linux-kernel - Re: [PATCH] sched_ext: Fix scx_enable() crash on helper kthread creation failure

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Message-ID: <b8391b8e-ec67-4e84-a0d7-529e85660f5a@linux.ibm.com>
Date: Wed, 19 Nov 2025 22:24:25 +0530
From: Samir M <samir@...ux.ibm.com>
To: Saket Kumar Bhaskar <skb99@...ux.ibm.com>, sched-ext@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Cc: hbathini@...ux.ibm.com, sachinpb@...ux.ibm.com, tj@...nel.org,
        void@...ifault.com, arighi@...dia.com, changwoo@...lia.com,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com
Subject: Re: [PATCH] sched_ext: Fix scx_enable() crash on helper kthread
 creation failure


On 19/11/25 4:07 pm, Saket Kumar Bhaskar wrote:
> A crash was observed when the sched_ext selftests runner was
> terminated with Ctrl+\ while test 15 was running:
>
> NIP [c00000000028fa58] scx_enable.constprop.0+0x358/0x12b0
> LR [c00000000028fa2c] scx_enable.constprop.0+0x32c/0x12b0
> Call Trace:
> scx_enable.constprop.0+0x32c/0x12b0 (unreliable)
> bpf_struct_ops_link_create+0x18c/0x22c
> __sys_bpf+0x23f8/0x3044
> sys_bpf+0x2c/0x6c
> system_call_exception+0x124/0x320
> system_call_vectored_common+0x15c/0x2ec
>
> kthread_run_worker() returns an ERR_PTR() on failure rather than NULL,
> but the current code in scx_alloc_and_add_sched() only checks for a NULL
> helper. Incase of failure on SIGQUIT, the error is not handled in
> scx_alloc_and_add_sched() and scx_enable() ends up dereferencing an
> error pointer.
>
> Error handling is fixed in scx_alloc_and_add_sched() to propagate
> PTR_ERR() into ret, so that scx_enable() jumps to the existing error
> path, avoiding random dereference on failure.
>
> Fixes: bff3b5aec1b7 ("sched_ext: Move disable machinery into scx_sched")
> Reported-by: Samir Mulani <samir@...ux.ibm.com>
> Signed-off-by: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
> ---
>   kernel/sched/ext.c | 5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 2b0e88206d07..7fc0cce68a1b 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -4392,8 +4392,11 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
>   		goto err_free_gdsqs;
>   
>   	sch->helper = kthread_run_worker(0, "sched_ext_helper");
> -	if (!sch->helper)
> +	if (IS_ERR(sch->helper)) {
> +		ret = PTR_ERR(sch->helper);
>   		goto err_free_pcpu;
> +	}
> +
>   	sched_set_fifo(sch->helper->task);
>   
>   	atomic_set(&sch->exit_kind, SCX_EXIT_NONE);
Hi,

I have validated this patch using the tools/testing/selftests/sched_ext 
test suite, and all tests passed without any issues.

Thank you for the fix ..!!
Please add below tag for the patch set.
Tested-by: Samir M <samir@...ux.ibm.com>

Regards,
Samir