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P37k4WRQRa2GV8YRxzlBfRU/NsuD2ZTpZ+JyRMFzbBAoh7UPSIuytEvznIOLc8+Zrov7Eyo41RFZhBxIJAhwgRNUbfPFqx6zxdqnTOSGSUpq+KDQ30areYbmHPyWIEttvWe7pIIcYNKXSrRukRYy1+cxylH8gnJwXcf3m X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > --- > 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 >