Re: [PATCH bpf-next v1 1/2] bpf: Allow calling static subprogs while holding a bpf_spin_lock

[David Vernet <void@manifault.com>]

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On Sun, Feb 04, 2024 at 12:02:05PM +0000, Kumar Kartikeya Dwivedi wrote:
> Currently, calling any helpers, kfuncs, or subprogs except the graph
> data structure (lists, rbtrees) API kfuncs while holding a bpf_spin_lock
> is not allowed. One of the original motivations of this decision was to
> force the BPF programmer's hand into keeping the bpf_spin_lock critical
> section small, and to ensure the execution time of the program does not
> increase due to lock waiting times. In addition to this, some of the
> helpers and kfuncs may be unsafe to call while holding a bpf_spin_lock.
> 
> However, when it comes to subprog calls, atleast for static subprogs,
> the verifier is able to explore their instructions during verification.
> Therefore, it is similar in effect to having the same code inlined into
> the critical section. Hence, not allowing static subprog calls in the
> bpf_spin_lock critical section is mostly an annoyance that needs to be
> worked around, without providing any tangible benefit.
> 
> Unlike static subprog calls, global subprog calls are not safe to permit
> within the critical section, as the verifier does not explore them
> during verification, therefore whether the same lock will be taken
> again, or unlocked, cannot be ascertained.
> 
> Therefore, allow calling static subprogs within a bpf_spin_lock critical
> section, and only reject it in case the subprog linkage is global.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Looks good, thanks for this improvement. I had the same suggestion as
Yonghong in [0], and also left a question below.

[0]: https://lore.kernel.org/all/2e008ab1-44b8-4d1b-a86d-1f347d7630e6@linux.dev/

Acked-by: David Vernet <void@manifault.com>

> ---
>  kernel/bpf/verifier.c                                  | 10 +++++++---
>  tools/testing/selftests/bpf/progs/verifier_spin_lock.c |  2 +-
>  2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 64fa188d00ad..f858c959753b 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9493,6 +9493,12 @@ static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  	if (subprog_is_global(env, subprog)) {
>  		const char *sub_name = subprog_name(env, subprog);
>  
> +		/* Only global subprogs cannot be called with a lock held. */
> +		if (env->cur_state->active_lock.ptr) {
> +			verbose(env, "function calls are not allowed while holding a lock\n");
> +			return -EINVAL;
> +		}
> +
>  		if (err) {
>  			verbose(env, "Caller passes invalid args into func#%d ('%s')\n",
>  				subprog, sub_name);
> @@ -17644,7 +17650,6 @@ static int do_check(struct bpf_verifier_env *env)
>  
>  				if (env->cur_state->active_lock.ptr) {
>  					if ((insn->src_reg == BPF_REG_0 && insn->imm != BPF_FUNC_spin_unlock) ||
> -					    (insn->src_reg == BPF_PSEUDO_CALL) ||
>  					    (insn->src_reg == BPF_PSEUDO_KFUNC_CALL &&
>  					     (insn->off != 0 || !is_bpf_graph_api_kfunc(insn->imm)))) {
>  						verbose(env, "function calls are not allowed while holding a lock\n");
> @@ -17692,8 +17697,7 @@ static int do_check(struct bpf_verifier_env *env)
>  					return -EINVAL;
>  				}
>  process_bpf_exit_full:
> -				if (env->cur_state->active_lock.ptr &&
> -				    !in_rbtree_lock_required_cb(env)) {
> +				if (env->cur_state->active_lock.ptr && !env->cur_state->curframe) {

Can we do the same thing here for the RCU check below? It seems like the
exact same issue, as we're already allowed to call subprogs from within
an RCU read region, but the verifier will get confused and think we
haven't unlocked by the time we return to the caller.

Assuming that's the case, we can take care of it in a separate patch
set.

>  					verbose(env, "bpf_spin_unlock is missing\n");
>  					return -EINVAL;
>  				}
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spin_lock.c b/tools/testing/selftests/bpf/progs/verifier_spin_lock.c
> index 9c1aa69650f8..fb316c080c84 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spin_lock.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spin_lock.c
> @@ -330,7 +330,7 @@ l1_%=:	r7 = r0;					\
>  
>  SEC("cgroup/skb")
>  __description("spin_lock: test10 lock in subprog without unlock")
> -__failure __msg("unlock is missing")
> +__success
>  __failure_unpriv __msg_unpriv("")
>  __naked void lock_in_subprog_without_unlock(void)
>  {
> -- 
> 2.40.1
> 

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