Re: [PATCH bpf-next v3 1/2] bpf: implement bpf_send_signal_task() kfunc

[Andrii Nakryiko]

On Mon, Oct 7, 2024 at 3:34 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 | 54 ++++++++++++++++++++++++++++++++++------
>  2 files changed, 47 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..ae8c9fa8b04d1 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;
> +       kernel_siginfo_t info;

group_send_sig_info() refers to this as `struct kernel_siginfo`, let's
use that and avoid unnecessary typedefs

>  };
>
>  static DEFINE_PER_CPU(struct send_signal_irq_work, send_signal_work);
> @@ -811,25 +813,43 @@ 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, work->type);
> +       if (work->has_siginfo)
> +               group_send_sig_info(work->sig, &work->info, work->task, work->type);
> +       else
> +               group_send_sig_info(work->sig, SEND_SIG_PRIV, work->task, work->type);

There is lots of duplication while the only difference is between
providing SEND_SIG_PRIV and our own &work->info. So maybe let's just
have something like

struct kernel_siginfo *siginfo;

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 *tsk, u64 value)

task? why tsk?

>  {
>         struct send_signal_irq_work *work = NULL;
> +       kernel_siginfo_t info;
> +       bool has_siginfo = false;
> +
> +       if (!tsk) {
> +               tsk = current;
> +       } else {
> +               has_siginfo = true;

nit: I find it less confusing for cases like with has_siginfo here,
for the variable to be explicitly assigned in both branches, instead
of defaulting to false and then reassigned in one of the branches

> +               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 *)value;
> +       }

kernel test bot complains that this should probably be (void
*)(unsigned long)value (which will truncate on 32-bit archtes, but oh
well)

but can you please double check that it's ok to set
info.si_value.sival_ptr for any signal? Because si_value.sival_ptr is
actually defined inside __sifields._rt._sigval, which clearly would
conflict with _kill, _timer, _sigchld and other groups of signals.

so I suspect we'd need to have a list of signals that are OK accepting
this extra u64 value, and reject it otherwise (instead of silently
corrupting data inside __sifields

pw-bot: cr

>
>         /* 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(tsk->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(tsk)))
>                 return -EPERM;
>
>         if (irqs_disabled()) {
> @@ -847,19 +867,24 @@ 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(tsk);
> +               work->has_siginfo = has_siginfo;
> +               work->info = info;

if you are using clear_siginfo(), you probably should use copy_siginfo() here?

>                 work->sig = sig;
>                 work->type = type;
>                 irq_work_queue(&work->irq_work);
>                 return 0;
>         }
>
> -       return group_send_sig_info(sig, SEND_SIG_PRIV, current, type);
> +       if (has_siginfo)
> +               return group_send_sig_info(sig, &info, tsk, type);
> +
> +       return group_send_sig_info(sig, SEND_SIG_PRIV, tsk, type);

Similarly to what I mentioned at the very top, the only difference is
a pointer to struct kernel_siginfo, so make it explicit?

struct kernel_siginfo *siginfo;

siginfo = task == current ? SEND_SIG_PRIV : &info;

?

>  }
>
>  BPF_CALL_1(bpf_send_signal, u32, sig)
>  {
> -       return bpf_send_signal_common(sig, PIDTYPE_TGID);
> +       return bpf_send_signal_common(sig, PIDTYPE_TGID, NULL, 0);
>  }
>
>  static const struct bpf_func_proto bpf_send_signal_proto = {
> @@ -871,7 +896,7 @@ static const struct bpf_func_proto bpf_send_signal_proto = {
>
>  BPF_CALL_1(bpf_send_signal_thread, u32, sig)
>  {
> -       return bpf_send_signal_common(sig, PIDTYPE_PID);
> +       return bpf_send_signal_common(sig, PIDTYPE_PID, NULL, 0);
>  }
>
>  static const struct bpf_func_proto bpf_send_signal_thread_proto = {
> @@ -3484,3 +3509,16 @@ static int __init bpf_kprobe_multi_kfuncs_init(void)
>  }
>
>  late_initcall(bpf_kprobe_multi_kfuncs_init);
> +
> +__bpf_kfunc_start_defs();
> +
> +__bpf_kfunc int bpf_send_signal_task(struct task_struct *task, int sig, enum pid_type type,
> +                                    u64 value)
> +{
> +       if (type != PIDTYPE_PID && type != PIDTYPE_TGID)
> +               return -EINVAL;
> +
> +       return bpf_send_signal_common(sig, type, task, value);
> +}
> +
> +__bpf_kfunc_end_defs();
> --
> 2.40.1
>