Date:   Thu, 23 May 2019 21:30:59 +0000
From:   Yonghong Song <yhs@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...com>, Kernel Team <Kernel-team@...com>,
        "Peter Zijlstra" <peterz@...radead.org>
Subject: Re: [PATCH bpf-next v2 1/3] bpf: implement bpf_send_signal() helper



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@...com>
>>>>> ---
>>>>>     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 = &current->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.
> 
>>
>> I think some more elaborate comment in the code would definitely be help.
> 
> Definitely will add some comments.
> 
>>
>> Btw, you probably need to wrap it under #ifdef CONFIG_IRQ_WORK.
> 
> I will check this. stackmaps.c use irq_work as well and did not have
> CONFIG_IRQ_WORK. Maybe we are missing there as well.

Looks like we do not need CONFIG_IRQ_WORK.

We have:
obj-$(CONFIG_BPF_EVENTS) += bpf_trace.o

config BPF_EVENTS
         depends on BPF_SYSCALL
         depends on (KPROBE_EVENTS || UPROBE_EVENTS) && PERF_EVENTS

config PERF_EVENTS
         bool "Kernel performance events and counters"
         default y if PROFILING
         depends on HAVE_PERF_EVENTS
         select IRQ_WORK
> 
>>
>>>>> +		work = this_cpu_ptr(&send_signal_work);
>>>>> +		if (work->irq_work.flags & IRQ_WORK_BUSY)
>>>>> +			return -EBUSY;
>>>>> +
>>>>> +		work->sig = sig;
>>>>> +		irq_work_queue(&work->irq_work);
>>>>> +		return 0;
>>>>> +	}
>>>>> +
>>>>> +	return group_send_sig_info(sig, SEND_SIG_PRIV, current, PIDTYPE_TGID);
>>>>> +
>>>>
>>>> Nit: extra newline slipped in
>>> Thanks. Will remove this in the next revision.
>>>>
>>>>> +}
>>>>> +
>>>>> +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 +669,8 @@ 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 +1397,18 @@ 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 */
>>>>>
>>>>
>>

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