On 5/24/19 2:59 PM, Daniel Borkmann wrote:
> On 05/24/2019 11:39 PM, Daniel Borkmann wrote:
>> On 05/23/2019 11:47 PM, Yonghong Song wrote:
>>> This patch tries to solve the following specific use case.
>>>
>>> Currently, bpf program can already collect stack traces
>>> through kernel function get_perf_callchain()
>>> when certain events happens (e.g., cache miss counter or
>>> cpu clock counter overflows). But such stack traces are
>>> not enough for jitted programs, e.g., hhvm (jited php).
>>> To get real stack trace, jit engine internal data structures
>>> need to be traversed in order to get the real user functions.
>>>
>>> bpf program itself may not be the best place to traverse
>>> the jit engine as the traversing logic could be complex and
>>> it is not a stable interface either.
>>>
>>> Instead, hhvm implements a signal handler,
>>> e.g. for SIGALARM, and a set of program locations which
>>> it can dump stack traces. When it receives a signal, it will
>>> dump the stack in next such program location.
>>>
>>> Such a mechanism can be implemented in the following way:
>>>    . a perf ring buffer is created between bpf program
>>>      and tracing app.
>>>    . once a particular event happens, bpf program writes
>>>      to the ring buffer and the tracing app gets notified.
>>>    . the tracing app sends a signal SIGALARM to the hhvm.
>>>
>>> But this method could have large delays and causing profiling
>>> results skewed.
>>>
>>> This patch implements bpf_send_signal() helper to send
>>> a signal to hhvm in real time, resulting in intended stack traces.
>>>
>>> Acked-by: Andrii Nakryiko <andr...@fb.com>
>>> Signed-off-by: Yonghong Song <y...@fb.com>
>>> ---
>>>   include/uapi/linux/bpf.h | 17 +++++++++-
>>>   kernel/trace/bpf_trace.c | 72 ++++++++++++++++++++++++++++++++++++++++
>>>   2 files changed, 88 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>>> index 63e0cf66f01a..68d4470523a0 100644
>>> --- a/include/uapi/linux/bpf.h
>>> +++ b/include/uapi/linux/bpf.h
>>> @@ -2672,6 +2672,20 @@ union bpf_attr {
>>>    *                0 on success.
>>>    *
>>>    *                **-ENOENT** if the bpf-local-storage cannot be found.
>>> + *
>>> + * int bpf_send_signal(u32 sig)
>>> + * Description
>>> + *         Send signal *sig* to the current task.
>>> + * Return
>>> + *         0 on success or successfully queued.
>>> + *
>>> + *         **-EBUSY** if work queue under nmi is full.
>>> + *
>>> + *         **-EINVAL** if *sig* is invalid.
>>> + *
>>> + *         **-EPERM** if no permission to send the *sig*.
>>> + *
>>> + *         **-EAGAIN** if bpf program can try again.
>>>    */
>>>   #define __BPF_FUNC_MAPPER(FN)             \
>>>     FN(unspec),                     \
>>> @@ -2782,7 +2796,8 @@ union bpf_attr {
>>>     FN(strtol),                     \
>>>     FN(strtoul),                    \
>>>     FN(sk_storage_get),             \
>>> -   FN(sk_storage_delete),
>>> +   FN(sk_storage_delete),          \
>>> +   FN(send_signal),
>>>   
>>>   /* integer value in 'imm' field of BPF_CALL instruction selects which 
>>> helper
>>>    * function eBPF program intends to call
>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>>> index f92d6ad5e080..70029eafc71f 100644
>>> --- a/kernel/trace/bpf_trace.c
>>> +++ b/kernel/trace/bpf_trace.c
>>> @@ -567,6 +567,63 @@ static const struct bpf_func_proto 
>>> bpf_probe_read_str_proto = {
>>>     .arg3_type      = ARG_ANYTHING,
>>>   };
>>>   
>>> +struct send_signal_irq_work {
>>> +   struct irq_work irq_work;
>>> +   struct task_struct *task;
>>> +   u32 sig;
>>> +};
>>> +
>>> +static DEFINE_PER_CPU(struct send_signal_irq_work, send_signal_work);
>>> +
>>> +static void do_bpf_send_signal(struct irq_work *entry)
>>> +{
>>> +   struct send_signal_irq_work *work;
>>> +
>>> +   work = container_of(entry, struct send_signal_irq_work, irq_work);
>>> +   group_send_sig_info(work->sig, SEND_SIG_PRIV, work->task, PIDTYPE_TGID);
>>> +}
>>> +
>>> +BPF_CALL_1(bpf_send_signal, u32, sig)
>>> +{
>>> +   struct send_signal_irq_work *work = NULL;
>>> +
> 
> Oh, and one more thing:
> 
>       if (!valid_signal(sig))
>               return -EINVAL;
> 
> Otherwise when deferring the work, you don't have any such feedback.

