A name under a directory that may not be searched or written is refused, permission denied, and a Save there says so, not no such directory

`pardes noperm/a/b/c` and `pardes /proc/1/root/x` took the missing
directory for one Save would make, opened a pane and exited 0; its Save
then said "no such directory". fs.deniedAbove finds the nearest
directory there and asks whether it may be searched and written:
forwarding refuses such a name, exit 1, "permission denied", and a
failed Save says "permission denied", which the writer's 9P error
carries as EPERM (9ns: EACCES).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

6 files changed, 45 insertions, 6 deletions

diff --git a/src/9p_io.zig b/src/9p_io.zig
index 9fc19df9..ea3f799c 100644
--- a/src/9p_io.zig
+++ b/src/9p_io.zig
@@ -328,7 +328,7 @@ pub const Listener = struct {
             // builtin's failure saying so is (ctl.failureErrno).
             const errno = if (std.mem.indexOf(u8, late, "no such") != null or std.mem.indexOf(u8, late, "not found") != null)
                 pardes.ctlfs.E.NOENT
-            else if (std.mem.indexOf(u8, late, "no space") != null) pardes.ctlfs.E.NOSPC else pardes.ctlfs.E.IO;
+            else if (std.mem.indexOf(u8, late, "no space") != null) pardes.ctlfs.E.NOSPC else if (std.mem.indexOf(u8, late, "permission denied") != null) pardes.ctlfs.E.PERM else pardes.ctlfs.E.IO;
             const failed = pardes.ctlfs.failText(req.tag, errno, late);
             // Its err record says it (the path in it), the one record: it
             // was said with no msg while this waited (fs.write_waits).
diff --git a/src/fs.zig b/src/fs.zig
index 965eb47c..e5980fe9 100644
--- a/src/fs.zig
+++ b/src/fs.zig
@@ -685,6 +685,26 @@ pub fn makeDirs(dir: []const u8) void {
     _ = libc.mkdir(z[0..dir.len :0], 0o755);
 }
 
