kbuild: generate KSYMTAB entries by modpost

Commit 7b4537199a4a ("kbuild: link symbol CRCs at final link, removing
CONFIG_MODULE_REL_CRCS") made modpost output CRCs in the same way
whether the EXPORT_SYMBOL() is placed in *.c or *.S.

For further cleanups, this commit applies a similar approach to the
entire data structure of EXPORT_SYMBOL().

The EXPORT_SYMBOL() compilation is split into two stages.

When a source file is compiled, EXPORT_SYMBOL() will be converted into
a dummy symbol in the .export_symbol section.

For example,

    EXPORT_SYMBOL(foo);
    EXPORT_SYMBOL_NS_GPL(bar, BAR_NAMESPACE);

will be encoded into the following assembly code:

    .section ".export_symbol","a"
    __export_symbol_foo:
            .asciz ""                      /* license */
            .asciz ""                      /* name space */
            .balign 8
            .quad foo                      /* symbol reference */
    .previous

    .section ".export_symbol","a"
    __export_symbol_bar:
            .asciz "GPL"                   /* license */
            .asciz "BAR_NAMESPACE"         /* name space */
            .balign 8
            .quad bar                      /* symbol reference */
    .previous

They are mere markers to tell modpost the name, license, and namespace
of the symbols. They will be dropped from the final vmlinux and modules
because the *(.export_symbol) will go into /DISCARD/ in the linker script.

Then, modpost extracts all the information about EXPORT_SYMBOL() from the
.export_symbol section, and generates the final C code:

    KSYMTAB_FUNC(foo, "", "");
    KSYMTAB_FUNC(bar, "_gpl", "BAR_NAMESPACE");

KSYMTAB_FUNC() (or KSYMTAB_DATA() if it is data) is expanded to struct
kernel_symbol that will be linked to the vmlinux or a module.

With this change, EXPORT_SYMBOL() works in the same way for *.c and *.S
files, providing the following benefits.

[1] Deprecate EXPORT_DATA_SYMBOL()

In the old days, EXPORT_SYMBOL() was only available in C files. To export
a symbol in *.S, EXPORT_SYMBOL() was placed in a separate *.c file.
arch/arm/kernel/armksyms.c is one example written in the classic manner.

Commit 22823ab419d8 ("EXPORT_SYMBOL() for asm") removed this limitation.
Since then, EXPORT_SYMBOL() can be placed close to the symbol definition
in *.S files. It was a nice improvement.

However, as that commit mentioned, you need to use EXPORT_DATA_SYMBOL()
for data objects on some architectures.

In the new approach, modpost checks symbol's type (STT_FUNC or not),
and outputs KSYMTAB_FUNC() or KSYMTAB_DATA() accordingly.

There are only two users of EXPORT_DATA_SYMBOL:

  EXPORT_DATA_SYMBOL_GPL(empty_zero_page)    (arch/ia64/kernel/head.S)
  EXPORT_DATA_SYMBOL(ia64_ivt)               (arch/ia64/kernel/ivt.S)

They are transformed as follows and output into .vmlinux.export.c

  KSYMTAB_DATA(empty_zero_page, "_gpl", "");
  KSYMTAB_DATA(ia64_ivt, "", "");

The other EXPORT_SYMBOL users in ia64 assembly are output as
KSYMTAB_FUNC().

EXPORT_DATA_SYMBOL() is now deprecated.

[2] merge <linux/export.h> and <asm-generic/export.h>

There are two similar header implementations:

  include/linux/export.h        for .c files
  include/asm-generic/export.h  for .S files

Ideally, the functionality should be consistent between them, but they
tend to diverge.

Commit 8651ec01daed ("module: add support for symbol namespaces.") did
not support the namespace for *.S files.

This commit shifts the essential implementation part to C, which supports
EXPORT_SYMBOL_NS() for *.S files.

<asm/export.h> and <asm-generic/export.h> will remain as a wrapper of
<linux/export.h> for a while.

They will be removed after #include <asm/export.h> directives are all
replaced with #include <linux/export.h>.

