Re: [PATCH] ring_buffer: wrap a list.next reference withrb_list_head()

[Steven Rostedt]

Just a FYI, it's best to send me email to my rostedt@xxxxxxxxxxx
account. I don't always read my messages on my redhat account.

On Wed, 2010-01-06 at 17:12 -0800, David Sharp wrote:
> This reference at the end of rb_get_reader_page() was causing off-by-one
> writes to the prev pointer of the page after the reader page when that
> page is the head page, and therefore the reader page has the RB_PAGE_HEAD
> flag in its list.next pointer. This eventually results in a GPF in a
> subsequent call to rb_set_head_page() (usually from rb_get_reader_page())
> when that prev pointer is dereferenced. The dereferenced register would
> characteristically have an address that appears shifted left by one byte
> (eg, ffxxxxxxxxxxxxyy instead of ffffxxxxxxxxxxxx) due to being written at
> an address one byte too high.
> 
> Signed-off-by: David Sharp <dhsharp@xxxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 2326b04..d5b7308 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -2906,7 +2906,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
>  	 *
>  	 * Now make the new head point back to the reader page.
>  	 */
> -	reader->list.next->prev = &cpu_buffer->reader_page->list;
> +	rb_list_head(reader->list.next)->prev = &cpu_buffer->reader_page->list;
>  	rb_inc_page(cpu_buffer, &cpu_buffer->head_page);

Wow, this is a nasty little race. The writer had to move the header page
forward by one between the time we swapped to now, which is the command
just outside the diff.

>  
>  	/* Finally update the reader page to the new head */

We probably should add a change at the top too, just to be safe.

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index 2326b04..edefe3b 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -2869,7 +2869,7 @@ rb_get_reader_page(struct ring_buffer_per_cpu *cpu_buffer)
 	 * Splice the empty reader page into the list around the head.
 	 */
 	reader = rb_set_head_page(cpu_buffer);
-	cpu_buffer->reader_page->list.next = reader->list.next;
+	cpu_buffer->reader_page->list.next = rb_list_head(reader->list.next);
 	cpu_buffer->reader_page->list.prev = reader->list.prev;
 
 	/*


Thanks, I'll apply this.

-- Steve


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