enforce permissions on all paths, use system user/group

- fileOwnerGroup: session internals owned by agent/agent,
  everything else by current system user and primary group
- checkPerm: always enforce, remove no-enforcement bypass
- Directory modes: /s/{sid}=755, /s/{sid}/t=500, /s=755,
  /u,/x=555, /p=664, /a=744, /sk=754, /tmp=777, /tr=766
- File modes: /p/=664, /sk/=664, /s/{sid}/t/*=500
- Session store: sh=550, b/bfg/bbg=555

server.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"net"
 	"os"
+	"os/user"
 	"path/filepath"
 	"strings"
 	"sync"
@@ -331,32 +332,32 @@ func (s *Server) InGroup(group, user string) bool {
 }
 
 // fileOwnerGroup returns the uid and gid for a given path.
-// Session files are owned by the session's principal with group "agent".
-// Prompt library (/p/) is owned by "ollie" with group "user".
-// Everything else is owned by "ollie" with group "ollie".
+// Session namespace (/s/{sid}/**) is owned by the agent principal with group "agent".
+// Everything else is owned by the current system user and their primary group.
 func (s *Server) fileOwnerGroup(path string) (uid, gid string) {
 	if strings.HasPrefix(path, "/s/") {
 		parts := strings.SplitN(strings.TrimPrefix(path, "/s/"), "/", 2)
-		if len(parts) >= 1 && parts[0] != "new" {
+		if len(parts) >= 1 && parts[0] != "new" && !isSessionStoreFile(path) {
 			if sess := s.sessionStore.Session(parts[0]); sess != nil {
 				return sess.Uname(), "agent"
 			}
 		}
 	}
-	if path == "/p" || strings.HasPrefix(path, "/p/") {
-		return "ollie", "user"
+	if u, err := user.Current(); err == nil {
+		gid := u.Gid
+		if g, err := user.LookupGroupId(u.Gid); err == nil {
+			gid = g.Name
+		}
+		return u.Username, gid
 	}
 	return "ollie", "ollie"
 }
 
 // checkPerm verifies that uname has the requested access (mode) to path.
 // mode is the 9P open mode: OREAD=0, OWRITE=1, ORDWR=2, OEXEC=3.
-// Enforced on session files and group-restricted paths (e.g. /p/).
+// Enforced on all paths.
 func (s *Server) checkPerm(uname, path string, mode uint8) error {
 	uid, gid := s.fileOwnerGroup(path)
-	if uid == "ollie" && gid == "ollie" {
-		return nil // no enforcement
-	}
 	dir := s.makeStat(path)
 	perm := uint32(dir.Mode) & 0777
 
@@ -1641,14 +1642,26 @@ func (s *Server) makeStat(path string) plan9.Dir {
 		if strings.HasPrefix(path, "/s/") {
 			parts := strings.SplitN(strings.TrimPrefix(path, "/s/"), "/", 3)
 			if (len(parts) == 2 && parts[1] == "t") || (len(parts) == 3 && parts[1] == "t") {
-				mode = plan9.DMDIR | 0700
+				mode = plan9.DMDIR | 0500 // rx owner only
 			} else {
-				mode = plan9.DMDIR | 0555
+				mode = plan9.DMDIR | 0755
 			}
-		} else if path == "/a" || path == "/m" || path == "/tmp" || path == "/u" {
+		} else if path == "/s" {
 			mode = plan9.DMDIR | 0755
-		} else {
+		} else if path == "/u" || path == "/x" {
 			mode = plan9.DMDIR | 0555
+		} else if path == "/p" {
+			mode = plan9.DMDIR | 0664
+		} else if path == "/a" {
+			mode = plan9.DMDIR | 0744
+		} else if path == "/sk" {
+			mode = plan9.DMDIR | 0754
+		} else if path == "/tmp" {
+			mode = plan9.DMDIR | 0777
+		} else if path == "/tr" {
+			mode = plan9.DMDIR | 0766
+		} else {
+			mode = plan9.DMDIR | 0755
 		}
 	} else {
 		switch base {
@@ -1671,19 +1684,19 @@ func (s *Server) makeStat(path string) plan9.Dir {
 			} else if strings.HasPrefix(path, "/a/") {
 				mode = 0666
 			} else if strings.HasPrefix(path, "/p/") {
-				mode = 0640
+				mode = 0664
 			} else if strings.HasPrefix(path, "/m/") {
 				mode = 0666
 			} else if path == "/sk/idx" {
 				mode = 0444
 			} else if strings.HasPrefix(path, "/sk/") {
-				mode = 0666
+				mode = 0664
 			} else if strings.HasPrefix(path, "/s/") {
 				parts := strings.SplitN(strings.TrimPrefix(path, "/s/"), "/", 3)
 				if len(parts) == 3 && parts[1] == "t" && parts[2] == "idx" {
 					mode = 0400
 				} else if len(parts) == 3 && parts[1] == "t" {
-					mode = 0700
+					mode = 0500
 				} else {
 					mode = 0444
 				}

store/session.go

@@ -83,7 +83,7 @@ var sessionStoreFiles = map[string]os.FileMode{
 	"idx":     0444,
 	"ls":      0555,
 	"kill":    0555,
-	"sh":      0555,
+	"sh":      0550,
 	"b":       0555,
 	"bfg":     0555,
 	"bbg":     0555,