Re: [PATCH bpf-next 1/5] bpf: Implement task local storage

[KP Singh]

On Fri, Oct 30, 2020 at 11:53 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
>
> Thanks for taking a look!
>
> On Wed, Oct 28, 2020 at 2:13 AM Martin KaFai Lau <kafai@xxxxxx> wrote:
> >
> > On Tue, Oct 27, 2020 at 06:03:13PM +0100, KP Singh wrote:
> > [ ... ]
> >
> > > diff --git a/kernel/bpf/bpf_task_storage.c b/kernel/bpf/bpf_task_storage.c
> > > new file mode 100644
> > > index 000000000000..774140c458cc
> > > --- /dev/null
> > > +++ b/kernel/bpf/bpf_task_storage.c
> > > @@ -0,0 +1,327 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +/*
> > > + * Copyright (c) 2019 Facebook
> > > + * Copyright 2020 Google LLC.
> > > + */
> > > +
> > > +#include "linux/pid.h"
> > > +#include "linux/sched.h"
> > > +#include <linux/rculist.h>
> > > +#include <linux/list.h>
> > > +#include <linux/hash.h>
> > > +#include <linux/types.h>
> > > +#include <linux/spinlock.h>
> > > +#include <linux/bpf.h>
> > > +#include <linux/bpf_local_storage.h>
> > > +#include <net/sock.h>
> > Is this required?
>
> Nope. Removed.
>
> >
> > > +#include <uapi/linux/sock_diag.h>
> > > +#include <uapi/linux/btf.h>
> > > +#include <linux/bpf_lsm.h>
> > > +#include <linux/btf_ids.h>
> > > +#include <linux/fdtable.h>
> > > +
> > > +DEFINE_BPF_STORAGE_CACHE(task_cache);
> > > +
> > > +static struct bpf_local_storage __rcu **task_storage_ptr(void *owner)
>
> [...]
>
> > > +             err = -EBADF;
> > > +             goto out_fput;
> > > +     }
> > > +
> > > +     pid = get_pid(f->private_data);
> > n00b question. Is get_pid(f->private_data) required?
> > f->private_data could be freed while holding f->f_count?
>
> I would assume that holding a reference to the file should also
> keep the private_data alive but I was not sure so I grabbed the
> extra reference.
>
> >
> > > +     task = get_pid_task(pid, PIDTYPE_PID);
> > Should put_task_struct() be called before returning?
>
> If we keep using get_pid_task then, yes, I see it grabs a reference to the task.
> We could also call pid_task under rcu locks but it might be cleaner to
> just get_pid_task
> and put_task_struct().

I refactored this to use pidfd_get_pid and it seems like we can simply call
pid_task since we are already in an RCU read side critical section.

And to be pedantic, I added a WARN_ON_ONCE(!rcu_read_lock_held());
(although this is not required as lockdep should pretty much handle it
by default)

- KP

>
> >
> > > +     if (!task || !task_storage_ptr(task)) {
> > "!task_storage_ptr(task)" is unnecessary, task_storage_lookup() should
> > have taken care of it.
> >
> >
> > > +             err = -ENOENT;
> > > +             goto out;
> > > +     }
> > > +
> > > +     sdata = task_storage_lookup(task, map, true);
> > > +     put_pid(pid);
>
> [...]
>
> > > +     .map_lookup_elem = bpf_pid_task_storage_lookup_elem,
> > > +     .map_update_elem = bpf_pid_task_storage_update_elem,
> > > +     .map_delete_elem = bpf_pid_task_storage_delete_elem,
> > Please exercise the syscall use cases also in the selftest.
>
> Will do. Thanks for the nudge :)

I also added another patch to exercise them for the other storage types too.

- KP

>
> >
> > > +     .map_check_btf = bpf_local_storage_map_check_btf,
> > > +     .map_btf_name = "bpf_local_storage_map",
> > > +     .map_btf_id = &task_storage_map_btf_id,
> > > +     .map_owner_storage_ptr = task_storage_ptr,
> > > +};
> > > +