[PATCH v3 4/6] livepatch: Add sample livepatch module

[Lukas Hruska <lhruska@suse.cz>]

From: Josh Poimboeuf <jpoimboe@redhat.com>

Add a new livepatch sample in samples/livepatch/ to make use of symbols
that must be post-processed to enable load-time relocation resolution.
As the new sample is to be used as an example, it is annotated with
KLP_RELOC_SYMBOL macro.

The livepatch sample updates the function cmdline_proc_show to print the
string referenced by the symbol saved_command_line appended by the
string "livepatch=1".

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Lukas Hruska <lhruska@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
---
 samples/livepatch/Makefile                  |  1 +
 samples/livepatch/livepatch-extern-symbol.c | 84 +++++++++++++++++++++
 2 files changed, 85 insertions(+)
 create mode 100644 samples/livepatch/livepatch-extern-symbol.c

diff --git a/samples/livepatch/Makefile b/samples/livepatch/Makefile
index 9f853eeb6140..5cc81d5db17c 100644
--- a/samples/livepatch/Makefile
+++ b/samples/livepatch/Makefile
@@ -6,3 +6,4 @@ obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-shadow-fix2.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-demo.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-mod.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-callbacks-busymod.o
+obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-extern-symbol.o
diff --git a/samples/livepatch/livepatch-extern-symbol.c b/samples/livepatch/livepatch-extern-symbol.c
new file mode 100644
index 000000000000..276a43d157b4
--- /dev/null
+++ b/samples/livepatch/livepatch-extern-symbol.c
@@ -0,0 +1,84 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2014 Seth Jennings <sjenning@redhat.com>
+ */
+
+/*
+ * livepatch-extern-symbol.c - Kernel Live Patching Sample Module
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+
+/*
+ * This (dumb) live patch overrides the function that prints the
+ * kernel boot cmdline when /proc/cmdline is read.
+ *
+ * This livepatch uses the symbol saved_command_line whose relocation
+ * must be resolved during load time. To enable that, this module
+ * must be post-processed by a tool called klp-convert, which embeds
+ * information to be used by the loader to solve the relocation.
+ *
+ * The module is annotated with KLP_RELOC_SYMBOL macros.
+ * These annotations are used by klp-convert to infer that the symbol
+ * saved_command_line is in the object vmlinux.
+ *
+ * Example:
+ *
+ * $ cat /proc/cmdline
+ * <your cmdline>
+ *
+ * $ insmod livepatch-sample.ko
+ * $ cat /proc/cmdline
+ * <your cmdline> livepatch=1
+ *
+ * $ echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
+ * $ cat /proc/cmdline
+ * <your cmdline>
+ */
+
+extern char *saved_command_line \
+	       KLP_RELOC_SYMBOL(vmlinux, vmlinux, saved_command_line);
+
+#include <linux/seq_file.h>
+static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "%s livepatch=1\n", saved_command_line);
+	return 0;
+}
+
+static struct klp_func funcs[] = {
+	{
+		.old_name = "cmdline_proc_show",
+		.new_func = livepatch_cmdline_proc_show,
+	}, { }
+};
+
+static struct klp_object objs[] = {
+	{
+		/* name being NULL means vmlinux */
+		.funcs = funcs,
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_init(void)
+{
+	return klp_enable_patch(&patch);
+}
+
+static void livepatch_exit(void)
+{
+}
+
+module_init(livepatch_init);
+module_exit(livepatch_exit);
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");
-- 
2.46.0