Good advice! Do you want me send a followup patch or
resend the whole series?

> 
>>> +   /* Similar to bpf_probe_write_user, task needs to be
>>> +    * in a sound condition and kernel memory access be
>>> +    * permitted in order to send signal to the current
>>> +    * task.
>>> +    */
>>> +   if (unlikely(current->flags & (PF_KTHREAD | PF_EXITING)))
>>> +           return -EPERM;
>>> +   if (unlikely(uaccess_kernel()))
>>> +           return -EPERM;
>>> +   if (unlikely(!nmi_uaccess_okay()))
>>> +           return -EPERM;
>>> +
>>> +   if (in_nmi()) {
>>> +           work = this_cpu_ptr(&send_signal_work);
>>> +           if (work->irq_work.flags & IRQ_WORK_BUSY)
>>
>> Given here and in stackmap are the only two users outside of 
>> kernel/irq_work.c,
>> it may probably be good to add a small helper to include/linux/irq_work.h and
>> use it for both.
>>
>> Perhaps something like ...
>>
>> static inline bool irq_work_busy(struct irq_work *work)
>> {
>>      return READ_ONCE(work->flags) & IRQ_WORK_BUSY;
>> }
>>
>>> +                   return -EBUSY;
>>> +
>>> +           /* Add the current task, which is the target of sending signal,
>>> +            * to the irq_work. The current task may change when queued
>>> +            * irq works get executed.
>>> +            */
>>> +           work->task = current;
>>> +           work->sig = sig;
>>> +           irq_work_queue(&work->irq_work);
>>> +           return 0;
>>> +   }
>>> +
>>> +   return group_send_sig_info(sig, SEND_SIG_PRIV, current, PIDTYPE_TGID);
>>> +}
>>> +
>>> +static const struct bpf_func_proto bpf_send_signal_proto = {
>>> +   .func           = bpf_send_signal,
>>> +   .gpl_only       = false,
>>> +   .ret_type       = RET_INTEGER,
>>> +   .arg1_type      = ARG_ANYTHING,
>>> +};
>>> +
>>>   static const struct bpf_func_proto *
>>>   tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>>>   {
>>> @@ -617,6 +674,8 @@ tracing_func_proto(enum bpf_func_id func_id, const 
>>> struct bpf_prog *prog)
>>>     case BPF_FUNC_get_current_cgroup_id:
>>>             return &bpf_get_current_cgroup_id_proto;
>>>   #endif
>>> +   case BPF_FUNC_send_signal:
>>> +           return &bpf_send_signal_proto;
>>>     default:
>>>             return NULL;
>>>     }
>>> @@ -1343,5 +1402,18 @@ static int __init bpf_event_init(void)
>>>     return 0;
>>>   }
>>>   
>>> +static int __init send_signal_irq_work_init(void)
>>> +{
>>> +   int cpu;
>>> +   struct send_signal_irq_work *work;
>>> +
>>> +   for_each_possible_cpu(cpu) {
>>> +           work = per_cpu_ptr(&send_signal_work, cpu);
>>> +           init_irq_work(&work->irq_work, do_bpf_send_signal);
>>> +   }
>>> +   return 0;
>>> +}
>>> +
>>>   fs_initcall(bpf_event_init);
>>> +subsys_initcall(send_signal_irq_work_init);
>>>   #endif /* CONFIG_MODULES */
>>>
>>
> 

Reply via email to