linux-kernel - [PATCH 7/7] modpost: use null string instead of NULL pointer for default namespace

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Date:   Sat, 11 Jun 2022 03:32:36 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     linux-kbuild@...r.kernel.org
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Nicolas Pitre <npitre@...libre.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-modules@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 7/7] modpost: use null string instead of NULL pointer for default namespace

The default namespace is the null string, "".

When set, the null string "" is converted to NULL:

  s->namespace = namespace[0] ? NOFAIL(strdup(namespace)) : NULL;

When printed, the NULL pointer is get back to the null string:

  sym->namespace ?: ""

This saves 1 byte memory allocated for "", but loses the readability.

In kernel-space, we strive to save memory, but modpost is a userspace
tool used to build the kernel. On modern systems, such small piece of
memory is not a big deal.

Handle the namespace string as is.

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 scripts/mod/modpost.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
index 0db2cbb74a2a..5a1785645943 100644
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -296,6 +296,13 @@ static bool contains_namespace(struct list_head *head, const char *namespace)
 {
 	struct namespace_list *list;
 
+	/*
+	 * The default namespace is null string "", which is always implicitly
+	 * contained.
+	 */
+	if (!namespace[0])
+		return true;
+
 	list_for_each_entry(list, head, list) {
 		if (!strcmp(list->namespace, namespace))
 			return true;
@@ -371,7 +378,7 @@ static struct symbol *sym_add_exported(const char *name, struct module *mod,
 	s = alloc_symbol(name);
 	s->module = mod;
 	s->is_gpl_only = gpl_only;
-	s->namespace = namespace[0] ? NOFAIL(strdup(namespace)) : NULL;
+	s->namespace = NOFAIL(strdup(namespace));
 	list_add_tail(&s->list, &mod->exported_symbols);
 	hash_add_symbol(s);
 
@@ -2117,8 +2124,7 @@ static void check_exports(struct module *mod)
 		else
 			basename = mod->name;
 
-		if (exp->namespace &&
-		    !contains_namespace(&mod->imported_namespaces, exp->namespace)) {
+		if (!contains_namespace(&mod->imported_namespaces, exp->namespace)) {
 			modpost_log(allow_missing_ns_imports ? LOG_WARN : LOG_ERROR,
 				    "module %s uses symbol %s from namespace %s, but does not import it.\n",
 				    basename, exp->name, exp->namespace);
@@ -2201,7 +2207,7 @@ static void add_exported_symbols(struct buffer *buf, struct module *mod)
 	list_for_each_entry(sym, &mod->exported_symbols, list)
 		buf_printf(buf, "KSYMTAB_ENTRY(%s, \"%s\", \"%s\");\n",
 			   sym->name, sym->is_gpl_only ? "_gpl" : "",
-			   sym->namespace ?: "");
+			   sym->namespace);
 
 	if (!modversions)
 		return;
@@ -2471,7 +2477,7 @@ static void write_dump(const char *fname)
 			buf_printf(&buf, "0x%08x\t%s\t%s\tEXPORT_SYMBOL%s\t%s\n",
 				   sym->crc, sym->name, mod->name,
 				   sym->is_gpl_only ? "_GPL" : "",
-				   sym->namespace ?: "");
+				   sym->namespace);
 		}
 	}
 	write_buf(&buf, fname);
-- 
2.32.0