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Date: Sat, 25 May 2019 19:14:20 +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 v5 1/3] bpf: implement bpf_send_signal() helper
On 5/24/19 3:37 PM, Daniel Borkmann wrote:
> On 05/25/2019 12:20 AM, Yonghong Song wrote:
>> On 5/24/19 2:39 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 <andriin@...com>
>>>> Signed-off-by: Yonghong Song <yhs@...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;
>>>> +
>>>> + /* 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;
>>> }
>>
>> Not sure whether READ_ONCE is needed here or not.
>>
>> The irq_work is per cpu data structure,
>> static DEFINE_PER_CPU(struct send_signal_irq_work, send_signal_work);
>> so presumably no collision for work->flags memory reference.
>
> The busy bit you're testing is cleared via cmpxchg(), kernel/irq_work.c +169:
>
> cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
Looks like for bpf case, we have bpf_prog_active guarding,
so nested nmi won't trigger nested bpf_send_signal() call,
so we should be fine without READ_ONCE here.
Also,
struct irq_work {
unsigned long flags;
struct llist_node llnode;
void (*func)(struct irq_work *);
};
-bash-4.4$ egrep -r 'work->flags' irq_work.c
flags = work->flags & ~IRQ_WORK_PENDING;
oflags = cmpxchg(&work->flags, flags, nflags);
if (work->flags & IRQ_WORK_LAZY) {
flags = work->flags & ~IRQ_WORK_PENDING;
xchg(&work->flags, flags);
(void)cmpxchg(&work->flags, flags, flags & ~IRQ_WORK_BUSY);
while (work->flags & IRQ_WORK_BUSY)
-bash-4.4$
From the above, `flags` is unsigned long and no READ_ONCE
is used, people may already assume atomic read_once is available,
so not using it.
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