9ns --mntgen: registry subdirectories are mount points too

A registry entry that is a directory is now served the way the root is:
a synthetic directory listing the real one, dialing the sockets inside
it on walk and recursing into further directories, to max_synth_depth
(8) levels across max_synth_dirs (64) synthetic nodes. That is the
plan9port mntgen shape and the layout zmx now posts under, so a live
session reads at /mnt/9p/zmx/<name>. Before this a directory in the
registry was dialed like a socket and answered EIO for good.

post gains the two entry points the traversal needs: postedDir (the
registry scan, against any directory) and dialPath (a dial by composed
path, no name validation).

Hardening, each from an attack that broke the code:

- BATCH_FORGET carries entries for many owners and puts 0 in the header
  nodeid, so routing it by the header dropped all of them: 32 of 64
  synthetic slots leaked in one close burst and the subdirectories that
  held them answered EIO forever. distributeForgets unpacks the body and
  hands each entry to its owner.
- probe() and connectBlocking() copied a caller's path into the kernel
  address with no bound: a path past sun_path overran the 110-byte stack
  sockaddr (a panic in Debug, silent corruption in ReleaseFast). Both
  refuse it now, probe as `.live` so a claim never deletes what it could
  not inspect.
- That bound then caught 9proc's own listener, which handed probe() the
  whole 108-byte sun_path array instead of the path inside it. The probe
  reads `.live` for anything it cannot ask about, so every stale socket
  became AlreadyListening and no server could ever take a dead
  predecessor's name back. It passes the path now.

Suites: 87/87 root (+7 post/serve attack regressions), 48/48 9ns,
51+88 9ns integration (+4 traversal and slot-recycling checks), 213/0
9ns adversarial, 60/60 9proc plus its adversarial suites with a new
stale-socket takeover check, freestanding green.

2 files changed, 361 insertions, 1 deletions

diff --git a/src/post.zig b/src/post.zig
index 47a7d3d..641970d 100644
--- a/src/post.zig
+++ b/src/post.zig
@@ -124,6 +124,14 @@ pub const PostedError = PathError || error{NoSpace} || Io.Dir.OpenError || Io.Di
 pub fn posted(io: Io, env: Env, out: []u8) PostedError!Names {
     var dir_buf: [std.fs.max_path_bytes]u8 = undefined;
     const dir_path = try registryDir(env, &dir_buf);
+    return postedDir(io, dir_path, out);
+}
+
+/// Lists one directory's entry names into `out` and returns an iterator
+/// over them — the registry scan, generalized for the registry
+/// subdirectories that 9ns mntgen serves. Nothing is dialed; a missing
+/// directory lists as empty.
+pub fn postedDir(io: Io, dir_path: [:0]const u8, out: []u8) PostedError!Names {
     const dir = Io.Dir.openDirAbsolute(io, dir_path, .{ .iterate = true }) catch |err| switch (err) {
         error.FileNotFound, error.NotDir => return .{ .bytes = out[0..0] },
         else => return err,
@@ -154,6 +162,10 @@ pub const Probe = enum { none, stale, live };
 /// `live`, because uncertainty must be owned by the server, never
 /// resolved by deleting what may be someone's socket.
 pub fn probe(path: [:0]const u8) Probe {
+    // A path that cannot fit a `sun_path` cannot be asked about at all;
+    // like `transport.isListening`, uncertainty is `.live` (occupied) so
+    // a claim never deletes a name it cannot inspect.
+    if (path.len >= sun_path_len) return .live;
     const fd = socketNonblocking() catch return .live;
     defer _ = linux.close(fd);
     var addr: linux.sockaddr.un = .{ .path = @splat(0) };
@@ -217,14 +229,37 @@ pub fn dial(io: Io, env: Env, name: []const u8) DialError!Io.net.Stream {
         error.AccessDenied => return error.AccessDenied,
         error.Loop => return error.SymLinkLoop,
         error.NotDir => return error.NotDir,
+        error.NameTooLong => return error.NameTooLong,
+    };
+    _ = io;
+    return .{ .socket = .{ .handle = fd, .address = .{ .ip4 = .loopback(0) } } };
+}
+
+/// Dials the socket at `path` — any registry path, including a
+/// subdirectory entry — and returns its stream. The same blocking,
+/// close-on-exec and refused-detection semantics as `dial`, with no
+/// name validation: the caller composed the path.
+pub fn dialPath(io: Io, path: [:0]const u8) DialError!Io.net.Stream {
+    const fd = connectBlocking(path) catch |err| switch (err) {
+        error.Socket => return error.SystemResources,
+        error.Noent => return error.NotPosted,
+        error.Refused => return error.Stale,
+        error.AccessDenied => return error.AccessDenied,
+        error.Loop => return error.SymLinkLoop,
+        error.NotDir => return error.NotDir,
+        error.NameTooLong => return error.NameTooLong,
     };
     _ = io;
     return .{ .socket = .{ .handle = fd, .address = .{ .ip4 = .loopback(0) } } };
 }
 
