From e03c713294a94fbd9b175c4f7b99607064e43529 Mon Sep 17 00:00:00 2001
From: Kovid Goyal <kovid@kovidgoyal.net>
Date: Fri, 5 Jan 2018 00:24:15 +0530
Subject: [PATCH] Remove a few more signal unsafe function calls between fork()
 and exec()

---
 kitty/child.c | 45 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 32 insertions(+), 13 deletions(-)

diff --git a/kitty/child.c b/kitty/child.c
index d5a84c975..051f3977b 100644
--- a/kitty/child.c
+++ b/kitty/child.c
@@ -23,54 +23,73 @@ serialize_string_tuple(PyObject *src) {
 
 extern char **environ;
 
+static inline void
+write_to_stderr(const char *text) {
+    size_t sz = strlen(text);
+    size_t written = 0;
+    while(written < sz) {
+        ssize_t amt = write(2, text + written, sz - written);
+        if (amt == 0) break;
+        if (amt < 0) {
+            if (errno == EAGAIN || errno == EINTR) continue;
+            break;
+        }
+        written += amt;
+    }
+}
+
 static PyObject*
 spawn(PyObject *self UNUSED, PyObject *args) {
     PyObject *argv_p, *env_p;
     int master, slave, stdin_read_fd, stdin_write_fd;
     char* cwd;
     if (!PyArg_ParseTuple(args, "sO!O!iiii", &cwd, &PyTuple_Type, &argv_p, &PyTuple_Type, &env_p, &master, &slave, &stdin_read_fd, &stdin_write_fd)) return NULL;
+    char name[2048] = {0};
+    if (ttyname_r(slave, name, sizeof(name) - 1) != 0) { PyErr_SetFromErrno(PyExc_OSError); return NULL; }
     char **argv = serialize_string_tuple(argv_p);
     char **env = serialize_string_tuple(env_p);
 
+#define exit_on_err(m) { write_to_stderr(m); write_to_stderr(": "); write_to_stderr(strerror(errno)); exit(EXIT_FAILURE); }
     pid_t pid = fork();
     if (pid == 0) {
         // child
-        // We cannot use malloc before exec() as it might deadlock if a thread in the parent process is in the middle of a malloc itself
+        // Use only signal-safe functions (man 7 signal-safety)
         if (chdir(cwd) != 0) { if (chdir("/") != 0) {} };  // ignore failure to chdir to /
-        if (setsid() == -1) { perror("setsid() in child process failed"); exit(EXIT_FAILURE); }
-        if (dup2(slave, 1) == -1) { perror("dup2() failed for fd number 1"); exit(EXIT_FAILURE); }
-        if (dup2(slave, 2) == -1) { perror("dup2() failed for fd number 2"); exit(EXIT_FAILURE); }
+        if (setsid() == -1) exit_on_err("setsid() in child process failed");
+        if (dup2(slave, 1) == -1) exit_on_err("dup2() failed for fd number 1");
+        if (dup2(slave, 2) == -1) exit_on_err("dup2() failed for fd number 2");
         if (stdin_read_fd > -1) {
-            if (dup2(stdin_read_fd, 0) == -1) { perror("dup2() failed for fd number 0"); exit(EXIT_FAILURE); }
+            if (dup2(stdin_read_fd, 0) == -1) exit_on_err("dup2() failed for fd number 0");
             close(stdin_read_fd);
             close(stdin_write_fd);
         } else {
-            if (dup2(slave, 0) == -1) { perror("dup2() failed for fd number 0"); exit(EXIT_FAILURE); }
+            if (dup2(slave, 0) == -1) exit_on_err("dup2() failed for fd number 0");
         }
         close(slave);
         close(master);
         for (int c = 3; c < 201; c++) close(c);
 
         // Establish the controlling terminal (see man 7 credentials)
-        char *name = ttyname(1);
-        if (name == NULL) { perror("Failed to call ttyname()"); exit(EXIT_FAILURE); }
         int tfd = open(name, O_RDWR);
-        if (tfd == -1) { perror("Failed to open controlling terminal"); exit(EXIT_FAILURE); }
+        if (tfd == -1) exit_on_err("Failed to open controlling terminal");
         close(tfd);
 
         environ = env;
         execvp(argv[0], argv);
         // Report the failure and exec a shell instead, so that we are not left
-        // with a forked but not execed process
-        fprintf(stderr, "Failed to launch child: %s\nWith error: %s [%d]\n", argv[0], strerror(errno), errno);
-        fprintf(stderr, "Press Enter to exit.\n");
-        fflush(stderr);
+        // with a forked but not exec'ed process
+        write_to_stderr("Failed to launch child: ");
+        write_to_stderr(argv[0]);
+        write_to_stderr("\nWith error: ");
+        write_to_stderr(strerror(errno));
+        write_to_stderr("\nPress Enter to exit.\n");
         execlp("sh", "sh", "-c", "read w", NULL);
         exit(EXIT_FAILURE);
     } else {
         free(argv);
         free(env);
     }
+#undef exit_on_err
     return PyLong_FromLong(pid);
 }