Add 0-length buffer checks to os.read & os.write

This helps prevent errors related to undefined pointers being passed
through to some OS apis when slices have 0 length.

Tests have also been added to catch these cases.

Author: moosichu

lib/std/os.zig

@@ -660,6 +660,7 @@ pub const ReadError = error{
 /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
 /// The corresponding POSIX limit is `math.maxInt(isize)`.
 pub fn read(fd: fd_t, buf: []u8) ReadError!usize {
+    if (buf.len == 0) return 0;
     if (builtin.os.tag == .windows) {
         return windows.ReadFile(fd, buf, null, std.io.default_mode);
     }
@@ -787,6 +788,7 @@ pub const PReadError = ReadError || error{Unseekable};
 /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
 /// The corresponding POSIX limit is `math.maxInt(isize)`.
 pub fn pread(fd: fd_t, buf: []u8, offset: u64) PReadError!usize {
+    if (buf.len == 0) return 0;
     if (builtin.os.tag == .windows) {
         return windows.ReadFile(fd, buf, offset, std.io.default_mode);
     }
@@ -1045,6 +1047,7 @@ pub const WriteError = error{
 /// The limit on Darwin is `0x7fffffff`, trying to read more than that returns EINVAL.
 /// The corresponding POSIX limit is `math.maxInt(isize)`.
 pub fn write(fd: fd_t, bytes: []const u8) WriteError!usize {
+    if (bytes.len == 0) return 0;
     if (builtin.os.tag == .windows) {
         return windows.WriteFile(fd, bytes, null, std.io.default_mode);
     }
@@ -1197,6 +1200,7 @@ pub const PWriteError = WriteError || error{Unseekable};
 /// The limit on Darwin is `0x7fffffff`, trying to write more than that returns EINVAL.
 /// The corresponding POSIX limit is `math.maxInt(isize)`.
 pub fn pwrite(fd: fd_t, bytes: []const u8, offset: u64) PWriteError!usize {
+    if (bytes.len == 0) return 0;
     if (builtin.os.tag == .windows) {
         return windows.WriteFile(fd, bytes, offset, std.io.default_mode);
     }

lib/std/os/test.zig

@@ -1073,3 +1073,89 @@ test "isatty" {
     var file = try tmp.dir.createFile("foo", .{});
     try expectEqual(os.isatty(file.handle), false);
 }
+
+test "read with empty buffer" {
+    if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
+    if (native_os == .wasi and !builtin.link_libc) try os.initPreopensWasi(std.heap.page_allocator, "/");
+
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    var bytes = try allocator.alloc(u8, 0);
+    defer testing.allocator.free(bytes);
+    // Open build.zig as we are just reading it anyway, and it's a text file
+    // that definitely exists.
+    var file = try fs.cwd().openFile("build.zig", .{ .mode = .read_only });
+    defer file.close();
+    _ = try os.read(file.handle, bytes);
+}
+
+test "pread with empty buffer" {
+    if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
+    if (native_os == .wasi and !builtin.link_libc) try os.initPreopensWasi(std.heap.page_allocator, "/");
+
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    var bytes = try allocator.alloc(u8, 0);
+    defer testing.allocator.free(bytes);
+    // Open build.zig as we are just reading it anyway, and it's a text file
+    // that definitely exists.
+    var file = try fs.cwd().openFile("build.zig", .{ .mode = .read_only });
+    defer file.close();
+    _ = try os.pread(file.handle, bytes, 0);
+}
+
+test "write with empty buffer" {
+    if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
+    if (native_os == .wasi and !builtin.link_libc) try os.initPreopensWasi(std.heap.page_allocator, "/");
+
+    var tmp = tmpDir(.{});
+    defer tmp.cleanup();
+
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    // Get base abs path
+    const base_path = blk: {
+        const relative_path = try fs.path.join(allocator, &[_][]const u8{ "zig-cache", "tmp", tmp.sub_path[0..] });
+        break :blk try fs.realpathAlloc(allocator, relative_path);
+    };
+
+    var file_path: []u8 = try fs.path.join(allocator, &[_][]const u8{ base_path, "some_file" });
+    var file = try fs.cwd().createFile(file_path, .{});
+    defer file.close();
+
+    var bytes = try allocator.alloc(u8, 0);
+
+    _ = try os.write(file.handle, bytes);
+}
+
+test "pwrite with empty buffer" {
+    if (native_os == .wasi and builtin.link_libc) return error.SkipZigTest;
+    if (native_os == .wasi and !builtin.link_libc) try os.initPreopensWasi(std.heap.page_allocator, "/");
+
+    var tmp = tmpDir(.{});
+    defer tmp.cleanup();
+
+    var arena = ArenaAllocator.init(testing.allocator);
+    defer arena.deinit();
+    const allocator = arena.allocator();
+
+    // Get base abs path
+    const base_path = blk: {
+        const relative_path = try fs.path.join(allocator, &[_][]const u8{ "zig-cache", "tmp", tmp.sub_path[0..] });
+        break :blk try fs.realpathAlloc(allocator, relative_path);
+    };
+
+    var file_path: []u8 = try fs.path.join(allocator, &[_][]const u8{ base_path, "some_file" });
+    var file = try fs.cwd().createFile(file_path, .{});
+    defer file.close();
+
+    var bytes = try allocator.alloc(u8, 0);
+
+    _ = try os.pwrite(file.handle, bytes, 0);
+}