main: Run 'ensure fds' code early during the program startup.

The files are apparently processed in alphabetic order, so cli_args.go is
processed before main.go. In order to run before the go-fuse imports, put
the 'ensure fds' code in a separate package. Debug messages are omitted
to avoid additional imports (that might contain other code messing up our
file descriptors).

2 files changed, 56 insertions, 0 deletions

diff --git a/internal/ensurefds012/ensurefds012.go b/internal/ensurefds012/ensurefds012.go
new file mode 100644
index 0000000..7872eb2
--- /dev/null
+++ b/internal/ensurefds012/ensurefds012.go
@@ -0,0 +1,52 @@
+package ensurefds012
+
+// Package ensurefds012 ensures that file descriptors 0,1,2 are open. It opens
+// multiple copies of /dev/null as required.
+// The Go stdlib as well as the gocryptfs code rely on the fact that
+// fds 0,1,2 are always open.
+//
+// Use like this:
+//
+//   import _ "github.com/rfjakob/gocryptfs/internal/ensurefds012"
+//
+// The import line MUST be in the alphabitcally first source code file of
+// package main!
+//
+// You can test if it works as expected by inserting a long sleep into main,
+// startings gocryptfs with all fds closed like this,
+//
+//   $ ./gocryptfs 0<&- 1>&- 2>&-
+//
+// and then checking the open fds. It should look like this:
+//
+//   $ ls -l /proc/$(pgrep gocryptfs)/fd
+//   total 0
+//   lrwx------. 1 jakob jakob 64 Jan  5 15:54 0 -> /dev/null
+//   lrwx------. 1 jakob jakob 64 Jan  5 15:54 1 -> /dev/null
+//   lrwx------. 1 jakob jakob 64 Jan  5 15:54 2 -> /dev/null
+//   l-wx------. 1 jakob jakob 64 Jan  5 15:54 3 -> /dev/null
+//   lrwx------. 1 jakob jakob 64 Jan  5 15:54 4 -> 'anon_inode:[eventpoll]'
+//
+// See https://github.com/rfjakob/gocryptfs/issues/320 for details.
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/exitcodes"
+)
+
+func init() {
+	fd, err := syscall.Open("/dev/null", syscall.O_RDWR, 0)
+	if err != nil {
+		os.Exit(exitcodes.DevNull)
+	}
+	for fd <= 2 {
+		fd, err = syscall.Dup(fd)
+		if err != nil {
+			os.Exit(exitcodes.DevNull)
+		}
+	}
+	// Close excess fd (usually fd 3)
+	syscall.Close(fd)
+}
diff --git a/internal/fusefrontend/dircache.go b/internal/fusefrontend/dircache.go
index 6d0d570..68fd99c 100644
--- a/internal/fusefrontend/dircache.go
+++ b/internal/fusefrontend/dircache.go
@@ -35,6 +35,8 @@ type dirCacheEntryStruct struct {
 func (e *dirCacheEntryStruct) Clear() {
 	// An earlier clear may have already closed the fd, or the cache
 	// has never been filled (fd is 0 in that case).
+	// Note: package ensurefds012, imported from main, guarantees that dirCache
+	// can never get fds 0,1,2.
 	if e.fd > 0 {
 		err := syscall.Close(e.fd)
 		if err != nil {
@@ -72,6 +74,8 @@ func (d *dirCacheStruct) Clear() {
 // Store the entry in the cache. The passed "fd" will be Dup()ed, and the caller
 // can close their copy at will.
 func (d *dirCacheStruct) Store(dirRelPath string, fd int, iv []byte) {
+	// Note: package ensurefds012, imported from main, guarantees that dirCache
+	// can never get fds 0,1,2.
 	if fd <= 0 || len(iv) != nametransform.DirIVLen {
 		log.Panicf("Store sanity check failed: fd=%d len=%d", fd, len(iv))
 	}