On Mon, Nov 19, 2018 at 2:33 AM Christian Brauner [off-list ref] wrote:

The kill() syscall operates on process identifiers. After a process has
exited its pid can be reused by another process. If a caller sends a signal
to a reused pid it will end up signaling the wrong process. This issue has
often surfaced and there has been a push [1] to address this problem.

A prior patch has introduced the ability to get a file descriptor
referencing struct pid by opening /proc/<pid>. This guarantees a stable
handle on a process which can be used to send signals to the referenced
process. Discussion has shown that a dedicated syscall is preferable over
ioctl()s. Thus, the  new syscall procfd_signal() is introduced to solve
this problem. It operates on a process file descriptor.
The syscall takes an additional siginfo_t and flags argument. If siginfo_t
is NULL then procfd_signal() behaves like kill() if it is not NULL it
behaves like rt_sigqueueinfo.
The flags argument is added to allow for future extensions of this syscall.
It currently needs to be passed as 0.

A few questions.  First: you've made this work on /proc/PID, but
should it also work on /proc/PID/task/TID to send signals to a
specific thread?

+bool proc_is_procfd(const struct file *file)
+{
+       return d_is_dir(file->f_path.dentry) &&
+              (file->f_op == &proc_tgid_base_operations);
+}

Maybe rename to proc_is_tgid_procfd() to leave room for proc_is_tid_procfd()?

+       if (info) {
+               ret = __copy_siginfo_from_user(sig, &kinfo, info);
+               if (unlikely(ret))
+                       goto err;
+               /*
+                * Not even root can pretend to send signals from the kernel.
+                * Nor can they impersonate a kill()/tgkill(), which adds
+                * source info.
+                */
+               ret = -EPERM;
+               if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
+                   (task_pid(current) != pid))
+                       goto err;

Is the exception for signaling yourself actually useful here?