On Wed, Jan 24, 2024 at 10:43:32AM -0500, Siddharth Chintamaneni wrote: > While we were working on some experiments with BPF trampoline, we came > across a deadlock scenario that could happen. > > A deadlock happens when two nested BPF programs tries to acquire the > same lock i.e, If a BPF program is attached using fexit to > bpf_spin_lock or using a fentry to bpf_spin_unlock, and it then > attempts to acquire the same lock as the previous BPF program, a > deadlock situation arises. > > Here is an example: > > SEC(fentry/bpf_spin_unlock) > int fentry_2{ > bpf_spin_lock(&x->lock); > bpf_spin_unlock(&x->lock); > } > > SEC(fentry/xxx) > int fentry_1{ > bpf_spin_lock(&x->lock); > bpf_spin_unlock(&x->lock); > } hi, looks like valid issue, could you add selftest for that? I wonder we could restrict just programs that use bpf_spin_lock/bpf_spin_unlock helpers? I'm not sure there's any useful use case for tracing spin lock helpers, but I think we should at least try this before we deny it completely > > To prevent these cases, a simple fix could be adding these helpers to > denylist in the verifier. This fix will prevent the BPF programs from > being loaded by the verifier. > > previously, a similar solution was proposed to prevent recursion. > https://lore.kernel.org/lkml/20230417154737.12740-2-laoar.shao@xxxxxxxxx/ the difference is that __rcu_read_lock/__rcu_read_unlock are called unconditionally (always) when executing bpf tracing probe, the problem you described above is only for programs calling spin lock helpers (on same spin lock) > > Signed-off-by: Siddharth Chintamaneni <sidchintamaneni@xxxxxx> > --- > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 65f598694d55..8f1834f27f81 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -20617,6 +20617,10 @@ BTF_ID(func, preempt_count_sub) > BTF_ID(func, __rcu_read_lock) > BTF_ID(func, __rcu_read_unlock) > #endif > +#if defined(CONFIG_DYNAMIC_FTRACE) why the CONFIG_DYNAMIC_FTRACE dependency? jirka > +BTF_ID(func, bpf_spin_lock) > +BTF_ID(func, bpf_spin_unlock) > +#endif > BTF_SET_END(btf_id_deny)