[PATCH v3] livepatch: old_name,number scheme in livepatch sysfs directory

[Chris J Arges <chris.j.arges@canonical.com>]

In cases of duplicate symbols in vmlinux, old_sympos will be used to
disambiguate instead of old_addr. Normally old_sympos will be 0, and
default to only returning the first found instance of that symbol. If an
incorrect symbol position is specified then livepatching will fail.
Finally, old_addr is now an internal structure element and not to be
specified by the user.

The following directory structure will allow for cases when the same
function name exists in a single object.
	/sys/kernel/livepatch/<patch>/<object>/<function.number>

The number corresponds to the nth occurrence of the symbol name in
kallsyms for the patched object.

An example of patching multiple symbols can be found here:
	https://github.com/dynup/kpatch/issues/493

Signed-off-by: Chris J Arges <chris.j.arges@canonical.com>
---
 Documentation/ABI/testing/sysfs-kernel-livepatch |  6 ++-
 include/linux/livepatch.h                        | 20 ++++---
 kernel/livepatch/core.c                          | 66 ++++++++++++++++--------
 3 files changed, 61 insertions(+), 31 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch
index 5bf42a8..21b6bc1 100644
--- a/Documentation/ABI/testing/sysfs-kernel-livepatch
+++ b/Documentation/ABI/testing/sysfs-kernel-livepatch
@@ -33,7 +33,7 @@ Description:
 		The object directory contains subdirectories for each function
 		that is patched within the object.
 
-What:		/sys/kernel/livepatch/<patch>/<object>/<function>
+What:		/sys/kernel/livepatch/<patch>/<object>/<function,number>
 Date:		Nov 2014
 KernelVersion:	3.19.0
 Contact:	live-patching@vger.kernel.org
@@ -41,4 +41,8 @@ Description:
 		The function directory contains attributes regarding the
 		properties and state of the patched function.
 
+		The directory name contains the patched function name and a
+		number corresponding to the nth occurrence of the symbol name
+		in kallsyms for the patched object.
+
 		There are currently no such attributes.
diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h
index 31db7a0..986e06d 100644
--- a/include/linux/livepatch.h
+++ b/include/linux/livepatch.h
@@ -37,8 +37,9 @@ enum klp_state {
  * struct klp_func - function structure for live patching
  * @old_name:	name of the function to be patched
  * @new_func:	pointer to the patched function code
- * @old_addr:	a hint conveying at what address the old function
+ * @old_sympos: a hint indicating which symbol position the old function
  *		can be found (optional, vmlinux patches only)
+ * @old_addr:	the address of the function being patched
  * @kobj:	kobject for sysfs resources
  * @state:	tracks function-level patch application state
  * @stack_node:	list node for klp_ops func_stack list
@@ -47,17 +48,20 @@ struct klp_func {
 	/* external */
 	const char *old_name;
 	void *new_func;
+
 	/*
-	 * The old_addr field is optional and can be used to resolve
-	 * duplicate symbol names in the vmlinux object.  If this
-	 * information is not present, the symbol is located by name
-	 * with kallsyms. If the name is not unique and old_addr is
-	 * not provided, the patch application fails as there is no
-	 * way to resolve the ambiguity.
+	 * The old_sympos field is optional and can be used to resolve duplicate
+	 * symbol names in the vmlinux object.  If this information is not
+	 * present, the first symbol located with kallsyms is used. This value
+	 * corresponds to the nth occurrence of the symbol name in kallsyms for
+	 * the patched object. If the name is not unique and old_sympos is not
+	 * provided, the patch application fails as there is no way to resolve
+	 * the ambiguity.
 	 */
-	unsigned long old_addr;
+	unsigned long old_sympos;
 
 	/* internal */
+	unsigned long old_addr;
 	struct kobject kobj;
 	enum klp_state state;
 	struct list_head stack_node;
diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 6e53441..1dd0d44 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -142,6 +142,7 @@ struct klp_find_arg {
 	 * name in the same object.
 	 */
 	unsigned long count;
+	unsigned long pos;
 };
 
 static int klp_find_callback(void *data, const char *name,
@@ -166,28 +167,39 @@ static int klp_find_callback(void *data, const char *name,
 	args->addr = addr;
 	args->count++;
 
