Commit b5fad3a40a

Andrew Kelley <andrew@ziglang.org>
2023-05-30 00:01:24
Revert "Windows: Support UNC, rooted, drive relative, and namespaced/device paths"
This reverts commit 1697d44809151e4759f6b5f9447a908c30ac1e84. Master branch started failing after this commit landed.
1 parent 138f125
Changed files (4)
lib/std/os/windows/ntdll.zig
@@ -158,16 +158,6 @@ pub extern "ntdll" fn RtlDosPathNameToNtPathName_U(
 ) callconv(WINAPI) BOOL;
 pub extern "ntdll" fn RtlFreeUnicodeString(UnicodeString: *UNICODE_STRING) callconv(WINAPI) void;
 
-/// Returns the number of bytes written to `Buffer`.
-/// If the returned count is larger than `BufferByteLength`, the buffer was too small.
-/// If the returned count is zero, an error occurred.
-pub extern "ntdll" fn RtlGetFullPathName_U(
-    FileName: [*:0]const u16,
-    BufferByteLength: ULONG,
-    Buffer: [*]u16,
-    ShortName: ?*[*:0]const u16,
-) callconv(windows.WINAPI) windows.ULONG;
-
 pub extern "ntdll" fn NtQueryDirectoryFile(
     FileHandle: HANDLE,
     Event: ?HANDLE,
lib/std/os/windows/test.zig
@@ -3,181 +3,7 @@ const builtin = @import("builtin");
 const windows = std.os.windows;
 const mem = std.mem;
 const testing = std.testing;
-
-/// Wrapper around RtlDosPathNameToNtPathName_U for use in comparing
-/// the behavior of RtlDosPathNameToNtPathName_U with wToPrefixedFileW
-/// Note: RtlDosPathNameToNtPathName_U is not used in the Zig implementation
-//        because it allocates.
-fn RtlDosPathNameToNtPathName_U(path: [:0]const u16) !windows.PathSpace {
-    var out: windows.UNICODE_STRING = undefined;
-    const rc = windows.ntdll.RtlDosPathNameToNtPathName_U(path, &out, null, null);
-    if (rc != windows.TRUE) return error.BadPathName;
-    defer windows.ntdll.RtlFreeUnicodeString(&out);
-
-    var path_space: windows.PathSpace = undefined;
-    const out_path = out.Buffer[0 .. out.Length / 2];
-    std.mem.copy(u16, path_space.data[0..], out_path);
-    path_space.len = out.Length / 2;
-    path_space.data[path_space.len] = 0;
-
-    return path_space;
-}
-
-/// Test that the Zig conversion matches the expected_path (for instances where
-/// the Zig implementation intentionally diverges from what RtlDosPathNameToNtPathName_U does).
-fn testToPrefixedFileNoOracle(comptime path: []const u8, comptime expected_path: []const u8) !void {
-    const path_utf16 = std.unicode.utf8ToUtf16LeStringLiteral(path);
-    const expected_path_utf16 = std.unicode.utf8ToUtf16LeStringLiteral(expected_path);
-    const actual_path = try windows.wToPrefixedFileW(path_utf16);
-    std.testing.expectEqualSlices(u16, expected_path_utf16, actual_path.span()) catch |e| {
-        std.debug.print("got '{s}', expected '{s}'\n", .{ std.unicode.fmtUtf16le(actual_path.span()), std.unicode.fmtUtf16le(expected_path_utf16) });
-        return e;
-    };
-}
-
-/// Test that the Zig conversion matches the expected_path and that the
-/// expected_path matches the conversion that RtlDosPathNameToNtPathName_U does.
-fn testToPrefixedFileWithOracle(comptime path: []const u8, comptime expected_path: []const u8) !void {
-    try testToPrefixedFileNoOracle(path, expected_path);
-    try testToPrefixedFileOnlyOracle(path);
-}
-
-/// Test that the Zig conversion matches the conversion that RtlDosPathNameToNtPathName_U does.
