Re: [PATCH v2 bpf-next 6/7] bpf: Allow bpf_spin_{lock,unlock} in sleepable progs

[Yonghong Song <yonghong.song@linux.dev>]

On 8/21/23 12:33 PM, Dave Marchevsky wrote:
> Commit 9e7a4d9831e8 ("bpf: Allow LSM programs to use bpf spin locks")
> disabled bpf_spin_lock usage in sleepable progs, stating:
> 
>   Sleepable LSM programs can be preempted which means that allowng spin
>   locks will need more work (disabling preemption and the verifier
>   ensuring that no sleepable helpers are called when a spin lock is
>   held).
> 
> This patch disables preemption before grabbing bpf_spin_lock. The second
> requirement above "no sleepable helpers are called when a spin lock is
> held" is implicitly enforced by current verifier logic due to helper
> calls in spin_lock CS being disabled except for a few exceptions, none
> of which sleep.
> 
> Due to above preemption changes, bpf_spin_lock CS can also be considered
> a RCU CS, so verifier's in_rcu_cs check is modified to account for this.
> 
> Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
> ---
>   kernel/bpf/helpers.c  | 2 ++
>   kernel/bpf/verifier.c | 9 +++------
>   2 files changed, 5 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 945a85e25ac5..8bd3812fb8df 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -286,6 +286,7 @@ static inline void __bpf_spin_lock(struct bpf_spin_lock *lock)
>   	compiletime_assert(u.val == 0, "__ARCH_SPIN_LOCK_UNLOCKED not 0");
>   	BUILD_BUG_ON(sizeof(*l) != sizeof(__u32));
>   	BUILD_BUG_ON(sizeof(*lock) != sizeof(__u32));
> +	preempt_disable();
>   	arch_spin_lock(l);
>   }
>   
> @@ -294,6 +295,7 @@ static inline void __bpf_spin_unlock(struct bpf_spin_lock *lock)
>   	arch_spinlock_t *l = (void *)lock;
>   
>   	arch_spin_unlock(l);
> +	preempt_enable();
>   }

preempt_disable()/preempt_enable() is not needed. Is it possible we can
have a different bpf_spin_lock proto, e.g, bpf_spin_lock_sleepable_proto
which implements the above with preempt_disable()/preempt_enable()?
Not sure how much difference my proposal will make since current
bpf_spin_lock() region does not support func calls except some
graph api kfunc operations.

>   
>   #else
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 55607ab30522..33e4b854d2d4 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -5062,7 +5062,9 @@ static int map_kptr_match_type(struct bpf_verifier_env *env,
>    */
>   static bool in_rcu_cs(struct bpf_verifier_env *env)
>   {
> -	return env->cur_state->active_rcu_lock || !env->prog->aux->sleepable;
> +	return env->cur_state->active_rcu_lock ||
> +	       env->cur_state->active_lock.ptr ||
> +	       !env->prog->aux->sleepable;
>   }
>   
>   /* Once GCC supports btf_type_tag the following mechanism will be replaced with tag check */
> @@ -16980,11 +16982,6 @@ static int check_map_prog_compatibility(struct bpf_verifier_env *env,
>   			verbose(env, "tracing progs cannot use bpf_spin_lock yet\n");
>   			return -EINVAL;
>   		}
> -
> -		if (prog->aux->sleepable) {
> -			verbose(env, "sleepable progs cannot use bpf_spin_lock yet\n");
> -			return -EINVAL;
> -		}
>   	}
>   
>   	if (btf_record_has_field(map->record, BPF_TIMER)) {