[PATCH 2/8] alpha: add ARCH_STACKWALK-based stacktrace support

[Magnus Lindholm <linmag7@gmail.com>]

Implement arch_stack_walk() for Alpha using a simple kernel
stack scanning walker. Start from regs+1 for current tasks
to skip pt_regs and use pcb.ksp for blocked tasks. Filter
candidates with __kernel_text_address() and stop at stack
bounds via kstack_end().

Enable CONFIG_STACKTRACE_SUPPORT and CONFIG_ARCH_STACKWALK
so generic stacktrace users (dump_stack(), /proc/*/stack,
SysRq backtraces, etc.) work on Alpha.

This provides functional in-kernel stack traces without
requiring frame pointer unwinding.

Signed-off-by: Magnus Lindholm <linmag7@gmail.com>
---
 arch/alpha/Kconfig              |  4 +++
 arch/alpha/kernel/Makefile      |  3 +-
 arch/alpha/kernel/stacktrace.c  | 61 +++++++++++++++++++++++++++++++++
 arch/alpha/kernel/vmlinux.lds.S |  2 ++
 4 files changed, 69 insertions(+), 1 deletion(-)
 create mode 100644 arch/alpha/kernel/stacktrace.c

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index f3b882835617..7ac435c56845 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -39,6 +39,7 @@ config ALPHA
 	select MODULES_USE_ELF_RELA
 	select ODD_RT_SIGACTION
 	select OLD_SIGSUSPEND
+	select ARCH_STACKWALK
 	select CPU_NO_EFFICIENT_FFS if !ALPHA_EV67
 	select MMU_GATHER_NO_RANGE
 	select MMU_GATHER_RCU_TABLE_FREE
@@ -80,6 +81,9 @@ config PGTABLE_LEVELS
 config AUDIT_ARCH
 	bool
 
+config STACKTRACE_SUPPORT
+	def_bool y
+
 menu "System setup"
 
 choice
diff --git a/arch/alpha/kernel/Makefile b/arch/alpha/kernel/Makefile
index 187cd8df2faf..4ea5c189e60e 100644
--- a/arch/alpha/kernel/Makefile
+++ b/arch/alpha/kernel/Makefile
@@ -9,7 +9,8 @@ ccflags-y	:= -Wno-sign-compare
 
 obj-y    := head.o entry.o traps.o process.o osf_sys.o irq.o \
 	    irq_alpha.o signal.o setup.o ptrace.o time.o \
-	    systbls.o err_common.o io.o bugs.o termios.o
+	    systbls.o err_common.o io.o bugs.o termios.o \
+	    stacktrace.o
 
 obj-$(CONFIG_VGA_HOSE)	+= console.o
 obj-$(CONFIG_SMP)	+= smp.o
diff --git a/arch/alpha/kernel/stacktrace.c b/arch/alpha/kernel/stacktrace.c
new file mode 100644
index 000000000000..74d95f591039
--- /dev/null
+++ b/arch/alpha/kernel/stacktrace.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/sched.h>
+#include <linux/sched/task_stack.h>
+#include <linux/stacktrace.h>
+#include <linux/kallsyms.h>
+
+#include <asm/thread_info.h>
+#include <asm/ptrace.h>
+
+static __always_inline unsigned long alpha_get_current_ksp(void)
+{
+	unsigned long sp;
+
+	asm volatile("mov $30, %0" : "=r"(sp));
+	return sp;
+}
+
+static void alpha_scan_kernel_stack(unsigned long ksp,
+				    stack_trace_consume_fn consume_entry,
+				    void *cookie)
+{
+	unsigned long *p = (unsigned long *)ksp;
+
+	if (unlikely(ksp & (sizeof(unsigned long) - 1)))
+		return;
+
+	while (!kstack_end(p)) {
+		unsigned long addr = READ_ONCE_NOCHECK(*p++);
+
+		if (!__kernel_text_address(addr))
+			continue;
+
+		if (!consume_entry(cookie, addr))
+			break;
+	}
+}
+
+noinline void arch_stack_walk(stack_trace_consume_fn consume_entry,
+				      void *cookie,
+				      struct task_struct *task,
+				      struct pt_regs *regs)
+{
+	unsigned long ksp;
+
+	if (!task)
+		task = current;
+
+	if (regs && task == current) {
+		/*
+		 * pt_regs is stored on the kernel stack; regs+1 matches
+		 * what arch/alpha/kernel/traps.c uses as the trace start.
+		 */
+		ksp = (unsigned long)(regs + 1);
+	} else if (task == current) {
+		ksp = alpha_get_current_ksp();
+	} else {
+		ksp = task_thread_info(task)->pcb.ksp;
+	}
+
+	alpha_scan_kernel_stack(ksp, consume_entry, cookie);
+}
diff --git a/arch/alpha/kernel/vmlinux.lds.S b/arch/alpha/kernel/vmlinux.lds.S
index 2d136c63db16..95704e64b6a6 100644
--- a/arch/alpha/kernel/vmlinux.lds.S
+++ b/arch/alpha/kernel/vmlinux.lds.S
@@ -28,6 +28,8 @@ SECTIONS
 		TEXT_TEXT
 		SCHED_TEXT
 		LOCK_TEXT
+		IRQENTRY_TEXT
+		SOFTIRQENTRY_TEXT
 		*(.fixup)
 		*(.gnu.warning)
 	} :text
-- 
2.53.0