Re: [PATCH 5.4] tracing/ring-buffer: Only do full wait when cpu != RING_BUFFER_ALL_CPUS

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 16 Dec 2022 14:42:41 +0100
Pratyush Yadav <ptyadav@amazon.de> wrote:

> full_hit() directly uses cpu as an array index. Since
> RING_BUFFER_ALL_CPUS == -1, calling full_hit() with cpu ==
> RING_BUFFER_ALL_CPUS will cause an invalid memory access.
> 
> The upstream commit 42fb0a1e84ff ("tracing/ring-buffer: Have polling
> block on watermark") already does this. This was missed when backporting
> to v5.4.y.
> 
> This bug was discovered and resolved using Coverity Static Analysis
> Security Testing (SAST) by Synopsys, Inc.

Nice.

> 
> Fixes: e65ac2bdda54 ("tracing/ring-buffer: Have polling block on watermark")
> Signed-off-by: Pratyush Yadav <ptyadav@amazon.de>
> ---
> 
> I am not familiar with this code. This was just pointed out by our
> static analysis tool and I wrote a quick patch fixing this. Only
> compile-tested.
> 
>  kernel/trace/ring_buffer.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 176d858903bd..11e8189dd8ae 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -727,6 +727,7 @@ __poll_t ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
> 
>  	if (cpu == RING_BUFFER_ALL_CPUS) {
>  		work = &buffer->irq_work;
> +		full = 0;

Good catch. This was indeed missed in the backport. The backported patch
even added the comment:

 * @full: wait until the percentage of pages are available, if @cpu != RING_BUFFER_ALL_CPUS

Greg, please take this patch.

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Thanks,

-- Steve

>  	} else {
>  		if (!cpumask_test_cpu(cpu, buffer->cpumask))
>  			return -EINVAL;
> --
> 2.38.1
> 
> 
> 
> 
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