fix(fs, model): improve symlink resilience in file shortcut (#10739)

Ensure file was a file before the shortcut as well as after... (This was
implied when talking to a correct implementation, but not enforced.)

Make our file opening operations safe by default by ensuring the last
path component is not a symlink.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/fs/basicfs.go

@@ -241,15 +241,7 @@ func (f *BasicFilesystem) DirNames(name string) ([]string, error) {
 }
 
 func (f *BasicFilesystem) Open(name string) (File, error) {
-	rootedName, err := f.rooted(name)
+	return f.OpenFile(name, os.O_RDONLY, 0)
-	if err != nil {
-		return nil, err
-	}
-	fd, err := os.Open(rootedName)
-	if err != nil {
-		return nil, err
-	}
-	return basicFile{fd, name}, err
 }
 
 func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File, error) {
@@ -257,6 +249,7 @@ func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File,
 	if err != nil {
 		return nil, err
 	}
+	flags |= alwaysOpenFlags // enforce extra bits in flags
 	fd, err := os.OpenFile(rootedName, flags, os.FileMode(mode))
 	if err != nil {
 		return nil, err
@@ -265,15 +258,7 @@ func (f *BasicFilesystem) OpenFile(name string, flags int, mode FileMode) (File,
 }
 
 func (f *BasicFilesystem) Create(name string) (File, error) {
-	rootedName, err := f.rooted(name)
+	return f.OpenFile(name, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0o666)
-	if err != nil {
-		return nil, err
-	}
-	fd, err := os.Create(rootedName)
-	if err != nil {
-		return nil, err
-	}
-	return basicFile{fd, name}, err
 }
 
 func (*BasicFilesystem) Walk(_ string, _ WalkFunc) error {

lib/fs/basicfs_unix.go

@@ -14,8 +14,11 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"syscall"
 )
 
+const alwaysOpenFlags = syscall.O_NOFOLLOW // never open symlinks as the final path component
+
 func (f *BasicFilesystem) CreateSymlink(target, name string) error {
 	name, err := f.rooted(name)
 	if err != nil {

lib/fs/basicfs_windows.go

@@ -21,6 +21,8 @@ import (
 	"golang.org/x/sys/windows"
 )
 
+const alwaysOpenFlags = 0 // no extra flags
+
 var errNotSupported = errors.New("symlinks not supported")
 
 func (BasicFilesystem) ReadSymlink(path string) (string, error) {

lib/model/folder_sendrecv.go

@@ -382,7 +382,7 @@ loop:
 			if err != nil {
 				return nil, nil, err
 			}
-			if hasCurFile && file.BlocksEqual(curFile) {
+			if hasCurFile && curFile.Type == file.Type && file.BlocksEqual(curFile) {
 				// We are supposed to copy the entire file, and then fetch nothing. We
 				// are only updating metadata, so we don't actually *need* to make the
 				// copy.