unix socket path length problems

[cfergeau]

unix socket paths have a very limited length, 104 bytes on macos, 108 on linux

Podman is currently hitting this limit with vfkit:

  time="2024-08-20T10:16:30-05:00" level=info msg="Using unix socket /var/folders/l0/3hgwcpld7h3gzm5s82dm48_w0000gn/T/podman/30247524d8c0-gvproxy.sock"
  Error: dial unixgram /var/folders/l0/3hgwcpld7h3gzm5s82dm48_w0000gn/T/podman_test2150468280/Library/Application Support/vfkit/net-5639-2529860791.sock->/var/folders/l0/3hgwcpld7h3gzm5s82dm48_w0000gn/T/podman/30247524d8c0-gvproxy.sock: bind: invalid argument

This path is generated in

vfkit/pkg/vf/virtionet.go

Lines 21 to 41 in 966a928

	func localUnixSocketPath() (string, error) {
	homeDir, err := os.UserHomeDir()
	if err != nil {
	return "", err
	}
	dir := filepath.Join(homeDir, "Library", "Application Support", "vfkit")
	if err := os.MkdirAll(dir, 0755); err != nil {
	return "", err
	}
	tmpFile, err := os.CreateTemp(dir, fmt.Sprintf("net-%d-*.sock", os.Getpid()))
	if err != nil {
	return "", err
	}
	// slightly racy, but this is in a directory only user-writable
	defer tmpFile.Close()
	defer os.Remove(tmpFile.Name())
	return tmpFile.Name(), nil
	}

[cfergeau]

We should also report a proper error when the path length is too long

[cfergeau]

Possible solutions:

• allow to override the path the socket goes to with a local variable
• put the vfkit socket in the same directory as the socket it's connecting to

[cfergeau]

Tentative patch for the 2nd option is

diff --git a/pkg/vf/virtionet.go b/pkg/vf/virtionet.go
index 72fd73c..28e257d 100644
--- a/pkg/vf/virtionet.go
+++ b/pkg/vf/virtionet.go
@@ -19,20 +19,12 @@ type VirtioNet struct {
 	localAddr *net.UnixAddr
 }
 
-func localUnixSocketPath() (string, error) {
-	homeDir, err := os.UserHomeDir()
+func localUnixSocketPath(dir string) (string, error) {
+	tmpFile, err := os.CreateTemp(dir, fmt.Sprintf("vfkit-%d-*.sock", os.Getpid()))
 	if err != nil {
 		return "", err
 	}
-	dir := filepath.Join(homeDir, "Library", "Application Support", "vfkit")
-	if err := os.MkdirAll(dir, 0755); err != nil {
-		return "", err
-	}
-	tmpFile, err := os.CreateTemp(dir, fmt.Sprintf("net-%d-*.sock", os.Getpid()))
-	if err != nil {
-		return "", err
-	}
-	// slightly racy, but this is in a directory only user-writable
+	// slightly racy, but hopefully this is in a directory only user-writable
 	defer tmpFile.Close()
 	defer os.Remove(tmpFile.Name())
 
@@ -44,7 +36,7 @@ func (dev *VirtioNet) connectUnixPath() error {
 		Name: dev.UnixSocketPath,
 		Net:  "unixgram",
 	}
-	localSocketPath, err := localUnixSocketPath()
+	localSocketPath, err := localUnixSocketPath(filepath.Dir(dev.UnixSocketPath))
 	if err != nil {
 		return err
 	}

(not even compile tested)

[nirs]

If the socket is created by vfkit, it can be under ~/.vfkit/. The is no need to hope that the directory is user writable, vfkit can enforce this on startup and fail to start if the permissions are unexpected.