unwind_user/x86: Enable frame pointer unwinding on x86

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>
Link: https://patch.msgid.link/20250827193828.347397433@kernel.org

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

arch/x86/include/asm/unwind_user.h

@@ -0,0 +1,25 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_UNWIND_USER_H
+#define _ASM_X86_UNWIND_USER_H
+
+#include <asm/ptrace.h>
+
+#define ARCH_INIT_USER_FP_FRAME(ws)			\
+	.cfa_off	=  2*(ws),			\
+	.ra_off		= -1*(ws),			\
+	.fp_off		= -2*(ws),			\
+	.use_fp		= true,
+
+static inline int unwind_user_word_size(struct pt_regs *regs)
+{
+	/* We can't unwind VM86 stacks */
+	if (regs->flags & X86_VM_MASK)
+		return 0;
+#ifdef CONFIG_X86_64
+	if (!user_64bit_mode(regs))
+		return sizeof(int);
+#endif
+	return sizeof(long);
+}
+
+#endif /* _ASM_X86_UNWIND_USER_H */