linux-kernel - pr_cat() + CATSTR(name, size)?

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Date:	Wed, 11 Jul 2012 12:33:28 +0200
From:	Kay Sievers <kay@...y.org>
To:	Joe Perches <joe@...ches.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: pr_cat() + CATSTR(name, size)?

Hey Joe,

what do you think this?

It would make composing continuation lines at the caller side entirely
race-free, and it might fit into the usual pattern.

The more interesting thing, this would allow us to completely race-free
use the dev_printk() calls with continuation content, which we should
avoid otherwise for integrity reasons.

The patch below is just hacked it into the printk.c file to illustrate
the idea. It prints:
  [    0.000000] Kernel command line: root=/dev/sda2 ...
  [    0.000000] 12 34 56 78
  [    0.000000] PID hash table entries: 2048 (order: 2, 16384 bytes)

  5,96,0;Kernel command line: root=/dev/sda2 ...
  4,97,0;12 34 56 78
  6,98,0;PID hash table entries: 2048 (order: 2, 16384 bytes)

Thanks,
Kay


diff --git a/kernel/printk.c b/kernel/printk.c
index dba1821..1fd00b0 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -48,6 +48,32 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/printk.h>
 
+#define CATSTR(name, max)		\
+	char name[max];			\
+	size_t _##name_len = 0;		\
+	size_t _##name_max = max;
+
+#define pr_cat(name, fmt, ...)		\
+	_catstr(name, &_##name_len, _##name_max, fmt, ##__VA_ARGS__)
+
+ssize_t _catstr(char *s, size_t *len, size_t size, const char *fmt, ...)
+{
+	va_list args;
+	size_t l = *len;
+	size_t r;
+
+	va_start(args, fmt);
+	r = vsnprintf(s + l, size - l, fmt, args);
+	va_end(args);
+
+	if (r >= size - l)
+		return -EINVAL;
+
+	*len += r;
+	s[*len] = '\0';
+	return r;
+}
+
 /*
  * Architectures can override it:
  */
@@ -668,6 +694,12 @@ void __init setup_log_buf(int early)
 	char *new_log_buf;
 	int free;
 
+	CATSTR(line, 80);
+	pr_cat(line, "%i ", 12);
+	pr_cat(line, "%i ", 34);
+	pr_cat(line, "%i ", 56);
+	pr_warn("%s%i\n", line, 78);
+
 	if (!new_log_buf_len)
 		return;
 

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