-const ConnectError = error{ Socket, Noent, Refused, AccessDenied, Loop, NotDir };
+const ConnectError = error{ Socket, Noent, Refused, AccessDenied, Loop, NotDir, NameTooLong };
 
 fn connectBlocking(path: [:0]const u8) ConnectError!i32 {
+    // The caller composed the path (dialPath runs no name validation), so
+    // it must still fit a `sun_path`; beyond it the copy into the kernel
+    // address would overrun the stack struct.
+    if (path.len >= sun_path_len) return error.NameTooLong;
     const rc = linux.socket(linux.AF.UNIX, linux.SOCK.STREAM | linux.SOCK.CLOEXEC, 0);
     if (rawErrno(@bitCast(rc)) != .SUCCESS) return error.Socket;
     const fd: i32 = @intCast(rc);
@@ -1022,3 +1057,104 @@ test "post watch: queue overflow surfaces, a deleted registry is gone" {
     try testing.expectEqualStrings("back", back.?.name);
     try Io.Dir.deleteFileAbsolute(io, try std.fmt.bufPrint(&name_buf, "{s}/back", .{reg}));
 }
+
+test "post probe/dialPath: a path past the sun_path budget is refused, never read" {
+    // A path longer than a sockaddr's `sun_path` cannot be handed to the
+    // kernel; the raw probe/dial copies must not overrun the stack
+    // address struct. `probe` owns the uncertainty as `.live` (never
+    // delete what cannot be inspected); `dialPath` names the failure.
+    var buf: [512]u8 = undefined;
+    const over = sun_path_len + 8;
+    @memset(buf[0..over], 'a');
+    buf[over] = 0;
+    const p: [:0]const u8 = buf[0..over :0];
+    try testing.expectEqual(Probe.live, probe(p));
+    try testing.expectError(error.NameTooLong, dialPath(testing.io, p));
+    // Exactly `sun_path_len` bytes leaves no room for a zero and is not a
+    // name either.
+    buf[sun_path_len] = 0;
+    const at: [:0]const u8 = buf[0..sun_path_len :0];
+    try testing.expectEqual(Probe.live, probe(at));
+    try testing.expectError(error.NameTooLong, dialPath(testing.io, at));
+}
+
+test "post dialPath: refused, missing, non-socket, loop and a CLOEXEC stream" {
+    if (builtin.os.tag != .linux) return error.SkipZigTest;
+    var s: Scratch = .{ .dir = undefined };
+    try s.start();
+    defer s.end();
+    const io = testing.io;
+    var real_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const rlen = try s.dir.dir.realPath(io, &real_buf);
+    const root = real_buf[0..rlen];
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+
+    // A live socket: dialPath yields a blocking, close-on-exec stream
+    // (the mounted PROGRAM must not inherit a dialed descriptor).
+    const sock_path = try std.fmt.bufPrintZ(&path_buf, "{s}/live.sock", .{root});
+    var server = try (try Io.net.UnixAddress.init(sock_path)).listen(io, .{});
+    {
+        var st = try dialPath(io, sock_path);
+        const fd = st.socket.handle;
+        try testing.expect(linux.fcntl(fd, linux.F.GETFD, 0) & linux.FD_CLOEXEC != 0);
+        st.close(io);
+    }
+    // The server dies without unlinking: refused, and the caller can tell.
+    server.deinit(io);
