linux-kernel - [PATCH] ftrace: fix current

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Date:	Tue, 28 Oct 2008 10:44:24 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linux Kernel <linux-kernel@...r.kernel.org>
cc:	Ingo Molnar <mingo@...e.hu>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] ftrace: fix current_tracer error return

The commit (in linux-tip) c2931e05ec5965597cbfb79ad332d4a29aeceb23
 ( ftrace: return an error when setting a nonexistent tracer )
added useful code that would error when a bad tracer was written into
the current_tracer file.

But this had a bug if the amount written was more than the amount read by 
that code. The first iteration would set the tracer correctly, but since
it did not consume the rest of what was written (usually whitespace), the 
userspace utility would continue to write what was not consumed. This 
second iteration would fail to find a tracer and return -EINVAL. Funny 
thing is that the tracer would have already been set.

This patch just consumes all the data that is written to the file.

Signed-off-by: Steven Rostedt <srostedt@...hat.com>
---
 kernel/trace/trace.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

Index: linux-tip.git/kernel/trace/trace.c
===================================================================
--- linux-tip.git.orig/kernel/trace/trace.c	2008-10-28 10:23:41.000000000 -0400
+++ linux-tip.git/kernel/trace/trace.c	2008-10-28 10:25:58.000000000 -0400
@@ -2399,9 +2399,10 @@ tracing_set_trace_write(struct file *fil
 	int i;
 	size_t ret;

+	ret = cnt;
+
 	if (cnt > max_tracer_type_len)
 		cnt = max_tracer_type_len;
-	ret = cnt;

 	if (copy_from_user(&buf, ubuf, cnt))
 		return -EFAULT;
@@ -2434,8 +2435,8 @@ tracing_set_trace_write(struct file *fil
  out:
 	mutex_unlock(&trace_types_lock);

-	if (ret == cnt)
-		filp->f_pos += cnt;
+	if (ret > 0)
+		filp->f_pos += ret;

 	return ret;
 }

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