+/// Whether `dir` is not there to make because one above it refuses: its
+/// nearest ancestor that is there may not be searched or written (EACCES:
+/// `noperm/a/b`, `/proc/1/root/x`).
+pub fn deniedAbove(dir: []const u8) bool {
+    if (comptime !platform_has_fs) return false;
+    var at: ?[]const u8 = dir;
+    while (at) |d| : (at = std.fs.path.dirname(d)) {
+        var z: [4096]u8 = undefined;
+        const dz = std.fmt.bufPrintSentinel(&z, "{s}", .{d}, 0) catch return false;
+        if (libc.access(dz.ptr, 0) != 0) continue; // F_OK
+        return libc.access(dz.ptr, 2 | 1) != 0; // W_OK | X_OK
+    }
+    return false;
+}
+
+test "a directory under one that may not be searched or written is denied, one merely missing is not" {
+    try std.testing.expect(deniedAbove("/proc/1/root/x/y"));
+    try std.testing.expect(!deniedAbove("/tmp/pardes-surely-missing-dir/a/b"));
+}
+
 pub fn localPath(path: []const u8) ?[]const u8 {
     if (std.mem.eql(u8, path, "/n/os")) return "/";
     if (std.mem.startsWith(u8, path, "/n/os/")) return path[5..];
diff --git a/src/main.zig b/src/main.zig
index a7d354b5..86558bae 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -256,7 +256,7 @@ fn forwardWords(err: anyerror, buf: []u8) []const u8 {
         error.NotOneLine => "a file name is one line, and this one holds a newline",
         error.NotAFileName => "names a directory, not a file",
         error.NoWorkingDirectory => "this shell's working directory is gone",
-        error.DirectoryNotWritable => "its directory may not be read or written, so it could never be saved",
+        error.DirectoryNotWritable => "permission denied: its directory may not be read or written, so it could never be saved",
         error.NotAPaneAddress => "not a pane address: @p and a pane's number",
         error.NoSuchPane => "this session has no such pane",
         else => if (pardes.Messages.dialReason(err)) |why| why else words: {
@@ -653,7 +653,11 @@ fn nativeMain(init: std.process.Init) !void {
                 var cwd_buf: [4096]u8 = undefined;
                 if (std.c.getcwd(&cwd_buf, cwd_buf.len) == null) Refuse.with(init.io, word, error.NoWorkingDirectory);
                 const cwd = std.mem.sliceTo(&cwd_buf, 0);
-                break :named std.fs.path.resolvePosix(arena, &.{ cwd, target.path }) catch |err| Refuse.with(init.io, word, err);
+                const absolute = std.fs.path.resolvePosix(arena, &.{ cwd, target.path }) catch |err| Refuse.with(init.io, word, err);
+                // Missing because one above may not be searched or written:
+                // its Save could never make it, so refused now.
+                if (pardes.filesystem.deniedAbove(std.fs.path.dirname(absolute) orelse "/")) Refuse.with(init.io, word, error.DirectoryNotWritable);
+                break :named absolute;
             };
             // A directory that is there but may not be read or written: a
             // pane for the name could only fail at its Save, so refused now.
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index c88eb328..d6d7a6ee 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -4056,3 +4056,12 @@ test "a command pane an exec open was answered is leased to it while it stays op
     try testing.expectEqual(Status.ok, wr(p, exec, "echo c\n").reply.status);
     try testing.expectEqual(a_pane, p.paneBySerial(p.fs.results[0]).?);
 }
+
+test "a Save whose directory is missing because one above refuses says permission denied, not no such directory" {
+    const p = try withFile(testing.allocator, "x\n");
+    defer p.deinit();
+    p.saveFailed(0, "/proc/1/root/x/f.txt", error.FileNotFound);
+    try testing.expect(th.logHas(p, "Save /proc/1/root/x/f.txt: permission denied"));
+    p.saveFailed(0, "/tmp/pardes-surely-missing-dir/f.txt", error.FileNotFound);
+    try testing.expect(th.logHas(p, "no such directory"));
+}
diff --git a/src/ninep/pane.zig b/src/ninep/pane.zig
index b9fa2857..3963c605 100644
--- a/src/ninep/pane.zig
+++ b/src/ninep/pane.zig
@@ -1535,13 +1535,15 @@ test "a name with a component over 255 bytes is refused, and a long path's faile
     try testing.expectEqual(Status.ok, wr(p, Node.of(serial, .name), "/tmp/" ++ "c" ** 255 ++ "\n").reply.status);
     // A Save of a 3000-byte path the host refuses: the waiting write's
     // reason is at the end, the path giving up its middle.
-    const long = "/nonexistent-pardes-root/" ++ ("d" ** 200 ++ "/") ** 15 ++ "f.txt";
+    // (Under /tmp: under / a user's Save is refused permission, not
+    // missing.)
+    const long = "/tmp/nonexistent-pardes-root/" ++ ("d" ** 200 ++ "/") ** 15 ++ "f.txt";
     const id = p.paneBySerial(serial).?;
     try nameBuffer(p, id, long, false);
     p.fs.late_failure_len = 0;
     p.saveFailed(@intCast(id), long, error.AccessDenied);
     const late = p.fs.late_failure[0..p.fs.late_failure_len];
-    try testing.expect(std.mem.startsWith(u8, late, "Save /nonexistent-pardes-root/"));
+    try testing.expect(std.mem.startsWith(u8, late, "Save /tmp/nonexistent-pardes-root/"));
     try testing.expect(std.mem.endsWith(u8, late, "/f.txt: no such directory"));
 }
 
diff --git a/src/pardes.zig b/src/pardes.zig
index 945fcafa..f5001797 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -5541,7 +5541,11 @@ pub const Pardes = struct {
         const parent = std.fs.path.dirname(path) orelse "/";
         const no_dir = err == error.FileNotFound or err == error.NotDir or
             (if (comptime hosted) (if (filesystem.localPath(parent)) |local| !exec.isDirectory(local) else false) else false);
-        if (no_dir)
+        // Not there because one above refuses: that is the reason, EACCES.
+        const denied = no_dir and (if (comptime hosted) (if (filesystem.localPath(parent)) |local| filesystem.deniedAbove(local) else false) else false);
+        if (denied)
+            p.reportFailure(id, std.fmt.bufPrint(&what, "Save {s}: permission denied", .{path}) catch "Save: permission denied")
+        else if (no_dir)
             p.reportFailure(id, std.fmt.bufPrint(&what, "Save {s}: no such directory", .{path}) catch "Save: no such directory")
         else
             p.reportError(id, std.fmt.bufPrint(&what, "Save {s}", .{path}) catch "Save", err);