On 05/23/2019 11:30 PM, Yonghong Song wrote:
> On 5/23/19 2:07 PM, Yonghong Song wrote:
>> On 5/23/19 9:28 AM, Daniel Borkmann wrote:
>>> On 05/23/2019 05:58 PM, Yonghong Song wrote:
>>>> On 5/23/19 8:41 AM, Daniel Borkmann wrote:
>>>>> On 05/22/2019 07:39 AM, 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.
>>>>>>
>>>>>> Signed-off-by: Yonghong Song <yhs@xxxxxx>
>>>>>> ---
>>>>>> include/uapi/linux/bpf.h | 17 +++++++++-
>>>>>> kernel/trace/bpf_trace.c | 67 ++++++++++++++++++++++++++++++++++++++++
>>>>>> 2 files changed, 83 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..f8cd0db7289f 100644
>>>>>> --- a/kernel/trace/bpf_trace.c
>>>>>> +++ b/kernel/trace/bpf_trace.c
>>>>>> @@ -567,6 +567,58 @@ 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;
>>>>>> + 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, current, PIDTYPE_TGID);
>>>>>> +}
>>>>>> +
>>>>>> +BPF_CALL_1(bpf_send_signal, u32, sig)
>>>>>> +{
>>>>>> + struct send_signal_irq_work *work = NULL;
>>>>>> +
>>>>>> + /* 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()) {
>>>>>
>>>>> Hm, bit confused, can't this only be done out of process context in
>>>>> general since only there current points to e.g. hhvm? I'm probably
>>>>> missing something. Could you elaborate?
>>>>
>>>> That is true. If in nmi, it is out of process context and in nmi
>>>> context, we use an irq_work here since group_send_sig_info() has
>>>> spinlock inside. The bpf program (e.g., a perf_event program) needs to
>>>> check it is with right current (e.g., by pid) before calling
>>>> this helper.
>>>>
>>>> Does this address your question?
>>
>> Thanks, Daniel. The below are really good questions which I did not
>> really think through with irq_work.
>>
>>> Hm, but how is it guaranteed that 'current' inside the callback is still
>>> the very same you intend to send the signal to?
>>
>> I went through irq_work infrastructure. It looks that "current" may
>> change. irq_work is registered as an interrupt on x86.
>> After nmi, some lower priority
>> interrupts get chances to execute including irq_work. But there are some
>> other interrupts like timer_interrupt and reschedule_interrupt may
>> change "current". But since we are still in interrupt context, task
>> should not be destroyed, so the task structure pointer is still valid.
>>
>> I will pass "current" task struct pointer to irq_work as well. This
>> is similar to what we did in stackmap.c:
>> work->sem = ¤t->mm->mmap_sem;
>> irq_work_queue(&work->irq_work);
>> At irq_work_run() time, the previous "current" in nmi should still be
>> valid.
>>
>>> What happens if you're in softirq and send SIGKILL to yourself? Is this
>>> ignored/handled gracefully in such case?
>>
>> It is not ignored. It handled gracefully in this case. I tried my
>> example to send SIGKILL. The call stack looks below.
>>
>> [ 24.211943] bpf_send_signal+0x9/0xb0
>> [ 24.212427] bpf_prog_fec6e7cc664d5b91_bpf_send_signal_test+0x228/0x1000
>> [ 24.213249] ? bpf_overflow_handler+0xb7/0x180
>> [ 24.213853] ? __perf_event_overflow+0x51/0xe0
>> [ 24.214385] ? perf_swevent_hrtimer+0x10a/0x160
>> [ 24.214965] ? __update_load_avg_cfs_rq+0x1a9/0x1c0
>> [ 24.215609] ? task_tick_fair+0x50/0x690
>> [ 24.216104] ? run_timer_softirq+0x208/0x490
>> [ 24.216637] ? timerqueue_del+0x1e/0x40
>> [ 24.217111] ? task_clock_event_del+0x10/0x10
>> [ 24.217658] ? __hrtimer_run_queues+0x10d/0x2c0
>> [ 24.218217] ? hrtimer_interrupt+0x122/0x270
>> [ 24.218765] ? rcu_irq_enter+0x31/0x110
>> [ 24.219223] ? smp_apic_timer_interrupt+0x67/0x160
>> [ 24.219842] ? apic_timer_interrupt+0xf/0x20
>> [ 24.220383] </IRQ>
>> [ 24.220655] ? event_sched_out.isra.108+0x150/0x150
>> [ 24.221271] ? smp_call_function_single+0xdc/0x100
>> [ 24.221898] ? perf_event_sysfs_show+0x20/0x20
>> [ 24.222469] ? cpu_function_call+0x42/0x60
>> [ 24.222982] ? cpu_clock_event_read+0x10/0x10
>> [ 24.223525] ? event_function_call+0xe6/0xf0
>> [ 24.224053] ? event_sched_out.isra.108+0x150/0x150
>> [ 24.224657] ? perf_remove_from_context+0x20/0x70
>> [ 24.225262] ? perf_event_release_kernel+0x106/0x2e0
>> [ 24.225896] ? perf_release+0xc/0x10
>> [ 24.226347] ? __fput+0xc9/0x230
>> [ 24.226767] ? task_work_run+0x83/0xb0
>> [ 24.227243] ? do_exit+0x300/0xc50
>> [ 24.227674] ? syscall_trace_enter+0x1c9/0x2d0
>> [ 24.228223] ? do_group_exit+0x39/0xb0
>> [ 24.228695] ? __x64_sys_exit_group+0x14/0x20
>> [ 24.229270] ? do_syscall_64+0x49/0x130
>> [ 24.229762] ? entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> I see the task is killed and other process is not impacted and
>> no kernel crash/warning.
Hm, but I rather meant when you have the case that we're in_serving_softirq()
e.g. when processing packets on rx and you send a signal to yourself. Shouldn't
we bail out from the helper in such situation as well?
Thanks,
Daniel
