linux-kernel - Re: [PATCH 1/2] remove all uses of printf's %n

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Date:	Mon, 16 Sep 2013 20:41:33 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	keescook@...omium.org, linux-kernel@...r.kernel.org
Cc:	joe@...ches.com, linux@...izon.com, dan.carpenter@...cle.com,
	viro@...iv.linux.org.uk, JBeulich@...e.com,
	kosaki.motohiro@...il.com, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/2] remove all uses of printf's %n

Kees Cook wrote:
> -	seq_printf(m, "%s%d%n", con->name, con->index, &len);
> -	len = 21 - len;
> +	len = m->count;
> +	seq_printf(m, "%s%d", con->name, con->index);
> +	len = 21 - (m->count - len);

Why not to create a new function which returns bytes written?
The new function does not need to return negative value for indicating errors.
---------- patch start ----------
diff --git a/include/linux/seq_file.h b/include/linux/seq_file.h
index 4e32edc..c889cf1 100644
--- a/include/linux/seq_file.h
+++ b/include/linux/seq_file.h
@@ -91,6 +91,7 @@ int seq_write(struct seq_file *seq, const void *data, size_t len);
 
 __printf(2, 3) int seq_printf(struct seq_file *, const char *, ...);
 __printf(2, 0) int seq_vprintf(struct seq_file *, const char *, va_list args);
+__printf(2, 3) int seq_new_printf(struct seq_file *m, const char *f, ...);
 
 int seq_path(struct seq_file *, const struct path *, const char *);
 int seq_dentry(struct seq_file *, struct dentry *, const char *);
diff --git a/fs/seq_file.c b/fs/seq_file.c
index 3135c25..7af75ec 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -419,6 +419,27 @@ int seq_printf(struct seq_file *m, const char *f, ...)
 EXPORT_SYMBOL(seq_printf);
 
 /**
+ *	seq_new_printf - seq_printf() which returns bytes written.
+ *	@m:	target buffer
+ *	@f:	format
+ *
+ *      Returns bytes written to @m.
+ */
+int seq_new_printf(struct seq_file *m, const char *f, ...)
+{
+	const int count = m->count;
+	int ret;
+	va_list args;
+
+	va_start(args, f);
+	ret = seq_vprintf(m, f, args);
+	va_end(args);
+
+	return ret ? 0 : m->count - count;
+}
+EXPORT_SYMBOL(seq_new_printf);
+
+/**
  *	mangle_path -	mangle and copy path to buffer beginning
  *	@s: buffer start
  *	@p: beginning of path in above buffer
---------- patch end ----------
With new function, we can do:

-	len = m->count;
-	seq_printf(m, "%s%d", con->name, con->index);
-	len = 21 - (m->count - len);
+	len = 21 - seq_new_printf(m, "%s%d", con->name, con->index);
--
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