kbuild: improved modversioning support for external modules

With following patch a second option is enabled to obtain
symbol information from a second external module when a
external module is build.
The recommended approach is to use a common kbuild file but
that may be impractical in certain cases.
With this patch one can copy over a Module.symvers from one
external module to make symbols (and symbol versions) available
for another external module.

Updated documentation in Documentation/kbuild/modules.txt

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

3 files changed, 158 insertions, 59 deletions

diff --git a/Documentation/kbuild/modules.txt b/Documentation/kbuild/modules.txt
index 87d858df4e3..fcccf2432f9 100644
--- a/Documentation/kbuild/modules.txt
+++ b/Documentation/kbuild/modules.txt
@@ -23,7 +23,10 @@ In this document you will find information about:
         === 6. Module installation
            --- 6.1 INSTALL_MOD_PATH
            --- 6.2 INSTALL_MOD_DIR
-        === 7. Module versioning
+        === 7. Module versioning & Module.symvers
+           --- 7.1 Symbols fron the kernel (vmlinux + modules)
+           --- 7.2 Symbols and external modules
+           --- 7.3 Symbols from another external module
         === 8. Tips & Tricks
            --- 8.1 Testing for CONFIG_FOO_BAR
 
@@ -89,7 +92,8 @@ when building an external module.
         make -C $KDIR M=$PWD modules_install
                 Install the external module(s).
                 Installation default is in /lib/modules/<kernel-version>/extra,
-                but may be prefixed with INSTALL_MOD_PATH - see separate chapter.
+                but may be prefixed with INSTALL_MOD_PATH - see separate
+                chapter.
 
         make -C $KDIR M=$PWD clean
                 Remove all generated files for the module - the kernel
@@ -433,7 +437,7 @@ External modules are installed in the directory:
                 => Install dir: /lib/modules/$(KERNELRELEASE)/gandalf
 
 
-=== 7. Module versioning
+=== 7. Module versioning & Module.symvers
 
 Module versioning is enabled by the CONFIG_MODVERSIONS tag.
 
@@ -443,11 +447,80 @@ when a module is loaded/used then the CRC values contained in the kernel are
 compared with similar values in the module. If they are not equal then the
 kernel refuses to load the module.
 
-During a kernel build a file named Module.symvers will be generated. This
-file includes the symbol version of all symbols within the kernel. If the 
-Module.symvers file is saved from the last full kernel compile one does not
-have to do a full kernel compile to build a module version's compatible module.
+Module.symvers contains a list of all exported symbols from a kernel build.
 
+--- 7.1 Symbols fron the kernel (vmlinux + modules)
+
+        During a kernel build a file named Module.symvers will be generated.
+        Module.symvers contains all exported symbols from the kernel and
+        compiled modules. For each symbols the corresponding CRC value
+        is stored too.
+
+        The syntax of the Module.symvers file is:
+                <CRC>       <Symbol>           <module>
+        Sample:
+                0x2d036834  scsi_remove_host   drivers/scsi/scsi_mod
+
+        For a kernel build without CONFIG_MODVERSIONING enabled the crc
+        would read: 0x00000000
+
+        Module.symvers serve two purposes.
+        1) It list all exported symbols both from vmlinux and all modules
+        2) It list CRC if CONFIG_MODVERSION is enabled
+
+--- 7.2 Symbols and external modules
+
+        When building an external module the build system needs access to
+        the symbols from the kernel to check if all external symbols are
+        defined. This is done in the MODPOST step and to obtain all
+        symbols modpost reads Module.symvers from the kernel.
+        If a Module.symvers file is present in the directory where
+        the external module is being build this file will be read too.
+        During the MODPOST step a new Module.symvers file will be written
+        containing all exported symbols that was not defined in the kernel.
+        
+--- 7.3 Symbols from another external module
+
+        Sometimes one external module uses exported symbols from another
+        external module. Kbuild needs to have full knowledge on all symbols
+        to avoid spitting out warnings about undefined symbols.
+        Two solutions exist to let kbuild know all symbols of more than
+        one external module.
+        The method with a top-level kbuild file is recommended but may be
+        impractical in certain situations.
+
+        Use a top-level Kbuild file
+                If you have two modules: 'foo', 'bar' and 'foo' needs symbols
+                from 'bar' then one can use a common top-level kbuild file so
+                both modules are compiled in same build.
+
+                Consider following directory layout:
+                ./foo/ <= contains the foo module
+                ./bar/ <= contains the bar module
+                The top-level Kbuild file would then look like:
+                
+                #./Kbuild: (this file may also be named Makefile)
+                        obj-y := foo/ bar/
+
+                Executing:
+                        make -C $KDIR M=`pwd`
+
+                will then do the expected and compile both modules with full
+                knowledge on symbols from both modules.
+
+        Use an extra Module.symvers file
+                When an external module is build a Module.symvers file is
+                generated containing all exported symbols which are not
+                defined in the kernel.
+                To get access to symbols from module 'bar' one can copy the
+                Module.symvers file from the compilation of the 'bar' module
+                to the directory where the 'foo' module is build.
+                During the module build kbuild will read the Module.symvers
+                file in the directory of the external module and when the
+                build is finished a new Module.symvers file is created
+                containing the sum of all symbols defined and not part of the
+                kernel.
+                
 === 8. Tips & Tricks
 
