bpf_task_storage improvement question

[Christian Brauner <brauner@xxxxxxxxxx>]

Hey,

I just had to take a quick look at kernel/bpf/bpf_task_storage.c and
realized that you're doing:


	fd = *(int *)key;
	pid = pidfd_get_pid(fd, &f_flags);

	// something something

	task = pid_task(pid, PIDTYPE_PID);

	bpf_task_storage_lock();
	// something something
	bpf_task_storage_unlock();
	put_pid(pid);

That reference count bump on struct pid seems unnecessary and I suspect
your lookup routines are supposed to be fast. So why don't you just
open-code this. Something like:

diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
index bf7fa15fdcc6..dc36a33c7b6d 100644
--- a/kernel/bpf/bpf_task_storage.c
+++ b/kernel/bpf/bpf_task_storage.c
@@ -92,10 +92,12 @@ static void *bpf_pid_task_storage_lookup_elem(struct bpf_map *map, void *key)
        struct task_struct *task;
        unsigned int f_flags;
        struct pid *pid;
-       int fd, err;

-       fd = *(int *)key;
-       pid = pidfd_get_pid(fd, &f_flags);
+       CLASS(fd, f)(*(int *)key);
+       if (fd_empty(f))
+               return -EBADF;
+
+       pid = pidfd_pid(f);
        if (IS_ERR(pid))
                return ERR_CAST(pid);

@@ -104,19 +106,13 @@ static void *bpf_pid_task_storage_lookup_elem(struct bpf_map *map, void *key)
         */
        WARN_ON_ONCE(!rcu_read_lock_held());
        task = pid_task(pid, PIDTYPE_PID);
-       if (!task) {
-               err = -ENOENT;
-               goto out;
-       }
+       if (!task)
+               return ERR_PTR(-ENOENT);

        bpf_task_storage_lock();
        sdata = task_storage_lookup(task, map, true);
        bpf_task_storage_unlock();
-       put_pid(pid);
        return sdata ? sdata->data : NULL;
-out:
-       put_pid(pid);
-       return ERR_PTR(err);
 }

which avoids the reference count bumps on @pid.
It remains pinned by the pidfd anyway.