Re: [PATCH bpf-next 1/2] bpf/docs: Document struct task_struct * kfuncs

David Vernet
2022-12-05T16:10:22Z

On Fri, Dec 02, 2022 at 06:15:00PM -0800, Alexei Starovoitov wrote:

[...]

> > +.. code-block:: c
> > +
> > + /**
> > + * A trivial example tracepoint program that shows how to
> > + * acquire and release a struct task_struct * pointer.
> > + */
> > + SEC("tp_btf/task_newtask")
> > + int BPF_PROG(task_acquire_release_example, struct task_struct *task, u64 clone_flags)
> > + {
> > + struct task_struct *acquired;
> > +
> > + acquired = bpf_task_acquire(task);
> > +
> > + /*
> > + * In a typical program you'd do something like store
> > + * the task in a map. Here, we just release it.
>
> There is a sentence later in this patch about what happens with the pointer
> that was stored in a map, but I would add some part of it here as well. Like:
>
> * In a typical program you'd do something like store
> * the task in a map and the map will automatically release it later.
> * Here, we release it manually.

Will do

> > + */
> > + bpf_task_release(acquired);
> > + return 0;
> > + }
> > +
> > +If you want to acquire a reference to a ``struct task_struct`` kptr that's
> > +already stored in a map, you can use bpf_task_kptr_get():
> > +
> > +.. kernel-doc:: kernel/bpf/helpers.c
> > + :identifiers: bpf_task_kptr_get
> > +
> > +Here's an example of how it can be used:
> > +
> > +.. code-block:: c
> > +
> > + /* struct containing the struct task_struct kptr which is actually stored in the map. */
> > + struct __tasks_kfunc_map_value {
> > + struct task_struct __kptr_ref * task;
> > + };
> > +
> > + /* The map containing struct __tasks_kfunc_map_value entries. */
> > + struct hash_map {
> > + __uint(type, BPF_MAP_TYPE_HASH);
> > + __type(key, int);
> > + __type(value, struct __tasks_kfunc_map_value);
> > + __uint(max_entries, 1);
> > + } __tasks_kfunc_map SEC(".maps");
> > +
> > + /* ... */
> > +
> > + /**
> > + * A simple example tracepoint program showing how a
> > + * struct task_struct kptr that is stored in a map can
> > + * be acquired using the bpf_task_kptr_get() kfunc.
> > + */
> > + SEC("tp_btf/task_newtask")
> > + int BPF_PROG(task_kptr_get_example, struct task_struct *task, u64 clone_flags)
> > + {
> > + struct task_struct *kptr;
> > + struct __tasks_kfunc_map_value *v;
> > + s32 pid;
> > + long status;
> > +
> > + status = bpf_probe_read_kernel(&pid, sizeof(pid), &task->pid);
>
> why use the slow bpf_probe_read_kernel() here?
> I think the example should follow modern coding practices.
> Just: pid = task->pid; instead ?

Yeah, I'll fix this.

[...]

> > + if (status)
> > + return status;
> > +
> > + /* Assume a task kptr was previously stored in the map. */
> > + v = bpf_map_lookup_elem(&__tasks_kfunc_map, &pid);
> > + if (!v)
> > + return -ENOENT;
> > +
> > + /* Acquire a reference to the task kptr that's already stored in the map. */
> > + kptr = bpf_task_kptr_get(&v->task);
> > + if (!kptr)
> > + /* If no task was present in the map, it's because
> > + * we're racing with another CPU that removed it with
> > + * bpf_kptr_xchg() between the bpf_map_lookup_elem()
> > + * above, and our call to bpf_task_kptr_get().
> > + * bpf_task_kptr_get() internally safely handles this
> > + * race, and will return NULL if the task is no longer
> > + * present in the map by the time we invoke the kfunc.
> > + */
> > + return -EBUSY;
> > +
> > + /* Free the reference we just took above. Note that the
> > + * original struct task_struct kptr is still in the map.
> > + * It will be freed either at a later time if another
> > + * context deletes it from the map, or automatically by
> > + * the BPF subsystem if it's still present when the map
> > + * is destroyed.
> > + */
> > + bpf_task_release(kptr);
> > +
> > + return 0;
> > + }
> > +
> > +Finally, a BPF program can also look up a task from a pid. This can be useful
> > +if the caller doesn't have a trusted pointer to a ``struct task_struct *``
> > +object that it can acquire a reference on with bpf_task_acquire().
> > +
> > +.. kernel-doc:: kernel/bpf/helpers.c
> > + :identifiers: bpf_task_from_pid
> > +
> > +Here is an example of it being used:
> > +
> > +.. code-block:: c
> > +
> > + SEC("tp_btf/task_newtask")
> > + int BPF_PROG(task_get_pid_example, struct task_struct *task, u64 clone_flags)
> > + {
> > + struct task_struct *lookup;
> > +
> > + lookup = bpf_task_from_pid(task->pid);
> > + if (!lookup)
> > + /* A task should always be found, as %task is a tracepoint arg. */
> > + return -ENOENT;
> > +
> > + if (lookup->pid != task->pid) {
> > + /* The pid of the lookup task should be the same as the input task. */
>
> I suspect both "errors" are actually possible in practice,
> since bpf_task_from_pid is using init_pid_ns.
> But this taskd might be in different pid_ns. See task_active_pid_ns.
> Probably worth mentioning this aspect of bpf_task_from_pid.

Yep, agreed. Will add

[...]

Thanks,
David