[3] Implement CONFIG_TRIM_UNUSED_KSYMS in one-pass algorithm (by a later commit)

When CONFIG_TRIM_UNUSED_KSYMS is enabled, Kbuild recursively traverses
the directory tree to determine which EXPORT_SYMBOL to trim. If an
EXPORT_SYMBOL turns out to be unused by anyone, Kbuild begins the
second traverse, where some source files are recompiled with their
EXPORT_SYMBOL() tuned into a no-op.

We can do this better now; modpost can selectively emit KSYMTAB entries
that are really used by modules.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

2 files changed, 74 insertions, 33 deletions

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 403ba4d923f5..ce37e6de5df7 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -217,6 +217,7 @@ struct symbol {
 	unsigned int crc;
 	bool crc_valid;
 	bool weak;
+	bool is_func;
 	bool is_gpl_only;	/* exported by EXPORT_SYMBOL_GPL */
 	char name[];
 };
@@ -533,6 +534,8 @@ static int parse_elf(struct elf_info *info, const char *filename)
 				fatal("%s has NOBITS .modinfo\n", filename);
 			info->modinfo = (void *)hdr + sechdrs[i].sh_offset;
 			info->modinfo_len = sechdrs[i].sh_size;
+		} else if (!strcmp(secname, ".export_symbol")) {
+			info->export_symbol_secndx = i;
 		}
 
 		if (sechdrs[i].sh_type == SHT_SYMTAB) {
@@ -655,18 +658,6 @@ static void handle_symbol(struct module *mod, struct elf_info *info,
 				   ELF_ST_BIND(sym->st_info) == STB_WEAK);
 		break;
 	default:
-		/* All exported symbols */
-		if (strstarts(symname, "__ksymtab_")) {
-			const char *name, *secname;
-
-			name = symname + strlen("__ksymtab_");
-			secname = sec_name(info, get_secindex(info, sym));
-
-			if (strstarts(secname, "___ksymtab_gpl+"))
-				sym_add_exported(name, mod, true);
-			else if (strstarts(secname, "___ksymtab+"))
-				sym_add_exported(name, mod, false);
-		}
 		if (strcmp(symname, "init_module") == 0)
 			mod->has_init = true;
 		if (strcmp(symname, "cleanup_module") == 0)
@@ -848,7 +839,6 @@ enum mismatch {
 	XXXEXIT_TO_SOME_EXIT,
 	ANY_INIT_TO_ANY_EXIT,
 	ANY_EXIT_TO_ANY_INIT,
-	EXPORT_TO_INIT_EXIT,
 	EXTABLE_TO_NON_TEXT,
 };
 
@@ -920,12 +910,6 @@ static const struct sectioncheck sectioncheck[] = {
 	.bad_tosec = { INIT_SECTIONS, NULL },
 	.mismatch = ANY_INIT_TO_ANY_EXIT,
 },
-/* Do not export init/exit functions or data */
-{
-	.fromsec = { "___ksymtab*", NULL },
-	.bad_tosec = { INIT_SECTIONS, EXIT_SECTIONS, NULL },
-	.mismatch = EXPORT_TO_INIT_EXIT,
-},
 {
 	.fromsec = { "__ex_table", NULL },
 	/* If you're adding any new black-listed sections in here, consider
@@ -1180,10 +1164,6 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
 		warn("%s: section mismatch in reference: %s (section: %s) -> %s (section: %s)\n",
 		     modname, fromsym, fromsec, tosym, tosec);
 		break;
-	case EXPORT_TO_INIT_EXIT:
-		warn("%s: EXPORT_SYMBOL used for init/exit symbol: %s (section: %s)\n",
-		     modname, tosym, tosec);
-		break;
 	case EXTABLE_TO_NON_TEXT:
 		warn("%s(%s+0x%lx): Section mismatch in reference to the %s:%s\n",
 		     modname, fromsec, (long)faddr, tosec, tosym);
@@ -1211,14 +1191,75 @@ static void default_mismatch_handler(const char *modname, struct elf_info *elf,
 	}
 }
 