+    try testing.expectError(error.Stale, dialPath(io, sock_path));
+
+    // No entry at all.
+    try testing.expectError(error.NotPosted, dialPath(io, try std.fmt.bufPrintZ(&path_buf, "{s}/missing", .{root})));
+
+    // An entry that is not a socket answers the same ECONNREFUSED the
+    // kernel gives a dead socket; the caller tells them apart by stat
+    // before dialing (post.post does exactly that).
+    try testing.expectError(error.Stale, dialPath(io, try std.fmt.bufPrintZ(&path_buf, "{s}", .{root})));
+    const file_path = try std.fmt.bufPrintZ(&path_buf, "{s}/plain", .{root});
+    {
+        var f = try Io.Dir.createFileAbsolute(io, file_path, .{});
+        f.close(io);
+    }
+    try testing.expectError(error.Stale, dialPath(io, file_path));
+
+    // A symlink loop is its own error, not a missing entry.
+    const loop_path = try std.fmt.bufPrintZ(&path_buf, "{s}/loop", .{root});
+    try Io.Dir.symLinkAbsolute(io, loop_path, loop_path, .{});
+    try testing.expectError(error.SymLinkLoop, dialPath(io, loop_path));
+}
+
+test "post postedDir: a 1000-entry directory lists fully or refuses cleanly" {
+    if (builtin.os.tag != .linux) return error.SkipZigTest;
+    var s: Scratch = .{ .dir = undefined };
+    try s.start();
+    defer s.end();
+    const io = testing.io;
+    var real_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const rlen = try s.dir.dir.realPath(io, &real_buf);
+    var dir_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir = try std.fmt.bufPrintZ(&dir_buf, "{s}", .{real_buf[0..rlen]});
+    for (0..1000) |i| {
+        var name_buf: [32]u8 = undefined;
+        var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+        const name = try std.fmt.bufPrint(&name_buf, "e{d:0>4}", .{i});
+        var f = try Io.Dir.createFileAbsolute(io, try std.fmt.bufPrintZ(&path_buf, "{s}/{s}", .{ dir, name }), .{});
+        f.close(io);
+    }
+    // A staging buffer that cannot hold every record is an explicit
+    // NoSpace — never a silently truncated listing.
+    var small: [512]u8 = undefined;
+    try testing.expectError(error.NoSpace, postedDir(io, dir, &small));
+    // Big enough lists every entry, with the first and last intact.
+    var big: [16 * 1024]u8 = undefined;
+    var names = try postedDir(io, dir, &big);
+    var count: usize = 0;
+    var have_first = false;
+    var have_last = false;
+    while (names.next()) |name| {
+        count += 1;
+        have_first = have_first or std.mem.eql(u8, name, "e0000");
+        have_last = have_last or std.mem.eql(u8, name, "e0999");
+    }
+    try testing.expectEqual(@as(usize, 1000), count);
+    try testing.expect(have_first and have_last);
+}
diff --git a/src/serve.zig b/src/serve.zig
index 19a8ca2..10c8d01 100644
--- a/src/serve.zig
+++ b/src/serve.zig
@@ -441,6 +441,7 @@ pub fn Runner(comptime Backend: type, comptime opts: fs.Options, comptime limits
 const testing = std.testing;
 const c9 = @import("root.zig");
 const Client = c9.Client;
+const linux = std.os.linux;
 