-fn testToPrefixedFileOnlyOracle(comptime path: []const u8) !void {
-    const path_utf16 = std.unicode.utf8ToUtf16LeStringLiteral(path);
-    const zig_result = try windows.wToPrefixedFileW(path_utf16);
-    const win32_api_result = try RtlDosPathNameToNtPathName_U(path_utf16);
-    std.testing.expectEqualSlices(u16, win32_api_result.span(), zig_result.span()) catch |e| {
-        std.debug.print("got '{s}', expected '{s}'\n", .{ std.unicode.fmtUtf16le(zig_result.span()), std.unicode.fmtUtf16le(win32_api_result.span()) });
-        return e;
-    };
-}
-
-test "toPrefixedFileW" {
-    if (builtin.os.tag != .windows)
-        return;
-
-    // Most test cases come from https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html
-    // Note that these tests do not actually touch the filesystem or care about whether or not
-    // any of the paths actually exist or are otherwise valid.
-
-    // Drive Absolute
-    try testToPrefixedFileWithOracle("X:\\ABC\\DEF", "\\??\\X:\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("X:\\", "\\??\\X:\\");
-    try testToPrefixedFileWithOracle("X:\\ABC\\", "\\??\\X:\\ABC\\");
-    // Trailing . and space characters are stripped
-    try testToPrefixedFileWithOracle("X:\\ABC\\DEF. .", "\\??\\X:\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("X:/ABC/DEF", "\\??\\X:\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("X:\\ABC\\..\\XYZ", "\\??\\X:\\XYZ");
-    try testToPrefixedFileWithOracle("X:\\ABC\\..\\..\\..", "\\??\\X:\\");
-    // Drive letter casing is unchanged
-    try testToPrefixedFileWithOracle("x:\\", "\\??\\x:\\");
-
-    // Drive Relative
-    // These tests depend on the CWD of the specified drive letter which can vary,
-    // so instead we just test that the Zig implementation matches the result of
-    // RtlDosPathNameToNtPathName_U.
-    // TODO: Setting the =X: environment variable didn't seem to affect
-    //       RtlDosPathNameToNtPathName_U, not sure why that is but getting that
-    //       to work could be an avenue to making these cases environment-independent.
-    // All -> are examples of the result if the X drive's cwd was X:\ABC
-    try testToPrefixedFileOnlyOracle("X:DEF\\GHI"); // -> \??\X:\ABC\DEF\GHI
-    try testToPrefixedFileOnlyOracle("X:"); // -> \??\X:\ABC
-    try testToPrefixedFileOnlyOracle("X:DEF. ."); // -> \??\X:\ABC\DEF
-    try testToPrefixedFileOnlyOracle("X:ABC\\..\\XYZ"); // -> \??\X:\ABC\XYZ
-    try testToPrefixedFileOnlyOracle("X:ABC\\..\\..\\.."); // -> \??\X:\
-    try testToPrefixedFileOnlyOracle("x:"); // -> \??\X:\ABC
-
-    // Rooted
-    // These tests depend on the drive letter of the CWD which can vary, so
-    // instead we just test that the Zig implementation matches the result of
-    // RtlDosPathNameToNtPathName_U.
-    // TODO: Getting the CWD path, getting the drive letter from it, and using it to
-    //       construct the expected NT paths could be an avenue to making these cases
-    //       environment-independent and therefore able to use testToPrefixedFileWithOracle.
-    // All -> are examples of the result if the CWD's drive letter was X
-    try testToPrefixedFileOnlyOracle("\\ABC\\DEF"); // -> \??\X:\ABC\DEF
-    try testToPrefixedFileOnlyOracle("\\"); // -> \??\X:\
-    try testToPrefixedFileOnlyOracle("\\ABC\\DEF. ."); // -> \??\X:\ABC\DEF
-    try testToPrefixedFileOnlyOracle("/ABC/DEF"); // -> \??\X:\ABC\DEF
-    try testToPrefixedFileOnlyOracle("\\ABC\\..\\XYZ"); // -> \??\X:\XYZ
-    try testToPrefixedFileOnlyOracle("\\ABC\\..\\..\\.."); // -> \??\X:\
-
-    // Relative
-    // These cases differ in functionality to RtlDosPathNameToNtPathName_U.