+static void check_export_symbol(struct module *mod, struct elf_info *elf,
+				Elf_Addr faddr, const char *secname,
+				Elf_Sym *sym)
+{
+	static const char *prefix = "__export_symbol_";
+	const char *label_name, *name, *data;
+	Elf_Sym *label;
+	struct symbol *s;
+	bool is_gpl;
+
+	label = find_fromsym(elf, faddr, elf->export_symbol_secndx);
+	label_name = sym_name(elf, label);
+
+	if (!strstarts(label_name, prefix)) {
+		error("%s: .export_symbol section contains strange symbol '%s'\n",
+		      mod->name, label_name);
+		return;
+	}
+
+	name = sym_name(elf, sym);
+	if (strcmp(label_name + strlen(prefix), name)) {
+		error("%s: .export_symbol section references '%s', but it does not seem to be an export symbol\n",
+		      mod->name, name);
+		return;
+	}
+
+	data = sym_get_data(elf, label);	/* license */
+	if (!strcmp(data, "GPL")) {
+		is_gpl = true;
+	} else if (!strcmp(data, "")) {
+		is_gpl = false;
+	} else {
+		error("%s: unknown license '%s' was specified for '%s'\n",
+		      mod->name, data, name);
+		return;
+	}
+
+	data += strlen(data) + 1;	/* namespace */
+	s = sym_add_exported(name, mod, is_gpl);
+	sym_update_namespace(name, data);
+
+	/*
+	 * We need to be aware whether we are exporting a function or
+	 * a data on some architectures.
+	 */
+	s->is_func = (ELF_ST_TYPE(sym->st_info) == STT_FUNC);
+
+	if (match(secname, PATTERNS(INIT_SECTIONS)))
+		warn("%s: %s: EXPORT_SYMBOL used for init symbol. Remove __init or EXPORT_SYMBOL.\n",
+		     mod->name, name);
+	else if (match(secname, PATTERNS(EXIT_SECTIONS)))
+		warn("%s: %s: EXPORT_SYMBOL used for exit symbol. Remove __exit or EXPORT_SYMBOL.\n",
+		     mod->name, name);
+}
+
 static void check_section_mismatch(struct module *mod, struct elf_info *elf,
 				   Elf_Sym *sym,
 				   unsigned int fsecndx, const char *fromsec,
 				   Elf_Addr faddr, Elf_Addr taddr)
 {
 	const char *tosec = sec_name(elf, get_secindex(elf, sym));
-	const struct sectioncheck *mismatch = section_mismatch(fromsec, tosec);
+	const struct sectioncheck *mismatch;
+
+	if (elf->export_symbol_secndx == fsecndx) {
+		check_export_symbol(mod, elf, faddr, tosec, sym);
+		return;
+	}
 
+	mismatch = section_mismatch(fromsec, tosec);
 	if (!mismatch)
 		return;
 
@@ -1698,15 +1739,6 @@ static void read_symbols(const char *modname)
 		handle_moddevtable(mod, &info, sym, symname);
 	}
 
-	for (sym = info.symtab_start; sym < info.symtab_stop; sym++) {
-		symname = remove_dot(info.strtab + sym->st_name);
-
-		/* Apply symbol namespaces from __kstrtabns_<symbol> entries. */
-		if (strstarts(symname, "__kstrtabns_"))
-			sym_update_namespace(symname + strlen("__kstrtabns_"),
-					     sym_get_data(&info, sym));
-	}
-
 	check_sec_ref(mod, &info);
 
 	if (!mod->is_vmlinux) {
@@ -1890,6 +1922,14 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
 {
 	struct symbol *sym;
 
+	/* generate struct for exported symbols */
+	buf_printf(buf, "\n");
+	list_for_each_entry(sym, &mod->exported_symbols, list)
+		buf_printf(buf, "KSYMTAB_%s(%s, \"%s\", \"%s\");\n",
+			   sym->is_func ? "FUNC" : "DATA", sym->name,
+			   sym->is_gpl_only ? "_gpl" : "",
+			   sym->namespace ?: "");
+
 	if (!modversions)
 		return;
 
diff --git a/scripts/mod/modpost.h b/scripts/mod/modpost.h
index b1e2d95f8047..dfdb9484e325 100644
--- a/scripts/mod/modpost.h
+++ b/scripts/mod/modpost.h
@@ -137,6 +137,7 @@ struct elf_info {
 	Elf_Shdr     *sechdrs;
 	Elf_Sym      *symtab_start;
 	Elf_Sym      *symtab_stop;
+	unsigned int export_symbol_secndx;	/* .export_symbol section */
 	char         *strtab;
 	char	     *modinfo;
 	unsigned int modinfo_len;