kbuild: check license compatibility when building modules

Modules that uses GPL symbols can no longer be build with kbuild,
the build will fail during the modpost step.
When a GPL-incompatible module uses a EXPORT_SYMBOL_GPL_FUTURE symbol
then warn during modpost so author are actually notified.

The actual license compatibility check is shared with the kernel
to make sure it is in sync.

Patch originally from: Andreas Gruenbacher <[e-mail]> and
Ram Pai <[e-mail]>

Signed-off-by: Sam Ravnborg <[e-mail]>

include/linux/license.h

···
+#ifndef __LICENSE_H
+#define __LICENSE_H
+
+static inline int license_is_gpl_compatible(const char *license)
+{
+	return (strcmp(license, "GPL") == 0
+		|| strcmp(license, "GPL v2") == 0
+		|| strcmp(license, "GPL and additional rights") == 0
+		|| strcmp(license, "Dual BSD/GPL") == 0
+		|| strcmp(license, "Dual MIT/GPL") == 0
+		|| strcmp(license, "Dual MPL/GPL") == 0);
+}
+
+#endif

kernel/module.c

···
 #include <asm/uaccess.h>
 #include <asm/semaphore.h>
 #include <asm/cacheflush.h>
+#include <linux/license.h>
 
 #if 0
 #define DEBUGP printk
···
 		if (m == 0)
 			mod->init_text_size = mod->init_size;
 	}
-}
-
-static inline int license_is_gpl_compatible(const char *license)
-{
-	return (strcmp(license, "GPL") == 0
-		|| strcmp(license, "GPL v2") == 0
-		|| strcmp(license, "GPL and additional rights") == 0
-		|| strcmp(license, "Dual BSD/GPL") == 0
-		|| strcmp(license, "Dual MIT/GPL") == 0
-		|| strcmp(license, "Dual MPL/GPL") == 0);
 }
 
 static void set_license(struct module *mod, const char *license)

scripts/mod/modpost.c

···
 
 #include <ctype.h>
 #include "modpost.h"
+#include "../../include/linux/license.h"
 
 /* Are we using CONFIG_MODVERSIONS? */
 int modversions = 0;
···
 
 	/* add to list */
 	mod->name = p;
+	mod->gpl_compatible = -1;
 	mod->next = modules;
 	modules = mod;
 
···
 	return string;
 }
 
-static char *get_modinfo(void *modinfo, unsigned long modinfo_len,
-			 const char *tag)
+static char *get_next_modinfo(void *modinfo, unsigned long modinfo_len,
+			      const char *tag, char *info)
 {
 	char *p;
 	unsigned int taglen = strlen(tag);
 	unsigned long size = modinfo_len;
+
+	if (info) {
+		size -= info - (char *)modinfo;
+		modinfo = next_string(info, &size);
+	}
 
 	for (p = modinfo; p; p = next_string(p, &size)) {
 		if (strncmp(p, tag, taglen) == 0 && p[taglen] == '=')
 			return p + taglen + 1;
 	}
 	return NULL;
+}
+
+static char *get_modinfo(void *modinfo, unsigned long modinfo_len,
+			 const char *tag)
+
+{
+	return get_next_modinfo(modinfo, modinfo_len, tag, NULL);
 }
 
 /**
···
 {
 	const char *symname;
 	char *version;
+	char *license;
 	struct module *mod;
 	struct elf_info info = { };
 	Elf_Sym *sym;
···
 	if (is_vmlinux(modname)) {
 		have_vmlinux = 1;
 		mod->skip = 1;
+	}
+
+	license = get_modinfo(info.modinfo, info.modinfo_len, "license");
+	while (license) {
+		if (license_is_gpl_compatible(license))
+			mod->gpl_compatible = 1;
+		else {
+			mod->gpl_compatible = 0;
+			break;
+		}
+		license = get_next_modinfo(info.modinfo, info.modinfo_len,
+					   "license", license);
 	}
 
 	for (sym = info.symtab_start; sym < info.symtab_stop; sym++) {
···
 	}
 	strncpy(buf->p + buf->pos, s, len);
 	buf->pos += len;
+}
+
+void check_license(struct module *mod)
+{
+	struct symbol *s, *exp;
+
+	for (s = mod->unres; s; s = s->next) {
+		if (mod->gpl_compatible == 1) {
+			/* GPL-compatible modules may use all symbols */
+			continue;
+		}
+		exp = find_symbol(s->name);
+		if (!exp || exp->module == mod)
+			continue;
+		const char *basename = strrchr(mod->name, '/');
+		if (basename)
+			basename++;
+		switch (exp->export) {
+			case export_gpl:
+				fatal("modpost: GPL-incompatible module %s "
+				      "uses GPL-only symbol '%s'\n",
+				 basename ? basename : mod->name,
+				exp->name);
+				break;
+			case export_gpl_future:
+				warn("modpost: GPL-incompatible module %s "
+				      "uses future GPL-only symbol '%s'\n",
+				      basename ? basename : mod->name,
+				      exp->name);
+				break;
+			case export_plain: /* ignore */ break;
+			case export_unknown: /* ignore */ break;
+		}
+        }
 }
 
 /**
···
 
 	while (optind < argc) {
 		read_symbols(argv[optind++]);
+	}
+
+	for (mod = modules; mod; mod = mod->next) {
+		if (mod->skip)
+			continue;
+		check_license(mod);
 	}
 
 	for (mod = modules; mod; mod = mod->next) {

scripts/mod/modpost.h

···
 struct module {
 	struct module *next;
 	const char *name;
+	int gpl_compatible;
 	struct symbol *unres;
 	int seen;
 	int skip;