-    // Relative paths remain relative if they don't have enough .. components
-    // to error with TooManyParentDirs
-    try testToPrefixedFileNoOracle("ABC\\DEF", "ABC\\DEF");
-    // TODO: enable this if trailing . and spaces are stripped from relative paths
-    //try testToPrefixedFileNoOracle("ABC\\DEF. .", "ABC\\DEF");
-    try testToPrefixedFileNoOracle("ABC/DEF", "ABC\\DEF");
-    try testToPrefixedFileNoOracle("./ABC/.././DEF", "DEF");
-    // TooManyParentDirs, so resolved relative to the CWD
-    // All -> are examples of the result if the CWD was X:\ABC\DEF
-    try testToPrefixedFileOnlyOracle("..\\GHI"); // -> \??\X:\ABC\GHI
-    try testToPrefixedFileOnlyOracle("GHI\\..\\..\\.."); // -> \??\X:\
-
-    // UNC Absolute
-    try testToPrefixedFileWithOracle("\\\\server\\share\\ABC\\DEF", "\\??\\UNC\\server\\share\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("\\\\server", "\\??\\UNC\\server");
-    try testToPrefixedFileWithOracle("\\\\server\\share", "\\??\\UNC\\server\\share");
-    try testToPrefixedFileWithOracle("\\\\server\\share\\ABC. .", "\\??\\UNC\\server\\share\\ABC");
-    try testToPrefixedFileWithOracle("//server/share/ABC/DEF", "\\??\\UNC\\server\\share\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("\\\\server\\share\\ABC\\..\\XYZ", "\\??\\UNC\\server\\share\\XYZ");
-    try testToPrefixedFileWithOracle("\\\\server\\share\\ABC\\..\\..\\..", "\\??\\UNC\\server\\share");
-
-    // Local Device
-    try testToPrefixedFileWithOracle("\\\\.\\COM20", "\\??\\COM20");
-    try testToPrefixedFileWithOracle("\\\\.\\pipe\\mypipe", "\\??\\pipe\\mypipe");
-    try testToPrefixedFileWithOracle("\\\\.\\X:\\ABC\\DEF. .", "\\??\\X:\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("\\\\.\\X:/ABC/DEF", "\\??\\X:\\ABC\\DEF");
-    try testToPrefixedFileWithOracle("\\\\.\\X:\\ABC\\..\\XYZ", "\\??\\X:\\XYZ");
-    // Can replace the first component of the path (contrary to drive absolute and UNC absolute paths)
-    try testToPrefixedFileWithOracle("\\\\.\\X:\\ABC\\..\\..\\C:\\", "\\??\\C:\\");
-    try testToPrefixedFileWithOracle("\\\\.\\pipe\\mypipe\\..\\notmine", "\\??\\pipe\\notmine");
-
-    // Special-case device names
-    // TODO: Enable once these are supported
-    //       more cases to test here: https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html
-    //try testToPrefixedFileWithOracle("COM1", "\\??\\COM1");
-    // Sometimes the special-cased device names are not respected
-    try testToPrefixedFileWithOracle("\\\\.\\X:\\COM1", "\\??\\X:\\COM1");
-    try testToPrefixedFileWithOracle("\\\\abc\\xyz\\COM1", "\\??\\UNC\\abc\\xyz\\COM1");
-
-    // Verbatim
-    // Left untouched except \\?\ is replaced by \??\
-    try testToPrefixedFileWithOracle("\\\\?\\X:", "\\??\\X:");
-    try testToPrefixedFileWithOracle("\\\\?\\X:\\COM1", "\\??\\X:\\COM1");
-    try testToPrefixedFileWithOracle("\\\\?\\X:/ABC/DEF. .", "\\??\\X:/ABC/DEF. .");
-    try testToPrefixedFileWithOracle("\\\\?\\X:\\ABC\\..\\..\\..", "\\??\\X:\\ABC\\..\\..\\..");
-    // NT Namespace
-    // Fully unmodified
-    try testToPrefixedFileWithOracle("\\??\\X:", "\\??\\X:");
-    try testToPrefixedFileWithOracle("\\??\\X:\\COM1", "\\??\\X:\\COM1");
-    try testToPrefixedFileWithOracle("\\??\\X:/ABC/DEF. .", "\\??\\X:/ABC/DEF. .");
-    try testToPrefixedFileWithOracle("\\??\\X:\\ABC\\..\\..\\..", "\\??\\X:\\ABC\\..\\..\\..");
-
-    // 'Fake' Verbatim
-    // If the prefix looks like the verbatim prefix but not all path separators in the
-    // prefix are backslashes, then it gets canonicalized and the prefix is dropped in favor
-    // of the NT prefix.
