Re: [PATCH bpf-next v4 1/2] bpf: implement bpf_send_signal_task() kfunc

[Puranjay Mohan]

Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> writes:

> On Tue, Oct 8, 2024 at 4:49 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>>
>> Implement bpf_send_signal_task kfunc that is similar to
>> bpf_send_signal_thread and bpf_send_signal helpers  but can be used to
>> send signals to other threads and processes. It also supports sending a
>> cookie with the signal similar to sigqueue().
>>
>> If the receiving process establishes a handler for the signal using the
>> SA_SIGINFO flag to sigaction(), then it can obtain this cookie via the
>> si_value field of the siginfo_t structure passed as the second argument
>> to the handler.
>>
>> Signed-off-by: Puranjay Mohan <puranjay@xxxxxxxxxx>
>> ---
>>  kernel/bpf/helpers.c     |  1 +
>>  kernel/trace/bpf_trace.c | 52 +++++++++++++++++++++++++++++++++-------
>>  2 files changed, 45 insertions(+), 8 deletions(-)
>>
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 4053f279ed4cc..2fd3feefb9d94 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -3035,6 +3035,7 @@ BTF_ID_FLAGS(func, bpf_task_get_cgroup1, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
>>  #endif
>>  BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
>>  BTF_ID_FLAGS(func, bpf_throw)
>> +BTF_ID_FLAGS(func, bpf_send_signal_task, KF_TRUSTED_ARGS)
>>  BTF_KFUNCS_END(generic_btf_ids)
>>
>>  static const struct btf_kfunc_id_set generic_kfunc_set = {
>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>> index a582cd25ca876..d9662e84510d3 100644
>> --- a/kernel/trace/bpf_trace.c
>> +++ b/kernel/trace/bpf_trace.c
>> @@ -802,6 +802,8 @@ struct send_signal_irq_work {
>>         struct task_struct *task;
>>         u32 sig;
>>         enum pid_type type;
>> +       bool has_siginfo;
>> +       struct kernel_siginfo info;
>>  };
>>
>>  static DEFINE_PER_CPU(struct send_signal_irq_work, send_signal_work);
>> @@ -809,27 +811,46 @@ 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;
>> +       struct kernel_siginfo *siginfo;
>>
>>         work = container_of(entry, struct send_signal_irq_work, irq_work);
>> -       group_send_sig_info(work->sig, SEND_SIG_PRIV, work->task, work->type);
>> +       siginfo = work->has_siginfo ? &work->info : SEND_SIG_PRIV;
>> +
>> +       group_send_sig_info(work->sig, siginfo, work->task, work->type);
>>         put_task_struct(work->task);
>>  }
>>
>> -static int bpf_send_signal_common(u32 sig, enum pid_type type)
>> +static int bpf_send_signal_common(u32 sig, enum pid_type type, struct task_struct *task, u64 value)
>>  {
>>         struct send_signal_irq_work *work = NULL;
>> +       struct kernel_siginfo info;
>> +       struct kernel_siginfo *siginfo;
>> +
>> +       if (!task) {
>> +               task = current;
>> +               siginfo = SEND_SIG_PRIV;
>> +       } else {
>> +               clear_siginfo(&info);
>> +               info.si_signo = sig;
>> +               info.si_errno = 0;
>> +               info.si_code = SI_KERNEL;
>> +               info.si_pid = 0;
>> +               info.si_uid = 0;
>> +               info.si_value.sival_ptr = (void *)(unsigned long)value;
>> +               siginfo = &info;
>> +       }
>>
>>         /* 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)))
>> +       if (unlikely(task->flags & (PF_KTHREAD | PF_EXITING)))
>>                 return -EPERM;
>>         if (unlikely(!nmi_uaccess_okay()))
>>                 return -EPERM;
>>         /* Task should not be pid=1 to avoid kernel panic. */
>> -       if (unlikely(is_global_init(current)))
>> +       if (unlikely(is_global_init(task)))
>>                 return -EPERM;
>>
>>         if (irqs_disabled()) {
>> @@ -847,19 +868,21 @@ static int bpf_send_signal_common(u32 sig, enum pid_type type)
>>                  * to the irq_work. The current task may change when queued
>>                  * irq works get executed.
>>                  */
>> -               work->task = get_task_struct(current);
>> +               work->task = get_task_struct(task);
>> +               work->has_siginfo = siginfo == &info;
>> +               copy_siginfo(&work->info, &info);
>
> we shouldn't copy_siginfo() if !work->has_siginfo, no?

Yes, but it is only used when has_siginfo is true, so copying it doesn't
cause any problems. I just didn't want to add another check here.

> other than that, lgtm
>
> Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

Thanks for the Ack. I hope this can go in now!

Thanks,
Puranjay Mohan

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