIND_USER_H */

Moo, and now you have me look at unwind/user.c:

        /* Make sure that the address is word aligned */
        shift = sizeof(long) == 4 ? 2 : 3;
        if (cfa & ((1 << shift) - 1))
                return -EINVAL;

Isn't that just:

	if (cfa & (sizeof(long) - 1))

?

Let me go add a patch to clean that up...

Re: [PATCH v2 1/2] unwind_user/x86: Enable frame pointer unwinding on x86

[Steven Rostedt]

On Tue, 23 Sep 2025 12:51:30 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> Moo, and now you have me look at unwind/user.c:
> 
>         /* Make sure that the address is word aligned */
>         shift = sizeof(long) == 4 ? 2 : 3;
>         if (cfa & ((1 << shift) - 1))
>                 return -EINVAL;
> 
> Isn't that just:
> 
> 	if (cfa & (sizeof(long) - 1))
> 
> ?
> 
> Let me go add a patch to clean that up...

Sure, as long as it's commented.

Thanks,

-- Steve

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

[Steven Rostedt]

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.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v1: https://lore.kernel.org/20250820190639.843442474@kernel.org

- Added #include <linux/compat.h> to use user_64bit_mode() in
  unwind_user.h. When adding this to the ftrace code, it failed to build
  due to the missing header.

 arch/x86/include/asm/unwind_user.h | 12 ++++++++++++
 include/linux/unwind_deferred.h    |  5 +++++
 kernel/unwind/deferred.c           |  3 +++
 3 files changed, 20 insertions(+)

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index 8597857bf896..0cef2e3b08c8 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -2,6 +2,18 @@
 #ifndef _ASM_X86_UNWIND_USER_H
 #define _ASM_X86_UNWIND_USER_H
 
+#ifdef CONFIG_IA32_EMULATION
+#include <linux/compat.h>
+/* 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 dc6040aae3ee..3601b2efe48d 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;
-- 
2.50.1

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

[Peter Zijlstra]

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?

Also, these two patches are in the wrong order, first you enable things,
including compat tasks, and then you go 'whoopsie, no compats'.

> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v1: https://lore.kernel.org/20250820190639.843442474@kernel.org
> 
> - Added #include <linux/compat.h> to use user_64bit_mode() in
>   unwind_user.h. When adding this to the ftrace code, it failed to build
>   due to the missing header.
> 
>  arch/x86/include/asm/unwind_user.h | 12 ++++++++++++
>  include/linux/unwind_deferred.h    |  5 +++++
>  kernel/unwind/deferred.c           |  3 +++
>  3 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
> index 8597857bf896..0cef2e3b08c8 100644
> --- a/arch/x86/include/asm/unwind_user.h
> +++ b/arch/x86/include/asm/unwind_user.h
> @@ -2,6 +2,18 @@
>  #ifndef _ASM_X86_UNWIND_USER_H
>  #define _ASM_X86_UNWIND_USER_H
>  
> +#ifdef CONFIG_IA32_EMULATION
> +#include <linux/compat.h>
> +/* 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 dc6040aae3ee..3601b2efe48d 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;

Should not this arch hook be after all the 'normal' early exits?

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

[Peter Zijlstra]

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 */

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

[Steven Rostedt]

On Tue, 23 Sep 2025 12:45:15 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> 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?
> 
> Also, these two patches are in the wrong order, first you enable things,
> including compat tasks, and then you go 'whoopsie, no compats'.

Sure, we can swap it. Yes, we had patches to support it, but they
were a bit complicated and when I tested them, they didn't work. But
then I also noticed that the current stack tracing didn't work on
compat either. Instead of adding complicated code that wasn't working
on my machine, I decided to remove the patches. But after I did that, I
realized I needed to make sure it wasn't even tried, which is this
patch.

I'm not sure who needs profiling on compat code, and I figured we can
not add the new deferred work to it if it's not needed. If in the
future we need it, we can add it then.

-- Steve