-    try testToPrefixedFileWithOracle("//?/C:/ABC", "\\??\\C:\\ABC");
-    // 'Fake' NT
-    // If the prefix looks like the NT prefix but not all path separators in the prefix
-    // are backslashes, then it gets canonicalized and the /??/ is not dropped but
-    // rather treated as part of the path. In other words, the path is treated
-    // as a rooted path, so the final path is resolved relative to the CWD's
-    // drive letter.
-    // The -> shows an example of the result if the CWD's drive letter was X
-    try testToPrefixedFileOnlyOracle("/??/C:/ABC"); // -> \??\X:\??\C:\ABC
-
-    // Root Local Device
-    // \\. and \\? always get converted to \??\
-    try testToPrefixedFileWithOracle("\\\\.", "\\??\\");
-    try testToPrefixedFileWithOracle("\\\\?", "\\??\\");
-    try testToPrefixedFileWithOracle("//?", "\\??\\");
-    try testToPrefixedFileWithOracle("//.", "\\??\\");
-}
+const expect = testing.expect;
 
 fn testRemoveDotDirs(str: []const u8, expected: []const u8) !void {
     const mutable = try testing.allocator.dupe(u8, str);
lib/std/os/windows.zig
@@ -1157,9 +1157,9 @@ pub fn GetFinalPathNameByHandle(
 
             // This surprising path is a filesystem path to the mount manager on Windows.
             // Source: https://stackoverflow.com/questions/3012828/using-ioctl-mountmgr-query-points
-            // This is the NT namespaced version of \\.\MountPointManager
-            const mgmt_path_u16 = std.unicode.utf8ToUtf16LeStringLiteral("\\??\\MountPointManager");
-            const mgmt_handle = OpenFile(mgmt_path_u16, .{
+            const mgmt_path = "\\MountPointManager";
+            const mgmt_path_u16 = sliceToPrefixedFileW(mgmt_path) catch unreachable;
+            const mgmt_handle = OpenFile(mgmt_path_u16.span(), .{
                 .access_mask = SYNCHRONIZE,
                 .share_access = FILE_SHARE_READ | FILE_SHARE_WRITE,
                 .creation = FILE_OPEN,
@@ -1997,248 +1997,43 @@ pub fn cStrToPrefixedFileW(s: [*:0]const u8) !PathSpace {
     return sliceToPrefixedFileW(mem.sliceTo(s, 0));
 }
 
-/// Same as `wToPrefixedFileW` but accepts a UTF-8 encoded path.
-pub fn sliceToPrefixedFileW(path: []const u8) !PathSpace {
-    var temp_path: PathSpace = undefined;
-    temp_path.len = try std.unicode.utf8ToUtf16Le(&temp_path.data, path);
-    temp_path.data[temp_path.len] = 0;
-    return wToPrefixedFileW(temp_path.span());
-}
-
-/// Converts the `path` to WTF16, null-terminated. If the path contains any
-/// namespace prefix, or is anything but a relative path (rooted, drive relative,
-/// etc) the result will have the NT-style prefix `\??\`.
-///
-/// Similar to RtlDosPathNameToNtPathName_U with a few differences:
-/// - Does not allocate on the heap.
-/// - Relative paths are kept as relative unless they contain too many ..
-///   components, in which case they are treated as drive-relative and resolved
-///   against the CWD.
