From 2a65f0ee4fa7d4eebd57a955fbd8db016c0c6fdb Mon Sep 17 00:00:00 2001
From: Andreas Kling <awesomekling@gmail.com>
Date: Thu, 2 May 2019 15:51:39 +0200
Subject: [PATCH] strace: Allow "strace command" to trace a process from start
 to finish.

Tracing a specific pid is now done via "strace -p PID".
To ensure we don't miss any syscalls, we fork and have the child immediately
SIGSTOP itself. Then when the parent has set up the systrace() fd, we send
SIGCONT to the child which then execs the command. :^)
---
 Userland/strace.cpp | 44 ++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 40 insertions(+), 4 deletions(-)

diff --git a/Userland/strace.cpp b/Userland/strace.cpp
index 374ac1cf80..bd8b9cf61b 100644
--- a/Userland/strace.cpp
+++ b/Userland/strace.cpp
@@ -1,22 +1,58 @@
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
+#include <string.h>
+#include <signal.h>
 #include <AK/Assertions.h>
 #include <AK/Types.h>
 #include <Kernel/Syscall.h>
 
+static int usage()
+{
+    printf("usage: strace [-p PID] [command...]\n");
+    return 0;
+}
+
 int main(int argc, char** argv)
 {
-    if (argc < 2)
-        return 1;
+    if (argc == 1)
+        return usage();
+
+    pid_t pid = -1;
+    bool pid_is_child = false;
+
+    if (!strcmp(argv[1], "-p")) {
+        if (argc != 3)
+            return usage();
+        pid = atoi(argv[2]);
+    } else {
+        pid_is_child = true;
+        pid = fork();
+        if (!pid) {
+            kill(getpid(), SIGSTOP);
+            int rc = execvp(argv[1], &argv[1]);
+            if (rc < 0) {
+                perror("execvp");
+                exit(1);
+            }
+            ASSERT_NOT_REACHED();
+        }
+    }
 
-    int pid = atoi(argv[1]);
     int fd = systrace(pid);
     if (fd < 0) {
         perror("systrace");
         return 1;
     }
 
+    if (pid_is_child) {
+        int rc = kill(pid, SIGCONT);
+        if (rc < 0) {
+            perror("kill(pid, SIGCONT)");
+            return 1;
+        }
+    }
+
     for (;;) {
         dword call[5];
         int nread = read(fd, &call, sizeof(call));
@@ -27,7 +63,7 @@ int main(int argc, char** argv)
             return 1;
         }
         ASSERT(nread == sizeof(call));
-        printf("%s(%#x, %#x, %#x) = %#x\n", Syscall::to_string((Syscall::Function)call[0]), call[1], call[2], call[3], call[4]);
+        fprintf(stderr, "%s(%#x, %#x, %#x) = %#x\n", Syscall::to_string((Syscall::Function)call[0]), call[1], call[2], call[3], call[4]);
     }
 
     int rc = close(fd);