lib: Make sure stdin/stdout/stderr have fd at startup and keep /dev/null always open

This guarantees we will always have a fd that can be used as /dev/null.
It works even inside chroots.

diff --git a/src/lib/lib.c b/src/lib/lib.c
index b0a4b63c32fd1b3598f5cebec86719c272ad0ca7..c1605cad8ce8bde98221409a27bfaa17b2957599 100644 (file)
--- a/src/lib/lib.c
+++ b/src/lib/lib.c
@@ -4,13 +4,16 @@
 #include "array.h"
 #include "env-util.h"
 #include "hostpid.h"
+#include "fd-close-on-exec.h"
 #include "ipwd.h"
 #include "process-title.h"
 
+#include <fcntl.h>
 #include <unistd.h>
 #include <sys/time.h>
 
 static bool lib_initialized = FALSE;
+int dev_null_fd = -1;
 
 struct atexit_callback {
        int priority;
@@ -115,6 +118,24 @@ void lib_atexit_run(void)
        }
 }
 
+static void lib_open_non_stdio_dev_null(void)
+{
+       dev_null_fd = open("/dev/null", O_WRONLY);
+       if (dev_null_fd == -1)
+               i_fatal("open(/dev/null) failed: %m");
+       /* Make sure stdin, stdout and stderr fds exist. We especially rely on
+          stderr being available and a lot of code doesn't like fd being 0.
+          We'll open /dev/null as write-only also for stdin, since if any
+          reads are attempted from it we'll want them to fail. */
+       while (dev_null_fd < STDERR_FILENO) {
+               dev_null_fd = dup(dev_null_fd);
+               if (dev_null_fd == -1)
+                       i_fatal("dup(/dev/null) failed: %m");
+       }
+       /* close the actual /dev/null fd on exec*(), but keep it in stdio fds */
+       fd_close_on_exec(dev_null_fd, TRUE);
+}
+
 void lib_init(void)
 {
        struct timeval tv;
@@ -127,6 +148,8 @@ void lib_init(void)
 
        data_stack_init();
        hostpid_init();
+       lib_open_non_stdio_dev_null();
+
        lib_initialized = TRUE;
 }
 
@@ -142,6 +165,7 @@ void lib_deinit(void)
        lib_atexit_run();
        ipwd_deinit();
        hostpid_deinit();
+       i_close_fd(&dev_null_fd);
        data_stack_deinit();
        env_deinit();
        failures_deinit();

diff --git a/src/lib/lib.h b/src/lib/lib.h
index 9a2eea9326561f20d2e950f9fadd907f55911adb..15819247599520d7cf58be0d4d7ff33a3313a047 100644 (file)
--- a/src/lib/lib.h
+++ b/src/lib/lib.h
@@ -49,6 +49,10 @@ typedef void lib_atexit_callback_t(void);
 #define LIB_ATEXIT_PRIORITY_DEFAULT 0
 #define LIB_ATEXIT_PRIORITY_LOW 10
 
+/* /dev/null opened as O_WRONLY. Opened at lib_init(), so it can be accessed
+   also inside chroots. */
+extern int dev_null_fd;
+
 int close_keep_errno(int *fd);
 /* Call unlink(). If it fails, log an error including the source filename
    and line number. */