linux-kernel - [PATCH 2/2] kobject: use kvasprintf

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Date:	Wed,  9 Sep 2015 23:45:52 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] kobject: use kvasprintf_const for formatting ->name

Sometimes kobject_set_name_vargs is called with a format string
conaining no %, or a format string of precisely "%s", where the single
vararg happens to point to .rodata. kvasprintf_const detects these
cases for us and returns a copy of that pointer instead of duplicating
the string, thus saving some run-time memory. Otherwise, it falls back
to kvasprintf. We just need to always deallocate ->name using
kfree_const.

Unfortunately, the dance we need to do to perform the '/' -> '!'
sanitization makes the resulting code rather ugly.

I instrumented kstrdup_const to provide some statistics on the memory
saved, and for me this gave an additional ~14KB after boot (306KB was
already saved; this patch bumped that to 320KB). I have
KMALLOC_SHIFT_LOW==3, and since 80% of the kvasprintf_const hits were
satisfied by an 8-byte allocation, the 14K would roughly be quadrupled
when KMALLOC_SHIFT_LOW==5. Whether these numbers are sufficient to
justify the ugliness I'll leave to others to decide.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
 lib/kobject.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/lib/kobject.c b/lib/kobject.c
index 3e3a5c3cb330..fee2fd950306 100644
--- a/lib/kobject.c
+++ b/lib/kobject.c
@@ -257,18 +257,32 @@ static int kobject_add_internal(struct kobject *kobj)
 int kobject_set_name_vargs(struct kobject *kobj, const char *fmt,
 				  va_list vargs)
 {
-	char *s;
+	const char *s;
 
 	if (kobj->name && !fmt)
 		return 0;
 
-	s = kvasprintf(GFP_KERNEL, fmt, vargs);
+	s = kvasprintf_const(GFP_KERNEL, fmt, vargs);
 	if (!s)
 		return -ENOMEM;
 
-	/* ewww... some of these buggers have '/' in the name ... */
-	strreplace(s, '/', '!');
-	kfree(kobj->name);
+	/*
+	 * ewww... some of these buggers have '/' in the name ... If
+	 * that's the case, we need to make sure we have an actual
+	 * allocated copy to modify, since kvasprintf_const may have
+	 * returned something from .rodata.
+	 */
+	if (strchr(s, '/')) {
+		char *t;
+
+		t = kstrdup(s, GFP_KERNEL);
+		kfree_const(s);
+		if (!t)
+			return -ENOMEM;
+		strreplace(t, '/', '!');
+		s = t;
+	}
+	kfree_const(kobj->name);
 	kobj->name = s;
 
 	return 0;
@@ -466,7 +480,7 @@ int kobject_rename(struct kobject *kobj, const char *new_name)
 	envp[0] = devpath_string;
 	envp[1] = NULL;
 
-	name = dup_name = kstrdup(new_name, GFP_KERNEL);
+	name = dup_name = kstrdup_const(new_name, GFP_KERNEL);
 	if (!name) {
 		error = -ENOMEM;
 		goto out;
@@ -486,7 +500,7 @@ int kobject_rename(struct kobject *kobj, const char *new_name)
 	kobject_uevent_env(kobj, KOBJ_MOVE, envp);
 
 out:
-	kfree(dup_name);
+	kfree_const(dup_name);
 	kfree(devpath_string);
 	kfree(devpath);
 	kobject_put(kobj);
@@ -632,7 +646,7 @@ static void kobject_cleanup(struct kobject *kobj)
 	/* free name if we allocated it */
 	if (name) {
 		pr_debug("kobject: '%s': free name\n", name);
-		kfree(name);
+		kfree_const(name);
 	}
 }
 
-- 
2.1.3

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