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@xxxxxxxxx>
> ---
> 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
>
