Re: [PATCH bpf-next v4 1/4] bpf: Add bpf_call_rcu() kfunc

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] bpf: deadlock when bpf_call_rcu() is called from NMI context
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commit af52c52f0edbfb5e1da2aca1dc782426e0ea9091
Author: Puranjay Mohan <puranjay@kernel.org>

bpf: Add bpf_call_rcu() kfunc

This patch adds a new bpf_call_rcu() kfunc that allows BPF programs to
execute a callback after an RCU grace period. This is primarily useful
for programs managing their own objects that need to defer free operations.

> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 051b6654e57c6..6debb92fc8740 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c

[ ... ]

> +__bpf_kfunc int bpf_call_rcu(struct bpf_rcu_head *rh, void *map__const_map,
> +			     bpf_rcu_callback_t callback, struct bpf_prog_aux *aux)
> +{
> +	struct bpf_rcu_head_kern *rhk = (void *)rh;
> +	struct bpf_map *map = map__const_map;
> +	struct bpf_prog *prog;

[ ... ]

> +	rhk->callback_fn = (bpf_callback_t)(void *)callback;
> +	rhk->map = map;
> +	rhk->prog = prog;
> +	call_rcu(&rhk->rcu, bpf_rcu_run_callback);

[Severity: High]
Could this lead to a deadlock if called from an NMI context?

Tracing BPF programs (like those attached to perf_events or NMI-based
kprobes) can execute in NMI context. If such a program calls this kfunc,
it unconditionally invokes call_rcu().

Since call_rcu() is not NMI-safe and acquires internal raw spinlocks
(like rdp->nocb_lock or rcu_node locks), an NMI interrupting a CPU already
holding one of these locks would try to acquire it again, deadlocking the
CPU.

Is there a way to prevent bpf_call_rcu() from being called in NMI
context, or use an NMI-safe alternative?

> +	return 0;
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20260915154248.3612028-1-puranjay@kernel.org?part=1