From 027a8de6f547115fd90b8ee612fc616b61d5000c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Sat, 1 Aug 2020 09:33:43 +0200
Subject: [PATCH] slave: use an event FD to communicate errors after fork
 before exec

Replace the pipe we used to communicate errors after fork(), but
before exec() to the parent process with an event FD.

The overhead in the kernel is much lower to setup an event fd,
compared to a pipe.
---
 slave.c | 37 +++++++++++++++++--------------------
 1 file changed, 17 insertions(+), 20 deletions(-)

diff --git a/slave.c b/slave.c
index ed0f94be..944297a5 100644
--- a/slave.c
+++ b/slave.c
@@ -12,6 +12,7 @@
 
 #include <sys/stat.h>
 #include <sys/ioctl.h>
+#include <sys/eventfd.h>
 #include <fcntl.h>
 
 #define LOG_MODULE "slave"
@@ -231,7 +232,7 @@ slave_exec(int ptmx, char *argv[], int err_fd, bool login_shell,
     execvp(file, argv);
 
 err:
-    (void)!write(err_fd, &errno, sizeof(errno));
+    (void)!write(err_fd, &(uint64_t){errno}, sizeof(uint64_t));
     if (pts != -1)
         close(pts);
     if (ptmx != -1)
@@ -245,9 +246,9 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
             const char *term_env, const char *conf_shell, bool login_shell,
             const user_notifications_t *notifications)
 {
-    int fork_pipe[2];
-    if (pipe2(fork_pipe, O_CLOEXEC) < 0) {
-        LOG_ERRNO("failed to create pipe");
+    int error_fd = eventfd(0, EFD_CLOEXEC);
+    if (error_fd < 0) {
+        LOG_ERRNO("failed to create event FD");
         return -1;
     }
 
@@ -255,18 +256,16 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
     switch (pid) {
     case -1:
         LOG_ERRNO("failed to fork");
-        close(fork_pipe[0]);
-        close(fork_pipe[1]);
+        close(error_fd);
         return -1;
 
     case 0:
         /* Child */
-        close(fork_pipe[0]);  /* Close read end */
 
         if (chdir(cwd) < 0) {
             const int _errno = errno;
             LOG_ERRNO("failed to change working directory");
-            (void)!write(fork_pipe[1], &_errno, sizeof(_errno));
+            (void)!write(error_fd, &(uint64_t){_errno}, sizeof(uint64_t));
             _exit(_errno);
         }
 
@@ -281,7 +280,7 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
         {
             const int _errno = errno;
             LOG_ERRNO_P("failed to restore signals", errno);
-            (void)!write(fork_pipe[1], &_errno, sizeof(_errno));
+            (void)!write(error_fd, &(uint64_t){_errno}, sizeof(uint64_t));
             _exit(_errno);
         }
 
@@ -293,9 +292,10 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
         if (argc == 0) {
             char *shell_copy = strdup(conf_shell);
             if (!tokenize_cmdline(shell_copy, &_shell_argv)) {
+                const int _errno = errno;
                 free(shell_copy);
-                (void)!write(fork_pipe[1], &errno, sizeof(errno));
-                _exit(0);
+                (void)!write(error_fd, &(uint64_t){_errno}, sizeof(uint64_t));
+                _exit(_errno);
             }
             shell_argv = _shell_argv;
         } else {
@@ -311,26 +311,23 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
         if (is_valid_shell(shell_argv[0]))
             setenv("SHELL", shell_argv[0], 1);
 
-        slave_exec(ptmx, shell_argv, fork_pipe[1], login_shell, notifications);
+        slave_exec(ptmx, shell_argv, error_fd, login_shell, notifications);
         assert(false);
         break;
 
     default: {
-        close(fork_pipe[1]); /* Close write end */
         LOG_DBG("slave has PID %d", pid);
 
-        int _errno;
-        static_assert(sizeof(errno) == sizeof(_errno), "errno size mismatch");
-
-        ssize_t ret = read(fork_pipe[0], &_errno, sizeof(_errno));
-        close(fork_pipe[0]);
+        uint64_t _errno;
+        ssize_t ret = read(error_fd, &_errno, sizeof(_errno));
+        close(error_fd);
 
         if (ret < 0) {
-            LOG_ERRNO("failed to read from pipe");
+            LOG_ERRNO("failed to read from error FD");
             return -1;
         } else if (ret == sizeof(_errno)) {
             LOG_ERRNO_P(
-                "%s: failed to execute", _errno, argc == 0 ? conf_shell : argv[0]);
+                "%s: failed to execute", (int)_errno, argc == 0 ? conf_shell : argv[0]);
             return -1;
         } else
             LOG_DBG("%s: successfully started", conf_shell);