Fix setsid(2) fails with EPERM when the binary has SUID bit

It is always good to read APUE before writing anything Unix.

1 files changed, 43 insertions, 24 deletions

diff --git a/main.c b/main.c
index 8c537d3..a05f0a1 100644
--- a/main.c
+++ b/main.c
@@ -5,6 +5,7 @@
 #include <string.h>
 #include <signal.h>
 #include <errno.h>
+#include <sys/wait.h>
 
 struct auth_env a_env;
 struct proc_env p_env = {
@@ -54,32 +55,50 @@ int main(int argc, char **argv) {
     }
     a_env.exe[len] = '\0';
 
-    /* Because we need to frequently dup(2). */
-    setbuf(stdout, NULL);
-    signal(SIGHUP, SIG_IGN);
-    struct sigaction sa = {
-            .sa_flags = 0,
-            .sa_handler = interrupt_int
-    };
-    sigemptyset(&sa.sa_mask);
-    /* Return EINTR on these signals to exit safely. */
-    sigaction(SIGINT, &sa, NULL);
-    sigaction(SIGTERM, &sa, NULL);
+    pid_t chld;
+    /* Detach from the current session. */
+    if (!(chld = fork())) {
+        /* Because we need to frequently dup(2). */
+        setbuf(stdout, NULL);
+        signal(SIGHUP, SIG_IGN);
+        struct sigaction sa = {
+                .sa_flags = 0,
+                .sa_handler = interrupt_int
+        };
+        sigemptyset(&sa.sa_mask);
+        /* Return EINTR on these signals to exit safely. */
+        sigaction(SIGINT, &sa, NULL);
+        sigaction(SIGTERM, &sa, NULL);
 
-    int r = sd_setup();
-    if (r == -1) {
-        sd_cleanup();
-        return 13;
+        int r = sd_setup();
+        if (r == -1) {
+            sd_cleanup();
+            return 13;
+        }
+        if (r) {
+            sd_cleanup();
+            r = main_consent(0);
+        } else {
+            dprintf(p_env.err, "Complete authorization on TTY %d by running `chvt %d`.\n",
+                    p_env.vt,
+                    p_env.vt);
+            r = main_consent(1);
+            sd_cleanup();
+        }
+        return r;
     }
-    if (r) {
-        sd_cleanup();
-        r = main_consent(0);
+    if (chld == -1) {
+        LOGFV("Cannot fork: %m", errno);
+        return errno;
+    }
+    int status;
+    if (waitpid(chld, &status, 0) == -1) {
+        LOGFV("Cannot wait for child: %m", errno);
+        return errno;
+    }
+    if (WIFEXITED(status)) {
+        return WEXITSTATUS(status);
     } else {
-        dprintf(p_env.err, "Complete authorization on TTY %d by running `chvt %d`.\n",
-                p_env.vt,
-                p_env.vt);
-        r = main_consent(1);
-        sd_cleanup();
+        return 255;
     }
-    return r;
 }