 --- 8.1 Testing for CONFIG_FOO_BAR
diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
index bf96a61d4b8..563e3c5bd8d 100644
--- a/scripts/Makefile.modpost
+++ b/scripts/Makefile.modpost
@@ -39,7 +39,8 @@ include .config
 include scripts/Kbuild.include
 include scripts/Makefile.lib
 
-symverfile := $(objtree)/Module.symvers
+kernelsymfile := $(objtree)/Module.symvers
+modulesymfile := $(KBUILD_EXTMOD)/Modules.symvers
 
 # Step 1), find all modules listed in $(MODVERDIR)/
 __modules := $(sort $(shell grep -h '\.ko' /dev/null $(wildcard $(MODVERDIR)/*.mod)))
@@ -54,7 +55,9 @@ quiet_cmd_modpost = MODPOST
       cmd_modpost = scripts/mod/modpost            \
         $(if $(CONFIG_MODVERSIONS),-m)             \
         $(if $(CONFIG_MODULE_SRCVERSION_ALL),-a,)  \
-        $(if $(KBUILD_EXTMOD),-i,-o) $(symverfile) \
+        $(if $(KBUILD_EXTMOD),-i,-o) $(kernelsymfile) \
+        $(if $(KBUILD_EXTMOD),-I $(modulesymfile)) \
+        $(if $(KBUILD_EXTMOD),-o $(modulesymfile)) \
         $(filter-out FORCE,$^)
 
 .PHONY: __modpost
diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 4a2f2e38d27..976adf152db 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -20,6 +20,8 @@ int modversions = 0;
 int have_vmlinux = 0;
 /* Is CONFIG_MODULE_SRCVERSION_ALL set? */
 static int all_versions = 0;
+/* If we are modposting external module set to 1 */
+static int external_module = 0;
 
 void fatal(const char *fmt, ...)
 {
@@ -45,6 +47,18 @@ void warn(const char *fmt, ...)
         va_end(arglist);
 }
 
+static int is_vmlinux(const char *modname)
+{
+        const char *myname;
+
+        if ((myname = strrchr(modname, '/')))
+                myname++;
+        else
+                myname = modname;
+
+        return strcmp(myname, "vmlinux") == 0;
+}
+
 void *do_nofail(void *ptr, const char *expr)
 {
         if (!ptr) {
@@ -100,6 +114,9 @@ struct symbol {
         unsigned int crc;
         int crc_valid;
         unsigned int weak:1;
+        unsigned int vmlinux:1;    /* 1 if symbol is defined in vmlinux */
+        unsigned int kernel:1;     /* 1 if symbol is from kernel
+                                    *  (only for external modules) **/
         char name[0];
 };
 
@@ -135,8 +152,7 @@ static struct symbol *alloc_symbol(const char *name, unsigned int weak,
 }
 
 /* For the hash of exported symbols */
-static void new_symbol(const char *name, struct module *module,
-                       unsigned int *crc)
+static struct symbol *new_symbol(const char *name, struct module *module)
 {
         unsigned int hash;
         struct symbol *new;
@@ -144,10 +160,7 @@ static void new_symbol(const char *name, struct module *module,
         hash = tdb_hash(name) % SYMBOL_HASH_SIZE;
         new = symbolhash[hash] = alloc_symbol(name, 0, symbolhash[hash]);
         new->module = module;
-        if (crc) {
-                new->crc = *crc;
-                new->crc_valid = 1;
-        }
+        return new;
 }
 
 static struct symbol *find_symbol(const char *name)
@@ -169,19 +182,27 @@ static struct symbol *find_symbol(const char *name)
  * Add an exported symbol - it may have already been added without a
  * CRC, in this case just update the CRC
  **/
-static void add_exported_symbol(const char *name, struct module *module,
-                                unsigned int *crc)
+static struct symbol *sym_add_exported(const char *name, struct module *mod)
 {
         struct symbol *s = find_symbol(name);
 
-        if (!s) {
-                new_symbol(name, module, crc);
-                return;
-        }
-        if (crc) {
-                s->crc = *crc;
-                s->crc_valid = 1;
-        }
+        if (!s)
+                s = new_symbol(name, mod);
+
+        s->vmlinux   = is_vmlinux(mod->name);
+        s->kernel    = 0;
+        return s;
+}
+
+static void sym_update_crc(const char *name, struct module *mod,
+                           unsigned int crc)
+{
+        struct symbol *s = find_symbol(name);
+
+        if (!s)
+                s = new_symbol(name, mod);
+        s->crc = crc;
+        s->crc_valid = 1;
 }
 
