allow patched modules to call external functions

When patching a kernel module, if we can't find a needed dynrela symbol,
we currently assume it's exported.  However, it's also possible that
it's provided by another .o in the patch module.  Add support for that.

Fixes #445.

kmod/core/core.c

@@ -534,16 +534,6 @@ static int kpatch_verify_symbol_match(const char *name, unsigned long addr)
 	return 0;
 }
 
-static unsigned long kpatch_find_exported_symbol(const char *name)
-{
-	const struct kernel_symbol *sym;
-
-	preempt_disable();
-	sym = find_symbol(name, NULL, NULL, true, true);
-	preempt_enable();
-	return sym ? sym->value : 0;
-}
-
 static unsigned long kpatch_find_module_symbol(struct module *mod,
 					       const char *name)
 {
@@ -564,6 +554,26 @@ static unsigned long kpatch_find_module_symbol(struct module *mod,
 	return kallsyms_lookup_name(buf);
 }
 
+/*
+ * External symbols are located outside the parent object (where the parent
+ * object is either vmlinux or the kmod being patched).
+ */
+static unsigned long kpatch_find_external_symbol(struct kpatch_module *kpmod,
+						 const char *name)
+{
+	const struct kernel_symbol *sym;
+
+	/* first, check if it's an exported symbol */
+	preempt_disable();
+	sym = find_symbol(name, NULL, NULL, true, true);
+	preempt_enable();
+	if (sym)
+		return sym->value;
+
+	/* otherwise check if it's in another .o within the patch module */
+	return kpatch_find_module_symbol(kpmod->mod, name);
+}
+
 static int kpatch_write_relocations(struct kpatch_module *kpmod,
 				    struct kpatch_object *object)
 {
@@ -584,8 +594,9 @@ static int kpatch_write_relocations(struct kpatch_module *kpmod,
 		} else {
 			/* module, dynrela->src needs to be discovered */
 
-			if (dynrela->exported)
+			if (dynrela->external)
-				src = kpatch_find_exported_symbol(dynrela->name);
+				src = kpatch_find_external_symbol(kpmod,
+								  dynrela->name);
 			else
 				src = kpatch_find_module_symbol(object->mod,
 								dynrela->name);

kmod/core/kpatch.h

@@ -53,7 +53,7 @@ struct kpatch_dynrela {
 	unsigned long type;
 	const char *name;
 	int addend;
-	int exported;
+	int external;
 	struct list_head list;
 };
 

kmod/patch/kpatch-patch-hook.c

@@ -318,7 +318,7 @@ static int patch_make_dynrelas_list(struct list_head *objects)
 		dynrela->src = p_dynrela->src;
 		dynrela->type = p_dynrela->type;
 		dynrela->name = p_dynrela->name;
-		dynrela->exported = p_dynrela->exported;
+		dynrela->external = p_dynrela->external;
 		dynrela->addend = p_dynrela->addend;
 		list_add_tail(&dynrela->list, &object->dynrelas);
 	}

kmod/patch/kpatch-patch.h

@@ -37,7 +37,7 @@ struct kpatch_patch_dynrela {
 	unsigned long type;
 	char *name;
 	char *objname;
-	int exported;
+	int external;
 	int addend;
 };
 

kpatch-build/create-diff-object.c

@@ -2213,7 +2213,7 @@ void kpatch_create_dynamic_rela_sections(struct kpatch_elf *kelf,
 	struct symbol *strsym;
 	struct lookup_result result;
 	struct kpatch_patch_dynrela *dynrelas;
-	int vmlinux, exported;
+	int vmlinux, external;
 
 	vmlinux = !strcmp(objname, "vmlinux");
 
@@ -2261,7 +2261,7 @@ void kpatch_create_dynamic_rela_sections(struct kpatch_elf *kelf,
 			if (kpatch_is_core_module_symbol(rela->sym->name))
 				continue;
 
-			exported = 0;
+			external = 0;
 
 			if (rela->sym->bind == STB_LOCAL) {
 				/* An unchanged local symbol */
@@ -2310,10 +2310,11 @@ void kpatch_create_dynamic_rela_sections(struct kpatch_elf *kelf,
 				if (lookup_global_symbol(table, rela->sym->name,
 							 &result))
 					/*
-					 * Not there, assume it's exported by
+					 * Not there, assume it's either an
-					 * another object.
+					 * exported symbol or provided by
+					 * another .o in the patch module.
 					 */
-					exported = 1;
+					external = 1;
 			}
 			log_debug("lookup for %s @ 0x%016lx len %lu\n",
 			          rela->sym->name, result.value, result.size);
@@ -2326,7 +2327,7 @@ void kpatch_create_dynamic_rela_sections(struct kpatch_elf *kelf,
 				dynrelas[index].src = 0;
 			dynrelas[index].addend = rela->addend;
 			dynrelas[index].type = rela->type;
-			dynrelas[index].exported = exported;
+			dynrelas[index].external = external;
 
 			/* add rela to fill in dest field */
 			ALLOC_LINK(dynrela, &relasec->relas);

test/integration/module-call-external.patch

@@ -0,0 +1,35 @@
+Index: src/fs/nfsd/export.c
+===================================================================
+--- src.orig/fs/nfsd/export.c
++++ src/fs/nfsd/export.c
+@@ -1241,6 +1241,8 @@ static void exp_flags(struct seq_file *m
+ 	}
+ }
+ 
++extern char *kpatch_string(void);
++
+ static int e_show(struct seq_file *m, void *p)
+ {
+ 	struct cache_head *cp = p;
+@@ -1250,6 +1252,7 @@ static int e_show(struct seq_file *m, vo
+ 	if (p == SEQ_START_TOKEN) {
+ 		seq_puts(m, "# Version 1.1\n");
+ 		seq_puts(m, "# Path Client(Flags) # IPs\n");
++		seq_puts(m, kpatch_string());
+ 		return 0;
+ 	}
+ 
+Index: src/net/netlink/af_netlink.c
+===================================================================
+--- src.orig/net/netlink/af_netlink.c
++++ src/net/netlink/af_netlink.c
+@@ -3228,4 +3228,9 @@ panic:
+ 	panic("netlink_init: Cannot allocate nl_table\n");
+ }
+ 
++char *kpatch_string(void)
++{
++	return "# kpatch\n";
++}
++
+ core_initcall(netlink_proto_init);