Re: [PATCH v6 mm-new 03/10] mm: thp: add a new kfunc bpf_mm_get_task()

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Tue, Aug 26, 2025 at 03:19:41PM +0800, Yafang Shao wrote:
> We will utilize this new kfunc bpf_mm_get_task() to retrieve the
> associated task_struct from the given @mm. The obtained task_struct must
> be released by calling bpf_task_release() as a paired operation.

You're basically describing the patch you're not saying why - yeah you're
getting a task struct from an mm (only if CONFIG_MEMCG which you don't
mention here), but not for what purpose you intend to use this?

>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> ---
>  mm/bpf_thp.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/mm/bpf_thp.c b/mm/bpf_thp.c
> index b757e8f425fd..46b3bc96359e 100644
> --- a/mm/bpf_thp.c
> +++ b/mm/bpf_thp.c
> @@ -205,11 +205,45 @@ __bpf_kfunc void bpf_put_mem_cgroup(struct mem_cgroup *memcg)
>  #endif
>  }
>
> +/**
> + * bpf_mm_get_task - Get the task struct associated with a mm_struct.
> + * @mm: The mm_struct to query
> + *
> + * The obtained task_struct must be released by calling bpf_task_release().

Hmmm so now bpf programs can cause kernel bugs by keeping a reference around?

This feels extremely dodgy, I don't like this at all.

I thought the whole point of BPF was that this kind of thing couldn't possibly
happen?

Or would this be a kernel bug?

If a bpf program can lead to a refcount not being put, this is not
upstreamable surely?

> + *
> + * Return: The associated task_struct on success, or NULL on failure. Note that
> + * this function depends on CONFIG_MEMCG being enabled - it will always return
> + * NULL if CONFIG_MEMCG is not configured.
> + */
> +__bpf_kfunc struct task_struct *bpf_mm_get_task(struct mm_struct *mm)
> +{
> +#ifdef CONFIG_MEMCG
> +	struct task_struct *task;
> +
> +	if (!mm)
> +		return NULL;
> +	rcu_read_lock();
> +	task = rcu_dereference(mm->owner);

> +	if (!task)
> +		goto out;
> +	if (!refcount_inc_not_zero(&task->rcu_users))
> +		goto out;
> +
> +	rcu_read_unlock();
> +	return task;
> +
> +out:
> +	rcu_read_unlock();
> +#endif

This #ifdeffery is horrid, can we please just have separate functions instead of
inside the one? Thanks.

> +	return NULL;

So we can't tell the difference between this failling due to CONFIG_MEMCG
not being set (in which case it will _always_ fail) or we couldn't get a
task or we couldn't get a refcount on the task.

Maybe this doesn't matter since perhaps we are only using this if
CONFIG_MEMCG but in that case why even expose this if !CONFIG_MEMCG?

> +}
> +
>  __bpf_kfunc_end_defs();
>
>  BTF_KFUNCS_START(bpf_thp_ids)
>  BTF_ID_FLAGS(func, bpf_mm_get_mem_cgroup, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_put_mem_cgroup, KF_RELEASE)
> +BTF_ID_FLAGS(func, bpf_mm_get_task, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_RET_NULL)
>  BTF_KFUNCS_END(bpf_thp_ids)
>
>  static const struct btf_kfunc_id_set bpf_thp_set = {
> --
> 2.47.3
>