-/// - Special case device names like COM1, NUL, etc are not handled specially (TODO)
-/// - . and space are not stripped from the end of relative paths (potential TODO)
-pub fn wToPrefixedFileW(path: [:0]const u16) !PathSpace {
-    const nt_prefix = [_]u16{ '\\', '?', '?', '\\' };
-    switch (getNamespacePrefix(u16, path)) {
-        // TODO: Figure out a way to design an API that can avoid the copy for .nt,
-        //       since it is always returned fully unmodified.
-        .nt, .verbatim => {
-            var path_space: PathSpace = undefined;
-            path_space.data[0..nt_prefix.len].* = nt_prefix;
-            const len_after_prefix = path.len - nt_prefix.len;
-            @memcpy(path_space.data[nt_prefix.len..][0..len_after_prefix], path[nt_prefix.len..]);
-            path_space.len = path.len;
-            path_space.data[path_space.len] = 0;
-            return path_space;
-        },
-        .local_device, .fake_verbatim => {
-            var path_space: PathSpace = undefined;
-            const path_byte_len = ntdll.RtlGetFullPathName_U(
-                path.ptr,
-                path_space.data.len * 2,
-                &path_space.data,
-                null,
-            );
-            if (path_byte_len == 0) {
-                // TODO: This may not be the right error
-                return error.BadPathName;
-            } else if (path_byte_len / 2 > path_space.data.len) {
-                return error.NameTooLong;
-            }
-            path_space.len = path_byte_len / 2;
-            // Both prefixes will be normalized but retained, so all
-            // we need to do now is replace them with the NT prefix
-            path_space.data[0..nt_prefix.len].* = nt_prefix;
-            return path_space;
-        },
-        .none => {
-            const path_type = getUnprefixedPathType(u16, path);
-            var path_space: PathSpace = undefined;
-            relative: {
-                if (path_type == .relative) {
-                    // TODO: Handle special case device names like COM1, AUX, NUL, CONIN$, CONOUT$, etc.
-                    //       See https://googleprojectzero.blogspot.com/2016/02/the-definitive-guide-on-win32-to-nt.html
-
-                    // TODO: Potentially strip all trailing . and space characters from the
-                    //       end of the path. This is something that both RtlDosPathNameToNtPathName_U
-                    //       and RtlGetFullPathName_U do. Technically, trailing . and spaces
-                    //       are allowed, but such paths may not interact well with Windows (i.e.
-                    //       files with these paths can't be deleted from explorer.exe, etc).
-                    //       This could be something that normalizePath may want to do.
-
-                    @memcpy(path_space.data[0..path.len], path);
-                    // Try to normalize, but if we get too many parent directories,
-                    // then this is effectively a 'drive relative' path, so we need to
-                    // start over and use RtlGetFullPathName_U instead.
-                    path_space.len = normalizePath(u16, path_space.data[0..path.len]) catch |err| switch (err) {
-                        error.TooManyParentDirs => break :relative,
-                    };
-                    path_space.data[path_space.len] = 0;
-                    return path_space;
-                }
-            }
-            // We now know we are going to return an absolute NT path, so
-            // we can unconditionally prefix it with the NT prefix.
-            path_space.data[0..nt_prefix.len].* = nt_prefix;
-            if (path_type == .root_local_device) {
-                // `\\.` and `\\?` always get converted to `\??\` exactly, so
-                // we can just stop here
-                path_space.len = nt_prefix.len;
-                path_space.data[path_space.len] = 0;
+/// Converts the path `s` to WTF16, null-terminated. If the path is absolute,
+/// it will get NT-style prefix `\??\` prepended automatically.
