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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Aug 21, 2023 at 07:53:22PM -0700, Yonghong Song wrote:
> 
> 
> 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

preempt_disable is needed in all cases. This mistake slipped in when
we converted preempt disabled bpf progs into migrate disabled.
For example, see how raw_spin_lock is doing it.