mm/util: deduplicate code in {kstrdup,kstrndup,kmemdup_nul}

These three functions follow the same pattern.  To deduplicate the code,
let's introduce a common helper __kmemdup_nul().

Link: https://lkml.kernel.org/r/20241007144911.27693-7-laoar.shao@gmail.com
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Simon Horman <horms@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Alejandro Colomar <alx@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Matus Jokay <matus.jokay@stuba.sk>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Ondrej Mosnacek <omosnace@redhat.com>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Quentin Monnet <qmo@kernel.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/util.c

@@ -44,6 +44,30 @@ void kfree_const(const void *x)
 }
 EXPORT_SYMBOL(kfree_const);
 
+/**
+ * __kmemdup_nul - Create a NUL-terminated string from @s, which might be unterminated.
+ * @s: The data to copy
+ * @len: The size of the data, not including the NUL terminator
+ * @gfp: the GFP mask used in the kmalloc() call when allocating memory
+ *
+ * Return: newly allocated copy of @s with NUL-termination or %NULL in
+ * case of error
+ */
+static __always_inline char *__kmemdup_nul(const char *s, size_t len, gfp_t gfp)
+{
+	char *buf;
+
+	/* '+1' for the NUL terminator */
+	buf = kmalloc_track_caller(len + 1, gfp);
+	if (!buf)
+		return NULL;
+
+	memcpy(buf, s, len);
+	/* Ensure the buf is always NUL-terminated, regardless of @s. */
+	buf[len] = '\0';
+	return buf;
+}
+
 /**
  * kstrdup - allocate space for and copy an existing string
  * @s: the string to duplicate
@@ -54,24 +78,7 @@ EXPORT_SYMBOL(kfree_const);
 noinline
 char *kstrdup(const char *s, gfp_t gfp)
 {
-	size_t len;
+	return s ? __kmemdup_nul(s, strlen(s), gfp) : NULL;
-	char *buf;
-
-	if (!s)
-		return NULL;
-
-	len = strlen(s) + 1;
-	buf = kmalloc_track_caller(len, gfp);
-	if (buf) {
-		memcpy(buf, s, len);
-		/*
-		 * During memcpy(), the string might be updated to a new value,
-		 * which could be longer than the string when strlen() is
-		 * called. Therefore, we need to add a NUL terminator.
-		 */
-		buf[len - 1] = '\0';
-	}
-	return buf;
 }
 EXPORT_SYMBOL(kstrdup);
 
@@ -107,19 +114,7 @@ EXPORT_SYMBOL(kstrdup_const);
  */
 char *kstrndup(const char *s, size_t max, gfp_t gfp)
 {
-	size_t len;
+	return s ? __kmemdup_nul(s, strnlen(s, max), gfp) : NULL;
-	char *buf;
-
-	if (!s)
-		return NULL;
-
-	len = strnlen(s, max);
-	buf = kmalloc_track_caller(len+1, gfp);
-	if (buf) {
-		memcpy(buf, s, len);
-		buf[len] = '\0';
-	}
-	return buf;
 }
 EXPORT_SYMBOL(kstrndup);
 
@@ -193,17 +188,7 @@ EXPORT_SYMBOL(kvmemdup);
  */
 char *kmemdup_nul(const char *s, size_t len, gfp_t gfp)
 {
-	char *buf;
+	return s ? __kmemdup_nul(s, len, gfp) : NULL;
-
-	if (!s)
-		return NULL;
-
-	buf = kmalloc_track_caller(len + 1, gfp);
-	if (buf) {
-		memcpy(buf, s, len);
-		buf[len] = '\0';
-	}
-	return buf;
 }
 EXPORT_SYMBOL(kmemdup_nul);