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

[Alexei Starovoitov]

On Tue, Oct 15, 2024 at 2:57 AM Puranjay Mohan <puranjay@xxxxxxxxxx> wrote:
>
> 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.

Still, let's avoid a pointless copy.
If I'm reading it correctly it will copy uninitialized memory
and sanitizers won't be happy.

Pls respin.