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SubjectRe: [PATCH] trace: Add NULL checks for buffer in ring_buffer_free_read_page()
On Fri, 13 Jan 2023 20:55:01 +0800
Jia-Ju Bai <baijiaju1990@gmail.com> wrote:

> In a previous commit 7433632c9ff6, buffer, buffer->buffers and
> buffer->buffers[cpu] in ring_buffer_wake_waiters() can be NULL,
> and thus the related checks are added.
>
> However, in the same call stack, these variables are also used in
> ring_buffer_free_read_page():
>
> tracing_buffers_release()
> ring_buffer_wake_waiters(iter->array_buffer->buffer)
> cpu_buffer = buffer->buffers[cpu] -> Add checks by previous commit
> ring_buffer_free_read_page(iter->array_buffer->buffer)
> cpu_buffer = buffer->buffers[cpu] -> No check
>
> Thus, to avod possible null-pointer derefernces, the related checks
> should be added.
>
> These results are reported by a static tool designed by myself.
>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>

Although I'm not adverse to this patch (it doesn't hurt), but the code has:

ring_buffer_wake_waiters(iter->array_buffer->buffer, iter->cpu_file);

if (info->spare)
ring_buffer_free_read_page(iter->array_buffer->buffer,
info->spare_cpu, info->spare);

Where none of those will be NULL if "info->spare" is set.

But as I said, I have no problem applying this, so I will ;-)

-- Steve


> ---
> kernel/trace/ring_buffer.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index c366a0a9ddba..45d4a23d6044 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5626,11 +5626,16 @@ EXPORT_SYMBOL_GPL(ring_buffer_alloc_read_page);
> */
> void ring_buffer_free_read_page(struct trace_buffer *buffer, int cpu, void *data)
> {
> - struct ring_buffer_per_cpu *cpu_buffer = buffer->buffers[cpu];
> + struct ring_buffer_per_cpu *cpu_buffer;
> struct buffer_data_page *bpage = data;
> struct page *page = virt_to_page(bpage);
> unsigned long flags;
>
> + if (!buffer || !buffer->buffers || !buffer->buffers[cpu])
> + return;
> +
> + cpu_buffer = buffer->buffers[cpu];
> +
> /* If the page is still in use someplace else, we can't reuse it */
> if (page_ref_count(page) > 1)
> goto out;

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