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Message-ID: <20250923114257.GV4068168@noisy.programming.kicks-ass.net>
Date: Tue, 23 Sep 2025 13:42:57 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>,
Mark Rutland <mark.rutland@....com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2 2/2] unwind deferred/x86: Do not defer stack tracing
for compat tasks
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@...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.
>
> 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@...radead.org>
Date: Tue Sep 23 13:27:34 CEST 2025
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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@...nel.org>
Date: Wed, 27 Aug 2025 15:36:45 -0400
From: Josh Poimboeuf <jpoimboe@...nel.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@...nel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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 */
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