[MODERATED] Re: [PATCH 3/5] SSB extra 1

[Dave Hansen <dave.hansen@linux.intel.com>]

On 05/04/2018 06:33 AM, speck for Thomas Gleixner wrote:
> On Thu, 3 May 2018, speck for Dave Hansen wrote:
>>  static void bpf_done_on_this_cpu(struct work_struct *work)
>>  {
>> -	if (!this_cpu_dec_return(bpf_prog_ran))
>> -		return;
>> +	if (this_cpu_dec_return(bpf_prog_ran)) {
>> +		/*
>> +		 * This is unexpected.  The elevated refcount indicates
>> +		 * being in the *middle* of a BPF program, which should
>> +		 * be impossible.  They are executed inside
>> +		 * rcu_read_lock() where we can not sleep and where
>> +		 * preemption is disabled.
> 
> Preemptible RCU exists in mainline since 2008, so your assumption that
> preemption is disabled is simply wrong. And please don't just slap a
> preempt_disable() somewhere as this really can be done without. It's per
> thread information not per cpu.

Ahhh, that's an interesting point.  Are you thinking that we add a BPF
"instruction" to enable or disable the mitigations, and then have the
verifier insert it as the first instruction and then before any exit
instructions?  Is there some precedent for doing this?