Re: [PATCH v3 bpf-next 2/5] bpf: Introduce sleepable BPF programs

[KP Singh <kpsingh@chromium.org>]

On Fri, Aug 28, 2020 at 12:01 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> From: Alexei Starovoitov <ast@kernel.org>
>
> Introduce sleepable BPF programs that can request such property for themselves
> via BPF_F_SLEEPABLE flag at program load time. In such case they will be able
> to use helpers like bpf_copy_from_user() that might sleep. At present only
> fentry/fexit/fmod_ret and lsm programs can request to be sleepable and only
> when they are attached to kernel functions that are known to allow sleeping.
>
> The non-sleepable programs are relying on implicit rcu_read_lock() and
> migrate_disable() to protect life time of programs, maps that they use and
> per-cpu kernel structures used to pass info between bpf programs and the
> kernel. The sleepable programs cannot be enclosed into rcu_read_lock().
> migrate_disable() maps to preempt_disable() in non-RT kernels, so the progs
> should not be enclosed in migrate_disable() as well. Therefore
> rcu_read_lock_trace is used to protect the life time of sleepable progs.
>
> There are many networking and tracing program types. In many cases the
> 'struct bpf_prog *' pointer itself is rcu protected within some other kernel
> data structure and the kernel code is using rcu_dereference() to load that
> program pointer and call BPF_PROG_RUN() on it. All these cases are not touched.
> Instead sleepable bpf programs are allowed with bpf trampoline only. The
> program pointers are hard-coded into generated assembly of bpf trampoline and
> synchronize_rcu_tasks_trace() is used to protect the life time of the program.
> The same trampoline can hold both sleepable and non-sleepable progs.
>
> When rcu_read_lock_trace is held it means that some sleepable bpf program is
> running from bpf trampoline. Those programs can use bpf arrays and preallocated
> hash/lru maps. These map types are waiting on programs to complete via
> synchronize_rcu_tasks_trace();
>
> Updates to trampoline now has to do synchronize_rcu_tasks_trace() and
> synchronize_rcu_tasks() to wait for sleepable progs to finish and for
> trampoline assembly to finish.
>
> This is the first step of introducing sleepable progs. Eventually dynamically
> allocated hash maps can be allowed and networking program types can become
> sleepable too.
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: KP Singh <kpsingh@google.com>

Thanks for kicking off the allow list.

I will continue my analysis looking at which hooks are sleepable
and we can, eventually, generalize the information into lsm_hook_defs.h