Re: [PATCH bpf-next v2 1/3] bpf: implement bpf_send_signal() helper

[Yonghong Song <yhs@fb.com>]

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@fb.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?

> 
>> +		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 */
>>
>