+pub fn sliceToPrefixedFileW(s: []const u8) !PathSpace {
+    // TODO https://github.com/ziglang/zig/issues/2765
+    var path_space: PathSpace = undefined;
+    const prefix = "\\??\\";
+    const prefix_index: usize = if (mem.startsWith(u8, s, prefix)) prefix.len else 0;
+    for (s[prefix_index..]) |byte| {
+        switch (byte) {
+            '*', '?', '"', '<', '>', '|' => return error.BadPathName,
+            else => {},
+        }
+    }
+    const prefix_u16 = [_]u16{ '\\', '?', '?', '\\' };
+    const start_index = if (prefix_index > 0 or !std.fs.path.isAbsolute(s)) 0 else blk: {
+        path_space.data[0..prefix_u16.len].* = prefix_u16;
+        break :blk prefix_u16.len;
+    };
+    path_space.len = start_index + try std.unicode.utf8ToUtf16Le(path_space.data[start_index..], s);
+    if (path_space.len > path_space.data.len) return error.NameTooLong;
+    path_space.len = start_index + (normalizePath(u16, path_space.data[start_index..path_space.len]) catch |err| switch (err) {
+        error.TooManyParentDirs => {
+            if (!std.fs.path.isAbsolute(s)) {
+                var temp_path: PathSpace = undefined;
+                temp_path.len = try std.unicode.utf8ToUtf16Le(&temp_path.data, s);
+                std.debug.assert(temp_path.len == path_space.len);
+                temp_path.data[path_space.len] = 0;
+                path_space.len = prefix_u16.len + try getFullPathNameW(&temp_path.data, path_space.data[prefix_u16.len..]);
+                path_space.data[0..prefix_u16.len].* = prefix_u16;
+                std.debug.assert(path_space.data[path_space.len] == 0);
                 return path_space;
             }
-            const path_buf_offset = switch (path_type) {
-                // UNC paths will always start with `\\`. However, we want to
-                // end up with something like `\??\UNC\server\share`, so to get
-                // RtlGetFullPathName to write into the spot we want the `server`
-                // part to end up, we need to provide an offset such that
-                // the `\\` part gets written where the `C\` of `UNC\` will be
-                // in the final NT path.
-                .unc_absolute => nt_prefix.len + 2,
-                else => nt_prefix.len,
-            };
-            const buf_len = @intCast(u32, path_space.data.len - path_buf_offset);
-            const path_byte_len = ntdll.RtlGetFullPathName_U(
-                path.ptr,
-                buf_len * 2,
-                path_space.data[path_buf_offset..].ptr,
-                null,
-            );
-            if (path_byte_len == 0) {
-                // TODO: This may not be the right error
-                return error.BadPathName;
-            } else if (path_byte_len / 2 > buf_len) {
-                return error.NameTooLong;
-            }
-            path_space.len = path_buf_offset + (path_byte_len / 2);
-            if (path_type == .unc_absolute) {
-                // Now add in the UNC, the `C` should overwrite the first `\` of the
-                // FullPathName, ultimately resulting in `\??\UNC\<the rest of the path>`
-                std.debug.assert(path_space.data[path_buf_offset] == '\\');
-                std.debug.assert(path_space.data[path_buf_offset + 1] == '\\');
-                const unc = [_]u16{ 'U', 'N', 'C' };
-                path_space.data[nt_prefix.len..][0..unc.len].* = unc;
-            }
-            return path_space;
+            return error.BadPathName;
         },
-    }
-}
-
-pub const NamespacePrefix = enum {
-    none,
-    /// `\\.\` (path separators can be `\` or `/`)
-    local_device,
-    /// `\\?\`
-    /// When converted to an NT path, everything past the prefix is left
-    /// untouched and `\\?\` is replaced by `\??\`.
-    verbatim,
-    /// `\\?\` without all path separators being `\`.
