libos+apps: Add kill

apps/CMakeLists.txt

@@ -40,3 +40,4 @@ luna_app(mktemp.cpp mktemp)
 luna_app(sysfuzz.cpp sysfuzz)
 luna_app(pivot_root.cpp pivot_root)
 luna_app(cp.cpp cp)
+luna_app(kill.cpp kill)

apps/kill.cpp

@@ -0,0 +1,23 @@
+#include <os/ArgumentParser.h>
+#include <os/Process.h>
+#include <stdlib.h>
+
+Result<int> luna_main(int argc, char** argv)
+{
+    StringView signo_sv = "15";
+    StringView process;
+
+    os::ArgumentParser parser;
+    parser.add_description("Send a signal to another process."_sv);
+    parser.add_system_program_info("kill"_sv);
+    parser.add_value_argument(signo_sv, 's', "signal", "the signal number to send");
+    parser.add_positional_argument(process, "pid", true);
+    parser.parse(argc, argv);
+
+    int signo = atoi(signo_sv.chars());
+    pid_t pid = atoi(process.chars());
+
+    TRY(os::Process::kill(pid, signo));
+
+    return 0;
+}

libos/include/os/Process.h

@@ -72,5 +72,14 @@ namespace os
          * child argument if ANY_CHILD was passed).
          */
         static Result<pid_t> wait(pid_t child, int* status, int options = 0);
+
+        /**
+         * @brief Send a signal to a process.
+         *
+         * @param pid The process ID of the process.
+         * @param signo The signal number to send. Can be 0 to simply test for a process's existence.
+         * @return Result<void> Whether the function succeeded.
+         */
+        static Result<void> kill(pid_t pid, int signo);
     };
 }

libos/src/Process.cpp

@@ -68,4 +68,10 @@ namespace os
         long rc = syscall(SYS_waitpid, child, status, options);
         return Result<pid_t>::from_syscall(rc);
     }
+
+    Result<void> Process::kill(pid_t pid, int signo)
+    {
+        long rc = syscall(SYS_kill, pid, signo);
+        return Result<void>::from_syscall(rc);
+    }
 }