ANDROID: android: Create debug_symbols driver

Introduce new API to expose symbols useful for debugging the GKI kernel.
Symbols exported from this driver would be difficult to maintain via the
traditional EXPORT_SYMBOL_GPL.

Bug: 160245776
Change-Id: I92053450bd74788889d1b7a569e291a35cd525ea
Signed-off-by: Elliot Berman <eberman@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>

drivers/android/Kconfig

@@ -54,6 +54,17 @@ config ANDROID_BINDER_IPC_SELFTEST
 	  exhaustively with combinations of various buffer sizes and
 	  alignments.
 
+config ANDROID_DEBUG_SYMBOLS
+	bool "Android Debug Symbols"
+	help
+	  Enables export of debug symbols that are useful for offline debugging
+	  of a kernel. These symbols would be used in vendor modules to find
+	  addresses of the core kernel symbols for vendor extensions.
+
+	  This driver is statically compiled into kernel and maintains all the
+	  required symbol addresses for vendor modules and provides necessary
+	  interface vendor modules.
+
 config ANDROID_VENDOR_HOOKS
 	bool "Android Vendor Hooks"
 	depends on TRACEPOINTS

drivers/android/Makefile

@@ -4,4 +4,5 @@ ccflags-y += -I$(src)			# needed for trace events
 obj-$(CONFIG_ANDROID_BINDERFS)		+= binderfs.o
 obj-$(CONFIG_ANDROID_BINDER_IPC)	+= binder.o binder_alloc.o
 obj-$(CONFIG_ANDROID_BINDER_IPC_SELFTEST) += binder_alloc_selftest.o
+obj-$(CONFIG_ANDROID_DEBUG_SYMBOLS)	+= debug_symbols.o
 obj-$(CONFIG_ANDROID_VENDOR_HOOKS) += vendor_hooks.o

drivers/android/debug_symbols.c

@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+/*
+ * Copyright (c) 2021, The Linux Foundation. All rights reserved.
+ */
+
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/android_debug_symbols.h>
+#include <asm/stacktrace.h>
+#include <asm/sections.h>
+
+struct ads_entry {
+	char *name;
+	void *addr;
+};
+
+#define _ADS_ENTRY(index, symbol)			\
+	[index] = { .name = #symbol, .addr = (void *)symbol }
+#define ADS_ENTRY(index, symbol) _ADS_ENTRY(index, symbol)
+
+#define _ADS_PER_CPU_ENTRY(index, symbol)			\
+	[index] = { .name = #symbol, .addr = (void *)&symbol }
+#define ADS_PER_CPU_ENTRY(index, symbol) _ADS_PER_CPU_ENTRY(index, symbol)
+
+/*
+ * This module maintains static array of symbol and address information.
+ * Add all required core kernel symbols and their addresses into ads_entries[] array,
+ * so that vendor modules can query and to find address of non-exported symbol.
+ */
+static const struct ads_entry ads_entries[ADS_END] = {
+	ADS_ENTRY(ADS_SDATA, _sdata),
+	ADS_ENTRY(ADS_BSS_END, __bss_stop),
+	ADS_ENTRY(ADS_PER_CPU_START, __per_cpu_start),
+	ADS_ENTRY(ADS_PER_CPU_END, __per_cpu_end),
+	ADS_ENTRY(ADS_START_RO_AFTER_INIT, __start_ro_after_init),
+	ADS_ENTRY(ADS_END_RO_AFTER_INIT, __end_ro_after_init),
+	ADS_ENTRY(ADS_LINUX_BANNER, linux_banner),
+};
+
+/*
+ * ads_per_cpu_entries array contains all the per_cpu variable address information.
+ */
+static const struct ads_entry ads_per_cpu_entries[ADS_DEBUG_PER_CPU_END] = {
+#ifdef CONFIG_ARM64
+	ADS_PER_CPU_ENTRY(ADS_IRQ_STACK_PTR, irq_stack_ptr),
+#endif
+#ifdef CONFIG_X86
+	ADS_PER_CPU_ENTRY(ADS_IRQ_STACK_PTR, hardirq_stack_ptr),
+#endif
+};
+
+/*
+ * android_debug_symbol - Provide address inforamtion of debug symbol.
+ * @symbol: Index of debug symbol array.
+ *
+ * Return address of core kernel symbol on success and a negative errno will be
+ * returned in error cases.
+ *
+ */
+void *android_debug_symbol(enum android_debug_symbol symbol)
+{
+	if (symbol >= ADS_END)
+		return ERR_PTR(-EINVAL);
+
+	return ads_entries[symbol].addr;
+}
+EXPORT_SYMBOL_GPL(android_debug_symbol);
+
+/*
+ * android_debug_per_cpu_symbol - Provide address inforamtion of per cpu debug symbol.
+ * @symbol: Index of per cpu debug symbol array.
+ *
+ * Return address of core kernel symbol on success and a negative errno will be
+ * returned in error cases.
+ *
+ */
+void *android_debug_per_cpu_symbol(enum android_debug_per_cpu_symbol symbol)
+{
+	if (symbol >= ADS_DEBUG_PER_CPU_END)
+		return ERR_PTR(-EINVAL);
+
+	return ads_per_cpu_entries[symbol].addr;
+}
+EXPORT_SYMBOL_GPL(android_debug_per_cpu_symbol);
+
+static int __init debug_symbol_init(void)
+{
+	return 0;
+}
+module_init(debug_symbol_init);
+
+static void __exit debug_symbol_exit(void)
+{ }
+module_exit(debug_symbol_exit);
+
+MODULE_DESCRIPTION("Debug Symbol Driver");
+MODULE_LICENSE("GPL v2");

include/linux/android_debug_symbols.h

@@ -0,0 +1,42 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (c) 2021, The Linux Foundation. All rights reserved.
+ */
+
+#ifndef _ANDROID_DEBUG_SYMBOLS_H
+#define _ANDROID_DEBUG_SYMBOLS_H
+
+enum android_debug_symbol {
+	ADS_SDATA = 0,
+	ADS_BSS_END,
+	ADS_PER_CPU_START,
+	ADS_PER_CPU_END,
+	ADS_START_RO_AFTER_INIT,
+	ADS_END_RO_AFTER_INIT,
+	ADS_LINUX_BANNER,
+	ADS_END
+};
+
+enum android_debug_per_cpu_symbol {
+	ADS_IRQ_STACK_PTR = 0,
+	ADS_DEBUG_PER_CPU_END
+};
+
+#ifdef CONFIG_ANDROID_DEBUG_SYMBOLS
+
+void *android_debug_symbol(enum android_debug_symbol symbol);
+void *android_debug_per_cpu_symbol(enum android_debug_per_cpu_symbol symbol);
+
+#else /* !CONFIG_ANDROID_DEBUG_SYMBOLS */
+
+static inline void *android_debug_symbol(enum android_debug_symbol symbol)
+{
+	return NULL;
+}
+static inline void *android_debug_per_cpu_symbol(enum android_debug_per_cpu_symbol symbol)
+{
+	return NULL;
+}
+#endif /* CONFIG_ANDROID_DEBUG_SYMBOLS */
+
+#endif /* _ANDROID_DEBUG_SYMBOLS_H */