-    /// This seems to be recognized as a prefix, but the 'verbatim' aspect
-    /// is not respected (i.e. if `//?/C:/foo` is converted to an NT path,
-    /// it will become `\??\C:\foo` [it will be canonicalized and the //?/ won't
-    /// be treated as part of the final path])
-    fake_verbatim,
-    /// `\??\`
-    nt,
-};
-
-pub fn getNamespacePrefix(comptime T: type, path: []const T) NamespacePrefix {
-    if (path.len < 4) return .none;
-    var all_backslash = switch (path[0]) {
-        '\\' => true,
-        '/' => false,
-        else => return .none,
-    };
-    all_backslash = all_backslash and switch (path[3]) {
-        '\\' => true,
-        '/' => false,
-        else => return .none,
-    };
-    switch (path[1]) {
-        '?' => if (path[2] == '?' and all_backslash) return .nt else return .none,
-        '\\' => {},
-        '/' => all_backslash = false,
-        else => return .none,
-    }
-    return switch (path[2]) {
-        '?' => if (all_backslash) .verbatim else .fake_verbatim,
-        '.' => .local_device,
-        else => .none,
-    };
-}
-
-test getNamespacePrefix {
-    try std.testing.expectEqual(NamespacePrefix.none, getNamespacePrefix(u8, ""));
-    try std.testing.expectEqual(NamespacePrefix.nt, getNamespacePrefix(u8, "\\??\\"));
-    try std.testing.expectEqual(NamespacePrefix.none, getNamespacePrefix(u8, "/??/"));
-    try std.testing.expectEqual(NamespacePrefix.none, getNamespacePrefix(u8, "/??\\"));
-    try std.testing.expectEqual(NamespacePrefix.none, getNamespacePrefix(u8, "\\?\\\\"));
-    try std.testing.expectEqual(NamespacePrefix.local_device, getNamespacePrefix(u8, "\\\\.\\"));
-    try std.testing.expectEqual(NamespacePrefix.local_device, getNamespacePrefix(u8, "\\\\./"));
-    try std.testing.expectEqual(NamespacePrefix.local_device, getNamespacePrefix(u8, "/\\./"));
-    try std.testing.expectEqual(NamespacePrefix.local_device, getNamespacePrefix(u8, "//./"));
-    try std.testing.expectEqual(NamespacePrefix.none, getNamespacePrefix(u8, "/.//"));
-    try std.testing.expectEqual(NamespacePrefix.verbatim, getNamespacePrefix(u8, "\\\\?\\"));
-    try std.testing.expectEqual(NamespacePrefix.fake_verbatim, getNamespacePrefix(u8, "\\/?\\"));
-    try std.testing.expectEqual(NamespacePrefix.fake_verbatim, getNamespacePrefix(u8, "\\/?/"));
-    try std.testing.expectEqual(NamespacePrefix.fake_verbatim, getNamespacePrefix(u8, "//?/"));
-}
-
-pub const UnprefixedPathType = enum {
-    unc_absolute,
-    drive_absolute,
-    drive_relative,
-    rooted,
-    relative,
-    root_local_device,
-};
-
-inline fn isSepW(c: u16) bool {
-    return c == '/' or c == '\\';
-}
-
-/// Get the path type of a path that is known to not have any namespace prefixes
-/// (`\\?\`, `\\.\`, `\??\`).
-pub fn getUnprefixedPathType(comptime T: type, path: []const T) UnprefixedPathType {
-    if (path.len < 1) return .relative;
-
-    if (std.debug.runtime_safety) {
-        std.debug.assert(getNamespacePrefix(T, path) == .none);
-    }
-
-    if (isSepW(path[0])) {
-        // \x
-        if (path.len < 2 or !isSepW(path[1])) return .rooted;
-        // exactly \\. or \\? with nothing trailing
-        if (path.len == 3 and (path[2] == '.' or path[2] == '?')) return .root_local_device;
-        // \\x
-        return .unc_absolute;
-    } else {
-        // x
-        if (path.len < 2 or path[1] != ':') return .relative;
-        // x:\
-        if (path.len > 2 and isSepW(path[2])) return .drive_absolute;
-        // x:
-        return .drive_relative;
-    }
-}
-
-test getUnprefixedPathType {
-    try std.testing.expectEqual(UnprefixedPathType.relative, getUnprefixedPathType(u8, ""));
-    try std.testing.expectEqual(UnprefixedPathType.relative, getUnprefixedPathType(u8, "x"));
-    try std.testing.expectEqual(UnprefixedPathType.relative, getUnprefixedPathType(u8, "x\\"));
