[release-branch.go1.12] path/filepath: don't discard .. in EvalSymlinks

EvalSymlinks was mishandling cases like "/x/../../y" or "../../../x"
where there is an extra ".." that goes past the start of the path.

Updates #30520
Fixes #30586

Change-Id: I07525575f83009032fa1a99aa270c8d42007d276
Reviewed-on: https://go-review.googlesource.com/c/go/+/164762Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 294edb27)
Reviewed-on: https://go-review.googlesource.com/c/go/+/165197Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/path/filepath/path_test.go

@@ -1410,3 +1410,103 @@ func TestIssue29372(t *testing.T) {
 		}
 	}
 }
+
+// Issue 30520 part 1.
+func TestEvalSymlinksAboveRoot(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	t.Parallel()
+
+	tmpDir, err := ioutil.TempDir("", "TestEvalSymlinksAboveRoot")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	evalTmpDir, err := filepath.EvalSymlinks(tmpDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.Mkdir(filepath.Join(evalTmpDir, "a"), 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(filepath.Join(evalTmpDir, "a"), filepath.Join(evalTmpDir, "b")); err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(evalTmpDir, "a", "file"), nil, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	// Count the number of ".." elements to get to the root directory.
+	vol := filepath.VolumeName(evalTmpDir)
+	c := strings.Count(evalTmpDir[len(vol):], string(os.PathSeparator))
+	var dd []string
+	for i := 0; i < c+2; i++ {
+		dd = append(dd, "..")
+	}
+
+	wantSuffix := strings.Join([]string{"a", "file"}, string(os.PathSeparator))
+
+	// Try different numbers of "..".
+	for _, i := range []int{c, c + 1, c + 2} {
+		check := strings.Join([]string{evalTmpDir, strings.Join(dd[:i], string(os.PathSeparator)), evalTmpDir[len(vol)+1:], "b", "file"}, string(os.PathSeparator))
+		if resolved, err := filepath.EvalSymlinks(check); err != nil {
+			t.Errorf("EvalSymlinks(%q) failed: %v", check, err)
+		} else if !strings.HasSuffix(resolved, wantSuffix) {
+			t.Errorf("EvalSymlinks(%q) = %q does not end with %q", check, resolved, wantSuffix)
+		} else {
+			t.Logf("EvalSymlinks(%q) = %q", check, resolved)
+		}
+	}
+}
+
+// Issue 30520 part 2.
+func TestEvalSymlinksAboveRootChdir(t *testing.T) {
+	testenv.MustHaveSymlink(t)
+
+	tmpDir, err := ioutil.TempDir("", "TestEvalSymlinksAboveRootChdir")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.Chdir(wd)
+
+	if err := os.Chdir(tmpDir); err != nil {
+		t.Fatal(err)
+	}
+
+	subdir := filepath.Join("a", "b")
+	if err := os.MkdirAll(subdir, 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(subdir, "c"); err != nil {
+		t.Fatal(err)
+	}
+	if err := ioutil.WriteFile(filepath.Join(subdir, "file"), nil, 0666); err != nil {
+		t.Fatal(err)
+	}
+
+	subdir = filepath.Join("d", "e", "f")
+	if err := os.MkdirAll(subdir, 0777); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chdir(subdir); err != nil {
+		t.Fatal(err)
+	}
+
+	check := filepath.Join("..", "..", "..", "c", "file")
+	wantSuffix := filepath.Join("a", "b", "file")
+	if resolved, err := filepath.EvalSymlinks(check); err != nil {
+		t.Errorf("EvalSymlinks(%q) failed: %v", check, err)
+	} else if !strings.HasSuffix(resolved, wantSuffix) {
+		t.Errorf("EvalSymlinks(%q) = %q does not end with %q", check, resolved, wantSuffix)
+	} else {
+		t.Logf("EvalSymlinks(%q) = %q", check, resolved)
+	}
+}

src/path/filepath/symlink.go

@@ -44,18 +44,26 @@ func walkSymlinks(path string) (string, error) {
 		} else if path[start:end] == ".." {
 			// Back up to previous component if possible.
 			// Note that volLen includes any leading slash.
+
+			// Set r to the index of the last slash in dest,
+			// after the volume.
 			var r int
 			for r = len(dest) - 1; r >= volLen; r-- {
 				if os.IsPathSeparator(dest[r]) {
 					break
 				}
 			}
-			if r < volLen {
+			if r < volLen || dest[r+1:] == ".." {
+				// Either path has no slashes
+				// (it's empty or just "C:")
+				// or it ends in a ".." we had to keep.
+				// Either way, keep this "..".
 				if len(dest) > volLen {
 					dest += pathSeparator
 				}
 				dest += ".."
 			} else {
+				// Discard everything since the last slash.
 				dest = dest[:r]
 			}
 			continue