+	/*
+	 * ensure count matches the symbol position
+	 */
+	if (args->pos == (args->count-1))
+		return 1;
+
 	return 0;
 }
 
 static int klp_find_object_symbol(const char *objname, const char *name,
-				  unsigned long *addr)
+				  unsigned long *addr, unsigned long sympos)
 {
 	struct klp_find_arg args = {
 		.objname = objname,
 		.name = name,
 		.addr = 0,
-		.count = 0
+		.count = 0,
+		.pos = sympos,
 	};
 
 	mutex_lock(&module_mutex);
 	kallsyms_on_each_symbol(klp_find_callback, &args);
 	mutex_unlock(&module_mutex);
 
-	if (args.count == 0)
+	/*
+	 * Ensure an address was found, then check that the symbol position
+	 * count matches sympos.
+	 */
+	if (args.addr == 0)
 		pr_err("symbol '%s' not found in symbol table\n", name);
-	else if (args.count > 1)
-		pr_err("unresolvable ambiguity (%lu matches) on symbol '%s' in object '%s'\n",
-		       args.count, name, objname);
+	else if (sympos != (args.count - 1))
+		pr_err("symbol position %lu for symbol '%s' in object '%s' not found\n",
+		       sympos, name, objname ? objname : "vmlinux");
 	else {
 		*addr = args.addr;
 		return 0;
@@ -239,20 +251,19 @@ static int klp_verify_vmlinux_symbol(const char *name, unsigned long addr)
 static int klp_find_verify_func_addr(struct klp_object *obj,
 				     struct klp_func *func)
 {
+	int sympos = 0;
 	int ret;
 
-#if defined(CONFIG_RANDOMIZE_BASE)
-	/* If KASLR has been enabled, adjust old_addr accordingly */
-	if (kaslr_enabled() && func->old_addr)
-		func->old_addr += kaslr_offset();
-#endif
+	if (func->old_sympos && !klp_is_module(obj))
+		sympos = func->old_sympos;
 
-	if (!func->old_addr || klp_is_module(obj))
-		ret = klp_find_object_symbol(obj->name, func->old_name,
-					     &func->old_addr);
-	else
-		ret = klp_verify_vmlinux_symbol(func->old_name,
-						func->old_addr);
+	/*
+	 * Verify the symbol, find old_addr, and write it to the structure.
+	 * By default sympos will be 0 and thus will only look for the first
+	 * occurrence. If another value is specified then that will be used.
+	 */
+	ret = klp_find_object_symbol(obj->name, func->old_name,
+				     &func->old_addr, sympos);
 
 	return ret;
 }
@@ -277,7 +288,7 @@ static int klp_find_external_symbol(struct module *pmod, const char *name,
 	preempt_enable();
 
 	/* otherwise check if it's in another .o within the patch module */
-	return klp_find_object_symbol(pmod->name, name, addr);
+	return klp_find_object_symbol(pmod->name, name, addr, 0);
 }
 
 static int klp_write_object_relocations(struct module *pmod,
@@ -307,7 +318,7 @@ static int klp_write_object_relocations(struct module *pmod,
 			else
 				ret = klp_find_object_symbol(obj->mod->name,
 							     reloc->name,
-							     &reloc->val);
+							     &reloc->val, 0);
 			if (ret)
 				return ret;
 		}
@@ -587,7 +598,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch);
  * /sys/kernel/livepatch/<patch>
  * /sys/kernel/livepatch/<patch>/enabled
  * /sys/kernel/livepatch/<patch>/<object>
- * /sys/kernel/livepatch/<patch>/<object>/<func>
+ * /sys/kernel/livepatch/<patch>/<object>/<func,number>
  */
 
 static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr,
@@ -732,8 +743,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func)
 	INIT_LIST_HEAD(&func->stack_node);
 	func->state = KLP_DISABLED;
 
-	return kobject_init_and_add(&func->kobj, &klp_ktype_func,
-				    &obj->kobj, "%s", func->old_name);
+	return 0;
 }
 
 /* parts of the initialization that is done only when the object is loaded */
@@ -755,6 +765,18 @@ static int klp_init_object_loaded(struct klp_patch *patch,
 			return ret;
 	}
 
+	/*
+	 * for each function initialize and add, old_sympos will be already
+	 * verified at this point
+	 */
+	klp_for_each_func(obj, func) {
+		ret = kobject_init_and_add(&func->kobj, &klp_ktype_func,
+				    &obj->kobj, "%s,%lu", func->old_name,
+				    func->old_sympos ? func->old_sympos : 0);
+		if (ret)
+			return ret;
+	}
+
 	return 0;
 }
 
-- 
1.9.1