On Fri, Aug 07, 2026 at 11:41:46PM +0800, Hui Su wrote:
> In test_ringbuffer()'s out_free cleanup loop, the check
> `!rb_threads[cpu]` only catches NULL entries and misses entries that
> hold an ERR_PTR.
>
> rb_threads[] is static, so unassigned slots are NULL. But when
> kthread_run_on_cpu() fails for a cpu, it stores ERR_PTR(-ENOMEM) (or
> -EINTR) in rb_threads[cpu] before the creation loop jumps to out_free.
> That entry is non-NULL, so the old `!ptr` check does not break, and the
> cleanup proceeds to call kthread_stop() on the ERR_PTR. kthread_stop()
> then dereferences the bogus pointer, crashing the kernel during the
> late_initcall self-test.
>
> crash logs:
> BUG: kernel NULL pointer dereference, address: 000000000000001c
> Oops: 0002 [#1] SMP NOPTI
> CPU: 1 PID: 1 Comm: swapper/0 Not tainted 7.2.0-rc6-dirty #7 PREEMPT(lazy)
> RIP: 0010:kthread_stop+0x2e/0x220
> RBX: fffffffffffffff4
> CR2: 000000000000001c
> Call Trace:
> <TASK>
> test_ringbuffer+0x1ec/0x650
> do_one_initcall+0x6c/0x2c0
> kernel_init_freeable+0x21d/0x420
> kernel_init+0x15/0x1c0
> ret_from_fork+0x21b/0x320
> </TASK>
> Kernel panic - not syncing: Fatal exception
>
> Fixes: 64ed3a049e3e ("ring-buffer: make use of the helper function
> kthread_run_on_cpu()")
> Signed-off-by: Hui Su <[email protected]>
> ---
> kernel/trace/ring_buffer.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 8e2485bb3aa8..6af7e36ca526 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -8214,7 +8214,7 @@ static __init int test_ringbuffer(void)
>
> out_free:
> for_each_online_cpu(cpu) {
> - if (!rb_threads[cpu])
> + if (IS_ERR_OR_NULL(rb_threads[cpu]))
> break;
> kthread_stop(rb_threads[cpu]);
> }
> --
> 2.43.0
>
>
Reviewed-by: Vincent Donnefort <[email protected]>
