livepatch: Add sample livepatch module

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_MODULE_RELOC and with KLP_SYMPOS macros.

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".

Update livepatch-sample.c to remove livepatch MODULE_INFO
statement.

[jmoreira:
* update module to use KLP_SYMPOS
* Comments on symbol resolution scheme
* Update Makefile
* Remove MODULE_INFO statement
* Changelog
]

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Joao Moreira <jmoreira@suse.de>

samples/livepatch/Makefile

@@ -2,6 +2,7 @@ LIVEPATCH_livepatch-sample.o := y
 LIVEPATCH_livepatch-shadow-fix1.o := y
 LIVEPATCH_livepatch-shadow-fix2.o := y
 LIVEPATCH_livepatch-callbacks-demo.o := y
+LIVEPATCH_livepatch-annotated-sample.o := y
 
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-sample.o
 obj-$(CONFIG_SAMPLE_LIVEPATCH) += livepatch-shadow-mod.o
@@ -10,3 +11,4 @@ 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-annotated-sample.o

samples/livepatch/livepatch-annotated-sample.c

@@ -0,0 +1,113 @@
+/*
+ * livepatch-annotated-sample.c - Kernel Live Patching Sample Module
+ *
+ * Copyright (C) 2014 Seth Jennings <sjenning@redhat.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/livepatch.h>
+#include <linux/seq_file.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_MODULE_RELOC/KLP_SYMPOS macros.
+ * These annotations are used by klp-convert to infer that the symbol
+ * saved_command_line is in the object vmlinux.
+ *
+ * As saved_command_line has no other homonimous symbol across
+ * kernel objects, this annotation is not a requirement, and can be
+ * suppressed with no harm to klp-convert. Yet, it is kept here as an
+ * example on how to annotate livepatch modules that contain symbols
+ * whose names are used in more than one kernel object.
+ *
+ * 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;
+
+static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "%s livepatch=1\n", saved_command_line);
+	return 0;
+}
+
+KLP_MODULE_RELOC(vmlinux) vmlinux_relocs[] = {
+	KLP_SYMPOS(saved_command_line, 0)
+};
+
+static struct klp_func vmlinux_funcs[] = {
+	{
+		.old_name = "cmdline_proc_show",
+		.new_func = livepatch_cmdline_proc_show,
+	}, { }
+};
+
+static struct klp_object objs[] = {
+	{
+		/* name being NULL means vmlinux */
+		.funcs = vmlinux_funcs,
+	}, { }
+};
+
+static struct klp_patch patch = {
+	.mod = THIS_MODULE,
+	.objs = objs,
+};
+
+static int livepatch_init(void)
+{
+	int ret;
+
+	ret = klp_register_patch(&patch);
+	if (ret)
+		return ret;
+	ret = klp_enable_patch(&patch);
+	if (ret) {
+		WARN_ON(klp_unregister_patch(&patch));
+		return ret;
+	}
+	return 0;
+}
+
+static void livepatch_exit(void)
+{
+	WARN_ON(klp_unregister_patch(&patch));
+}
+
+module_init(livepatch_init);
+module_exit(livepatch_exit);
+MODULE_LICENSE("GPL");