 /// A read-only tree in the style of the engine's StubFs: `/index` is a
 /// file, `/event` parks its reads until `post()` and `/dir` is a directory.
@@ -936,6 +937,7 @@ test "serve: listenPosted serves the registry name and stop() unposts" {
     while (names.next()) |n| seen = seen or std.mem.eql(u8, n, "posted");
     try testing.expect(seen);
     rig.runner.stop();
+    rig.runner.stop(); // idempotent: the second stop unposts nothing more
     try testing.expectError(error.FileNotFound, Io.Dir.statFile(.cwd(), io, path, .{}));
     names = try post.posted(io, envp, &stage);
     try testing.expect(names.next() == null);
@@ -1013,3 +1015,225 @@ test "serve: listenPosted beside listen(): stop unposts only the registry name"
     try Io.Dir.deleteFileAbsolute(io, unix_path);
     rig.dir.cleanup();
 }
+
+// ---- runner capacity and process hygiene ----
+
+/// Counts the process's open descriptors through /proc, no allocation.
+fn openFdCount(io: Io) !usize {
+    var rb: [Io.Dir.Iterator.reader_buffer_len]u8 align(@alignOf(usize)) = undefined;
+    const dir = try Io.Dir.openDirAbsolute(io, "/proc/self/fd", .{ .iterate = true });
+    defer Io.Dir.close(dir, io);
+    var it = Io.Dir.Reader.init(dir, &rb);
+    var n: usize = 0;
+    while (try it.next(io)) |_| n += 1;
+    return n;
+}
+
+/// Counts the process's OS threads through /proc, no allocation.
+fn taskCount(io: Io) !usize {
+    var rb: [Io.Dir.Iterator.reader_buffer_len]u8 align(@alignOf(usize)) = undefined;
+    const dir = try Io.Dir.openDirAbsolute(io, "/proc/self/task", .{ .iterate = true });
+    defer Io.Dir.close(dir, io);
+    var it = Io.Dir.Reader.init(dir, &rb);
+    var n: usize = 0;
+    while (try it.next(io)) |_| n += 1;
+    return n;
+}
+
+fn waitCount(runner: anytype, n: usize) !void {
+    var tries: usize = 0;
+    while (runner.count() != n) : (tries += 1) {
+        if (tries == 5000) return error.Timeout;
+        try testing.io.sleep(.fromMilliseconds(1), .awake);
+    }
+}
+
+const BigRunner = Runner(Stub, .{ .fid_capacity = 8, .slot_capacity = 4 }, .{ .msize = 4096, .connections = 16, .listeners = 1 });
+
+fn serveNowBig(ctx: ?*anyopaque, conn: *BigRunner.Conn, req: fs.Req) void {
+    const a = Stub.of(ctx).handle(req);
+    conn.reply(&a.reply, a.bytes);
+}
+
+fn countClosedBig(ctx: ?*anyopaque, conn: *BigRunner.Conn) void {
+    _ = conn;
+    const st = Stub.of(ctx);
+    st.mutex.lockUncancelable(testing.io);
+    defer st.mutex.unlock(testing.io);
+    st.closed += 1;
+}
+
+test "serve: sixteen connections fill the table, the seventeenth is closed, stop ends all" {
+    const io = testing.io;
+    var dir = testing.tmpDir(.{});
+    defer dir.cleanup();
+    var path_buffer: [std.fs.max_path_bytes]u8 = undefined;
+    var unix_buffer: [transport.sun_path_len]u8 = undefined;
+    const plen = try dir.dir.realPath(io, &path_buffer);
+    const unix = try std.fmt.bufPrintSentinel(&unix_buffer, "{s}/9p", .{path_buffer[0..plen]}, 0);
+    var stub: Stub = .{};
+    var runner: BigRunner = undefined;
+    runner.init(.{ .io = io, .root = Stub.root_node, .handler = .{ .ctx = &stub, .serve = serveNowBig, .closed = countClosedBig } });
+    _ = try runner.listen(.{ .unix = unix }, 16);
+    defer runner.stop();
+
+    var clients: [16]TestClient = undefined;
+    for (&clients) |*tc| {
+        tc.* = .{ .io = undefined, .stream = undefined };
+        try tc.open(io, .{ .unix = unix });
+        try tc.handshake();
+    }
+    defer for (&clients) |*tc| tc.close();
+    try waitCount(&runner, 16);
+
+    // The seventeenth connection is accepted and closed at once: it never
+    // gets a version reply, and the sixteen keep their slots.
+    var extra: TestClient = .{ .io = undefined, .stream = undefined };
+    try extra.open(io, .{ .unix = unix });
+    defer extra.close();
+    try expectHangup(extra.one(.{ .version = .{} }));
+    try waitCount(&runner, 16);
+
+    // stop() hangs every live connection up and frees every slot.
+    const closed0 = stub.closed;
+    runner.stop();
+    try testing.expectEqual(@as(usize, 0), runner.count());
+    try testing.expectEqual(@as(u32, 16), stub.closed - closed0);
+    for (&clients) |*tc| try expectHangup(tc.one(.{ .stat = .{ .fid = 0 } }));
+}
+
+test "serve: repeated connect/disconnect leaves descriptors and threads flat" {
+    var rig: Rig = .{ .dir = undefined };
+    try rig.start(.{ .serve = serveNow }, 0);
+    defer rig.end();
+    const io = testing.io;
+    const Cycle = struct {
+        fn run(r: *Rig) !void {
+            var tc: TestClient = .{ .io = undefined, .stream = undefined };
+            try tc.open(testing.io, .{ .unix = r.unix });
+            try tc.handshake();
+            try tc.readIndex(1);
+            tc.close();
+        }
+    };
+    // Warm the task pool to its steady state (the first connections grow
+    // it; later ones must not).
+    for (0..400) |_| try Cycle.run(&rig);
+    try rig.settle(0);
