backtrace: Add a library to access the backtrace

Provide an API to access the backtrace, in an arch-neutral way.

The backtrace can be retrieved, examined and printed.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

lib/Kconfig

@@ -28,6 +28,14 @@ config PHYSMEM
 	  Enable this to access this basic support, which only supports clearing
 	  the memory.
 
+config BACKTRACE
+	bool "Enable backtrace support"
+	depends on SANDBOX
+	help
+	  Enables support for printing a backtrace showing the current call
+	  stack. This is currently only available on sandbox. The backtrace
+	  command can be used to print the backtrace.
+
 config BCH
 	bool "Enable Software based BCH ECC"
 	help

lib/Makefile

@@ -147,6 +147,7 @@ obj-$(CONFIG_TRACE) += trace.o
 obj-$(CONFIG_LIB_UUID) += uuid.o
 obj-$(CONFIG_LIB_RAND) += rand.o
 obj-y += panic.o
+obj-$(CONFIG_BACKTRACE) += backtrace.o
 
 ifeq ($(CONFIG_XPL_BUILD),y)
 # SPL U-Boot may use full-printf, tiny-printf or none at all

lib/backtrace.c

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Stack-backtrace support
+ *
+ * Copyright 2025 Canonical Ltd
+ * Written by Simon Glass <simon.glass@canonical.com>
+ */
+
+#include <backtrace.h>
+#include <stdio.h>
+
+int backtrace_show(void)
+{
+	char buf[BACKTRACE_BUFSZ];
+	struct backtrace_ctx ctx;
+	uint i;
+	int ret;
+
+	ret = backtrace_init(&ctx, 1);
+	if (ret < 0)
+		return ret;
+
+	ret = backtrace_get_syms(&ctx, buf, sizeof(buf));
+	if (ret) {
+		backtrace_uninit(&ctx);
+		return ret;
+	}
+
+	printf("backtrace: %d addresses\n", ctx.count);
+	for (i = 0; i < ctx.count; i++) {
+		if (ctx.syms[i])
+			printf("  %s\n", ctx.syms[i]);
+		else
+			printf("  %p\n", ctx.addrs[i]);
+	}
+
+	backtrace_uninit(&ctx);
+
+	return 0;
+}