-    try std.testing.expectEqual(UnprefixedPathType.root_local_device, getUnprefixedPathType(u8, "//."));
-    try std.testing.expectEqual(UnprefixedPathType.root_local_device, getUnprefixedPathType(u8, "/\\?"));
-    try std.testing.expectEqual(UnprefixedPathType.root_local_device, getUnprefixedPathType(u8, "\\\\?"));
-    try std.testing.expectEqual(UnprefixedPathType.unc_absolute, getUnprefixedPathType(u8, "\\\\x"));
-    try std.testing.expectEqual(UnprefixedPathType.unc_absolute, getUnprefixedPathType(u8, "//x"));
-    try std.testing.expectEqual(UnprefixedPathType.rooted, getUnprefixedPathType(u8, "\\x"));
-    try std.testing.expectEqual(UnprefixedPathType.rooted, getUnprefixedPathType(u8, "/"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_relative, getUnprefixedPathType(u8, "x:"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_relative, getUnprefixedPathType(u8, "x:abc"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_relative, getUnprefixedPathType(u8, "x:a/b/c"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_absolute, getUnprefixedPathType(u8, "x:\\"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_absolute, getUnprefixedPathType(u8, "x:\\abc"));
-    try std.testing.expectEqual(UnprefixedPathType.drive_absolute, getUnprefixedPathType(u8, "x:/a/b/c"));
+    });
+    path_space.data[path_space.len] = 0;
+    return path_space;
 }
 
 fn getFullPathNameW(path: [*:0]const u16, out: []u16) !usize {
@@ -2251,6 +2046,34 @@ fn getFullPathNameW(path: [*:0]const u16, out: []u16) !usize {
     return result;
 }
 
+/// Assumes an absolute path.
+pub fn wToPrefixedFileW(s: []const u16) !PathSpace {
+    // TODO https://github.com/ziglang/zig/issues/2765
+    var path_space: PathSpace = undefined;
+
+    const start_index = if (mem.startsWith(u16, s, &[_]u16{ '\\', '?' })) 0 else blk: {
+        const prefix = [_]u16{ '\\', '?', '?', '\\' };
+        path_space.data[0..prefix.len].* = prefix;
+        break :blk prefix.len;
+    };
+    path_space.len = start_index + s.len;
+    if (path_space.len > path_space.data.len) return error.NameTooLong;
+    @memcpy(path_space.data[start_index..][0..s.len], s);
+    // > File I/O functions in the Windows API convert "/" to "\" as part of
+    // > converting the name to an NT-style name, except when using the "\\?\"
+    // > prefix as detailed in the following sections.
+    // from https://docs.microsoft.com/en-us/windows/desktop/FileIO/naming-a-file#maximum-path-length-limitation
+    // Because we want the larger maximum path length for absolute paths, we
+    // convert forward slashes to backward slashes here.
+    for (path_space.data[0..path_space.len]) |*elem| {
+        if (elem.* == '/') {
+            elem.* = '\\';
+        }
+    }
+    path_space.data[path_space.len] = 0;
+    return path_space;
+}
+
 inline fn MAKELANGID(p: c_ushort, s: c_ushort) LANGID {
     return (s << 10) | p;
 }
lib/std/child_process.zig
@@ -957,12 +957,15 @@ fn windowsCreateProcessPathExt(
     // NtQueryDirectoryFile calls.
 
     var dir = dir: {
+        if (fs.path.isAbsoluteWindowsWTF16(dir_buf.items[0..dir_path_len])) {
+            const prefixed_path = try windows.wToPrefixedFileW(dir_buf.items[0..dir_path_len]);
+            break :dir fs.cwd().openDirW(prefixed_path.span().ptr, .{}, true) catch return error.FileNotFound;
+        }
         // needs to be null-terminated
         try dir_buf.append(allocator, 0);
-        defer dir_buf.shrinkRetainingCapacity(dir_path_len);
+        defer dir_buf.shrinkRetainingCapacity(dir_buf.items[0..dir_path_len].len);
         const dir_path_z = dir_buf.items[0 .. dir_buf.items.len - 1 :0];
-        const prefixed_path = try windows.wToPrefixedFileW(dir_path_z);
-        break :dir fs.cwd().openDirW(prefixed_path.span().ptr, .{}, true) catch return error.FileNotFound;
+        break :dir std.fs.cwd().openDirW(dir_path_z.ptr, .{}, true) catch return error.FileNotFound;
     };
     defer dir.close();