+    const fds0 = try openFdCount(io);
+    const threads0 = try taskCount(io);
+    for (0..1200) |_| try Cycle.run(&rig);
+    try rig.settle(0);
+    // Descriptors are exactly flat: no stream, buffer or address is leaked
+    // per connection.
+    try testing.expectEqual(fds0, try openFdCount(io));
+    // Threads come from the shared Io worker pool and may lazily add a
+    // worker; they must not scale with the connection count (which would be
+    // a per-connection thread leak).
+    try testing.expect(try taskCount(io) <= threads0 + 2);
+}
+
+test "serve: a connection storm is absorbed and the runner keeps serving" {
+    var rig: Rig = .{ .dir = undefined };
+    try rig.start(.{ .serve = serveNow }, 0);
+    defer rig.end();
+    const io = testing.io;
+    const fds0 = try openFdCount(io);
+    const Storm = struct {
+        fn run(r: *Rig, refused: *std.atomic.Value(u32)) void {
+            for (0..32) |_| {
+                var tc: TestClient = .{ .io = undefined, .stream = undefined };
+                tc.open(testing.io, .{ .unix = r.unix }) catch {
+                    _ = refused.fetchAdd(1, .monotonic);
+                    continue;
+                };
+                // No handshake: the connection is torn down as soon as it
+                // is accepted (or refused a slot by the runner).
+                tc.close();
+            }
+        }
+    };
+    var group: Io.Group = .init;
+    var refused = std.atomic.Value(u32).init(0);
+    for (0..16) |_| try group.concurrent(io, Storm.run, .{ &rig, &refused });
+    try group.await(io);
+    try rig.settle(0);
+    // 512 connections, two slots: the extra ones are closed by the runner,
+    // not refused by the kernel backlog.
+    try testing.expectEqual(@as(u32, 0), refused.load(.monotonic));
+    try testing.expectEqual(fds0, try openFdCount(io));
+    // The runner still accepts and serves a full session.
+    var tc: TestClient = .{ .io = undefined, .stream = undefined };
+    try tc.open(io, .{ .unix = rig.unix });
+    defer tc.close();
+    try tc.handshake();
+    try tc.readIndex(1);
+}
+
+/// The open descriptors of this process, from /proc, for the CLOEXEC audit.
+fn collectFds(io: Io, out: []i32) ![]i32 {
+    var rb: [Io.Dir.Iterator.reader_buffer_len]u8 align(@alignOf(usize)) = undefined;
+    const dir = try Io.Dir.openDirAbsolute(io, "/proc/self/fd", .{ .iterate = true });
+    defer Io.Dir.close(dir, io);
+    var it = Io.Dir.Reader.init(dir, &rb);
+    var n: usize = 0;
+    while (try it.next(io)) |e| {
+        const fd = std.fmt.parseInt(i32, e.name, 10) catch continue;
+        if (n < out.len) {
+            out[n] = fd;
+            n += 1;
+        }
+    }
+    return out[0..n];
+}
+
+fn hasFd(fds: []const i32, fd: i32) bool {
+    for (fds) |f| if (f == fd) return true;
+    return false;
+}
+
+test "serve: every descriptor opened by the post/serve path is close-on-exec" {
+    if (@import("builtin").os.tag != .linux) return error.SkipZigTest;
+    const io = testing.io;
+    var dir = testing.tmpDir(.{});
+    defer dir.cleanup();
+    var real_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const rlen = try dir.dir.realPath(io, &real_buf);
+    var env_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const value = try std.fmt.bufPrintZ(&env_buf, "XDG_RUNTIME_DIR={s}", .{real_buf[0..rlen]});
+    const env = [2]?[*:0]const u8{ @ptrCast(value.ptr), null };
+    const envp: post.Env = @ptrCast(&env);
+
+    var before_buf: [64]i32 = undefined;
+    const before = try collectFds(io, &before_buf);
+
+    // Post a runner, dial its name, scan the registry and accept a client:
+    // every descriptor this opens must not survive into an exec.
+    var stub: Stub = .{};
+    var runner: TestRunner = undefined;
+    runner.init(.{ .io = io, .root = Stub.root_node, .handler = .{ .ctx = &stub, .serve = serveNow } });
+    defer runner.stop();
+    try runner.listenPosted(envp, "cloexec", 4);
+    var path_buf: [transport.sun_path_len]u8 = undefined;
+    const posted_path = try post.registryPath(envp, "cloexec", &path_buf);
+    var dialed = try post.dialPath(io, posted_path);
+    defer dialed.close(io);
+    var stage: [1024]u8 = undefined;
+    var names = try post.posted(io, envp, &stage);
+    try testing.expect(names.next() != null);
+    var tc: TestClient = .{ .io = undefined, .stream = undefined };
+    try tc.open(io, .{ .unix = posted_path });
+    defer tc.close();
+    try tc.handshake();
+
+    var after_buf: [64]i32 = undefined;
+    const after = try collectFds(io, &after_buf);
+    for (after) |fd| {
+        if (hasFd(before, fd)) continue;
+        const flags = linux.fcntl(fd, linux.F.GETFD, 0);
+        try testing.expect(flags & linux.FD_CLOEXEC != 0);
+    }
+    // Sanity: the audit saw the new descriptors (the listener, the dialed
+    // socket, the accepted connection).
+    try testing.expect(after.len > before.len);
+}