Re: [PATCH bpf-next 1/2] bpf: Allow invoking kfuncs from BPF_PROG_TYPE_SYSCALL progs

[Andrii Nakryiko]

On Wed, Apr 3, 2024 at 6:03 PM David Vernet <void@xxxxxxxxxxxxx> wrote:
>
> Currently, a set of core BPF kfuncs (e.g. bpf_task_*, bpf_cgroup_*,
> bpf_cpumask_*, etc) cannot be invoked from BPF_PROG_TYPE_SYSCALL
> programs. The whitelist approach taken for enabling kfuncs makes sense:
> it not safe to call these kfuncs from every program type. For example,
> it may not be safe to call bpf_task_acquire() in an fentry to
> free_task().
>
> BPF_PROG_TYPE_SYSCALL, on the other hand, is a perfectly safe program
> type from which to invoke these kfuncs, as it's a very controlled
> environment, and we should never be able to run into any of the typical
> problems such as recursive invoations, acquiring references on freeing
> kptrs, etc. Being able to invoke these kfuncs would be useful, as
> BPF_PROG_TYPE_SYSCALL can be invoked with BPF_PROG_RUN, and would
> therefore enable user space programs to synchronously call into BPF to
> manipulate these kptrs.
>
> This patch therefore enables invoking the aforementioned core kfuncs
> from BPF_PROG_TYPE_SYSCALL progs.
>
> Signed-off-by: David Vernet <void@xxxxxxxxxxxxx>
> ---
>  kernel/bpf/cpumask.c | 1 +
>  kernel/bpf/helpers.c | 1 +
>  2 files changed, 2 insertions(+)
>

Makes sense, but see my comments on patch #2.

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> diff --git a/kernel/bpf/cpumask.c b/kernel/bpf/cpumask.c
> index dad0fb1c8e87..33c473d676a5 100644
> --- a/kernel/bpf/cpumask.c
> +++ b/kernel/bpf/cpumask.c
> @@ -474,6 +474,7 @@ static int __init cpumask_kfunc_init(void)
>         ret = bpf_mem_alloc_init(&bpf_cpumask_ma, sizeof(struct bpf_cpumask), false);
>         ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &cpumask_kfunc_set);
>         ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &cpumask_kfunc_set);
> +       ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &cpumask_kfunc_set);
>         return  ret ?: register_btf_id_dtor_kfuncs(cpumask_dtors,
>                                                    ARRAY_SIZE(cpumask_dtors),
>                                                    THIS_MODULE);
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index d9e7aca8ae9e..8cde717137bd 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2653,6 +2653,7 @@ static int __init kfunc_init(void)
>         ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SCHED_CLS, &generic_kfunc_set);
>         ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_XDP, &generic_kfunc_set);
>         ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &generic_kfunc_set);
> +       ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &generic_kfunc_set);
>         ret = ret ?: register_btf_id_dtor_kfuncs(generic_dtors,
>                                                   ARRAY_SIZE(generic_dtors),
>                                                   THIS_MODULE);
> --
> 2.44.0
>