linux-kernel - Re: [tip:tracing/urgent] tracing: Fix trace

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Date:	Sun, 06 Dec 2009 14:02:44 +0100
From:	Carsten Emde <Carsten.Emde@...dl.org>
To:	Olof Johansson <olof@...om.net>
CC:	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:tracing/urgent] tracing: Fix trace_marker output

Olof,

> On Sun, Nov 22, 2009 at 10:24:48AM +0000, tip-bot for Carsten Emde wrote:
>> Commit-ID:  c13d2f7c3231e873f30db92b96c8caa48f100f33
>> Gitweb:     http://git.kernel.org/tip/c13d2f7c3231e873f30db92b96c8caa48f100f33
>> Author:     Carsten Emde <Carsten.Emde@...dl.org>
>> AuthorDate: Mon, 16 Nov 2009 20:56:13 +0100
>> Committer:  Steven Rostedt <rostedt@...dmis.org>
>> CommitDate: Tue, 17 Nov 2009 09:19:06 -0500
>>
>> tracing: Fix trace_marker output
>>
>> When a string was written to <debugfs>/tracing/trace_marker, some
>> strange characters appeared in the trace output instead of the
>> string, since a vprint function erroneously called a vararg print
>> function with a va_list argument. This patch fixes the problem and
>> simplifies the related code.
> 
> [...]
> 
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index 03c7fd5..12b49ca 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -1361,10 +1361,11 @@ int trace_array_vprintk(struct trace_array *tr,
>>  	pause_graph_tracing();
>>  	raw_local_irq_save(irq_flags);
>>  	__raw_spin_lock(&trace_buf_lock);
>> -	len = vsnprintf(trace_buf, TRACE_BUF_SIZE, fmt, args);
>> -
>> -	len = min(len, TRACE_BUF_SIZE-1);
>> -	trace_buf[len] = 0;
>> +	if (args == NULL) {
>> +		strncpy(trace_buf, fmt, TRACE_BUF_SIZE);
>> +		len = strlen(trace_buf);
>> +	} else
>> +		len = vsnprintf(trace_buf, TRACE_BUF_SIZE, fmt, args);
> 
> 
> Comparing a va_list with NULL is bogus. It's supposed to be treated like
> an opaque type and only be manipulated with va_* accessors.
> 
> I wouldn't really care much, but it broke builds on some ARM platforms:
> 
> kernel/trace/trace.c: In function 'trace_array_vprintk':
> kernel/trace/trace.c:1364: error: invalid operands to binary == (have 'va_list' and 'void *')
> kernel/trace/trace.c: In function 'tracing_mark_write':
> kernel/trace/trace.c:3349: error: incompatible type for argument 3 of 'trace_vprintk'
Having looked at other possible solutions, I find that the best thing to do
is to revert this part of the patch. Sorry that I didn't check it on ARM
platforms.

	Carsten.

-=-

This patch partly reverts c13d2f7c3231e873f30db92b96c8caa48f100f33 and
re-installs Steven's original mark_printk() mechanism.

Signed-off-by: Carsten Emde <C.Emde@...dl.org>

Index: linux-2.6-tip/kernel/trace/trace.c
===================================================================
--- linux-2.6-tip.orig/kernel/trace/trace.c
+++ linux-2.6-tip/kernel/trace/trace.c
@@ -1361,11 +1361,7 @@ int trace_array_vprintk(struct trace_arr
 	pause_graph_tracing();
 	raw_local_irq_save(irq_flags);
 	__raw_spin_lock(&trace_buf_lock);
-	if (args == NULL) {
-		strncpy(trace_buf, fmt, TRACE_BUF_SIZE);
-		len = strlen(trace_buf);
-	} else
-		len = vsnprintf(trace_buf, TRACE_BUF_SIZE, fmt, args);
+	len = vsnprintf(trace_buf, TRACE_BUF_SIZE, fmt, args);
 
 	size = sizeof(*entry) + len + 1;
 	buffer = tr->buffer;
@@ -3320,6 +3316,16 @@ tracing_entries_write(struct file *filp,
 	return cnt;
 }
 
+static int mark_printk(const char *fmt, ...)
+{
+	int ret;
+	va_list args;
+	va_start(args, fmt);
+	ret = trace_vprintk(0, fmt, args);
+	va_end(args);
+	return ret;
+}
+
 static ssize_t
 tracing_mark_write(struct file *filp, const char __user *ubuf,
 					size_t cnt, loff_t *fpos)
@@ -3346,7 +3352,7 @@ tracing_mark_write(struct file *filp, co
 	} else
 		buf[cnt] = '\0';
 
-	cnt = trace_vprintk(0, buf, NULL);
+	cnt = mark_printk("%s", buf);
 	kfree(buf);
 	*fpos += cnt;
 
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