linux-kernel - [PATCH 2/2] tracing: Fix returning of duplicate data after EOF in trace_pipe

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Message-Id: <20111017175023.569695684@goodmis.org>
Date:	Mon, 17 Oct 2011 13:49:40 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable <stable@...nel.org>
Subject: [PATCH 2/2] tracing: Fix returning of duplicate data after EOF in trace_pipe_raw

From: Steven Rostedt <srostedt@...hat.com>

The trace_pipe_raw handler holds a cached page from the time the file
is opened to the time it is closed. The cached page is used to handle
the case of the user space buffer being smaller than what was read from
the ring buffer. The left over buffer is held in the cache so that the
next read will continue where the data left off.

After EOF is returned (no more data in the buffer), the index of
the cached page is set to zero. If a user app reads the page again
after EOF, the check in the buffer will see that the cached page
is less than page size and will return the cached page again. This
will cause reading the trace_pipe_raw again after EOF to return
duplicate data, making the output look like the time went backwards
but instead data is just repeated.

The fix is to not reset the index right after all data is read
from the cache, but to reset it after all data is read and more
data exists in the ring buffer.

Cc: stable <stable@...nel.org>
Reported-by: Jeremy Eder <jeder@...hat.com>
Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
---
 kernel/trace/trace.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index cea1605..b24a72d 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3903,8 +3903,6 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 	if (info->read < PAGE_SIZE)
 		goto read;

-	info->read = 0;
-
 	trace_access_lock(info->cpu);
 	ret = ring_buffer_read_page(info->tr->buffer,
 				    &info->spare,
@@ -3914,6 +3912,8 @@ tracing_buffers_read(struct file *filp, char __user *ubuf,
 	if (ret < 0)
 		return 0;

+	info->read = 0;
+
 read:
 	size = PAGE_SIZE - info->read;
 	if (size > count)
-- 
1.7.6.3

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