 void *grab_file(const char *filename, unsigned long *size)
@@ -332,8 +353,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
                 /* CRC'd symbol */
                 if (memcmp(symname, CRC_PFX, strlen(CRC_PFX)) == 0) {
                         crc = (unsigned int) sym->st_value;
-                        add_exported_symbol(symname + strlen(CRC_PFX),
-                                            mod, &crc);
+                        sym_update_crc(symname + strlen(CRC_PFX), mod, crc);
                 }
                 break;
         case SHN_UNDEF:
@@ -377,8 +397,7 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
         default:
                 /* All exported symbols */
                 if (memcmp(symname, KSYMTAB_PFX, strlen(KSYMTAB_PFX)) == 0) {
-                        add_exported_symbol(symname + strlen(KSYMTAB_PFX),
-                                            mod, NULL);
+                        sym_add_exported(symname + strlen(KSYMTAB_PFX), mod);
                 }
                 if (strcmp(symname, MODULE_SYMBOL_PREFIX "init_module") == 0)
                         mod->has_init = 1;
@@ -388,18 +407,6 @@ static void handle_modversions(struct module *mod, struct elf_info *info,
         }
 }
 
-static int is_vmlinux(const char *modname)
-{
-        const char *myname;
-
-        if ((myname = strrchr(modname, '/')))
-                myname++;
-        else
-                myname = modname;
-
-        return strcmp(myname, "vmlinux") == 0;
-}
-
 /**
  * Parse tag=value strings from .modinfo section
  **/
@@ -450,9 +457,7 @@ static void read_symbols(char *modname)
         /* When there's no vmlinux, don't print warnings about
          * unresolved symbols (since there'll be too many ;) */
         if (is_vmlinux(modname)) {
-                unsigned int fake_crc = 0;
                 have_vmlinux = 1;
-                add_exported_symbol("struct_module", mod, &fake_crc);
                 mod->skip = 1;
         }
 
@@ -665,7 +670,7 @@ static void write_if_changed(struct buffer *b, const char *fname)
         fclose(file);
 }
 
-static void read_dump(const char *fname)
+static void read_dump(const char *fname, unsigned int kernel)
 {
         unsigned long size, pos = 0;
         void *file = grab_file(fname, &size);
@@ -679,6 +684,7 @@ static void read_dump(const char *fname)
                 char *symname, *modname, *d;
                 unsigned int crc;
                 struct module *mod;
+                struct symbol *s;
 
                 if (!(symname = strchr(line, '\t')))
                         goto fail;
@@ -699,13 +705,28 @@ static void read_dump(const char *fname)
                         mod = new_module(NOFAIL(strdup(modname)));
                         mod->skip = 1;
                 }
-                add_exported_symbol(symname, mod, &crc);
+                s = sym_add_exported(symname, mod);
+                s->kernel = kernel;
+                sym_update_crc(symname, mod, crc);
         }
         return;
 fail:
         fatal("parse error in symbol dump file\n");
 }
 
+/* For normal builds always dump all symbols.
+ * For external modules only dump symbols
+ * that are not read from kernel Module.symvers.
+ **/
+static int dump_sym(struct symbol *sym)
+{
+        if (!external_module)
+                return 1;
+        if (sym->vmlinux || sym->kernel)
+                return 0;
+        return 1;
+}
+                
 static void write_dump(const char *fname)
 {
         struct buffer buf = { };
@@ -715,15 +736,10 @@ static void write_dump(const char *fname)
         for (n = 0; n < SYMBOL_HASH_SIZE ; n++) {
                 symbol = symbolhash[n];
                 while (symbol) {
-                        symbol = symbol->next;
-                }
-        }
-
-        for (n = 0; n < SYMBOL_HASH_SIZE ; n++) {
-                symbol = symbolhash[n];
-                while (symbol) {
-                        buf_printf(&buf, "0x%08x\t%s\t%s\n", symbol->crc,
-                                symbol->name, symbol->module->name);
+                        if (dump_sym(symbol))
+                                buf_printf(&buf, "0x%08x\t%s\t%s\n",
+                                        symbol->crc, symbol->name, 
+                                        symbol->module->name);
                         symbol = symbol->next;
                 }
         }
@@ -735,13 +751,18 @@ int main(int argc, char **argv)
         struct module *mod;
         struct buffer buf = { };
         char fname[SZ];
-        char *dump_read = NULL, *dump_write = NULL;
+        char *kernel_read = NULL, *module_read = NULL;
+        char *dump_write = NULL;
         int opt;
 
-        while ((opt = getopt(argc, argv, "i:mo:a")) != -1) {
+        while ((opt = getopt(argc, argv, "i:I:mo:a")) != -1) {
                 switch(opt) {
                         case 'i':
-                                dump_read = optarg;
+                                kernel_read = optarg;
+                                break;
+                        case 'I':
+                                module_read = optarg;
+                                external_module = 1;
                                 break;
                         case 'm':
                                 modversions = 1;
@@ -757,8 +778,10 @@ int main(int argc, char **argv)
                 }
         }
 
-        if (dump_read)
-                read_dump(dump_read);
+        if (kernel_read)
+                read_dump(kernel_read, 1);
+        if (module_read)
+                read_dump(module_read, 0);
 
         while (optind < argc) {
                 read_symbols(argv[optind++]);