linux-kernel - [patch v2] tracing: truncated output is better than nothing

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Date:	Thu, 27 Nov 2014 18:57:52 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: [patch v2] tracing: truncated output is better than nothing

The initial reason for this patch is that I noticed that:

	if (len > TRACE_BUF_SIZE)

is off by one.  In this code, if len == TRACE_BUF_SIZE, then it means we
have truncated the last character off the output string.  If we truncate
two or more characters then we exit without printing.

After some discussion, we decided that printing truncated data is better
than not printing at all so we should just use vscnprintf() and remove
the test entirely.  Also I have updated memcpy() to copy the NUL char
instead of setting the NUL in a separate step.

Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 42a822d..ab76b7b 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2160,9 +2160,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
 		goto out;
 	}
 
-	len = vsnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
-	if (len > TRACE_BUF_SIZE)
-		goto out;
+	len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
 
 	local_save_flags(flags);
 	size = sizeof(*entry) + len + 1;
@@ -2173,8 +2171,7 @@ __trace_array_vprintk(struct ring_buffer *buffer,
 	entry = ring_buffer_event_data(event);
 	entry->ip = ip;
 
-	memcpy(&entry->buf, tbuffer, len);
-	entry->buf[len] = '\0';
+	memcpy(&entry->buf, tbuffer, len + 1);
 	if (!call_filter_check_discard(call, entry, buffer, event)) {
 		__buffer_unlock_commit(buffer, event);
 		ftrace_trace_stack(buffer, flags, 6, pc);
--
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