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?
> + 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
> +}
> +
> +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 */
>
