linux-kernel - Re: [PATCH v10 03/14] unwind_user: Add compat mode frame pointer support

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Message-ID: <20250618111840.24a940f6@gandalf.local.home>
Date: Wed, 18 Jun 2025 11:18:40 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.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>
Subject: Re: [PATCH v10 03/14] unwind_user: Add compat mode frame pointer
 support

On Wed, 18 Jun 2025 15:47:58 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jun 10, 2025 at 08:54:24PM -0400, Steven Rostedt wrote:
> 
> > +#ifndef arch_unwind_user_init
> > +static inline void arch_unwind_user_init(struct unwind_user_state *state, struct pt_regs *reg) {}
> > +#endif
> > +
> > +#ifndef arch_unwind_user_next
> > +static inline void arch_unwind_user_next(struct unwind_user_state *state) {}
> > +#endif  
> 
> The purpose of these arch hooks is so far mysterious. No comments, no
> changelog, no nothing.

I'll add comments.

It's used later in the x86 compat code to allow the architecture to do any
special initialization or to handling moving to the next frame.

From patch 14:

+#define in_compat_mode(regs) !user_64bit_mode(regs)
+
+static inline void arch_unwind_user_init(struct unwind_user_state *state,
+					 struct pt_regs *regs)
+{
+	unsigned long cs_base, ss_base;
+
+	if (state->type != UNWIND_USER_TYPE_COMPAT_FP)
+		return;
+
+	scoped_guard(irqsave) {
+		cs_base = segment_base_address(regs->cs);
+		ss_base = segment_base_address(regs->ss);
+	}
+
+	state->arch.cs_base = cs_base;
+	state->arch.ss_base = ss_base;
+
+	state->ip += cs_base;
+	state->sp += ss_base;
+	state->fp += ss_base;
+}
+#define arch_unwind_user_init arch_unwind_user_init
+
+static inline void arch_unwind_user_next(struct unwind_user_state *state)
+{
+	if (state->type != UNWIND_USER_TYPE_COMPAT_FP)
+		return;
+
+	state->ip += state->arch.cs_base;
+	state->fp += state->arch.ss_base;
+}
+#define arch_unwind_user_next arch_unwind_user_next

-- Steve