QStorageInfo/Linux: rewrite the label retriever to use device IDs

Instead of the previous realpath() comparison resulting from the symlink
processing. parseMountInfo() was extracting the device number from
/proc, so this information was already readily available.

We must take care of anonymous block devices (major == 0). Certain
filesystems, such as btrfs, always use them, so we must still stat() the
device path to get the real block device.

This implementation assumes that udev only creates entries in the
/dev/disks/by-label directory that are symlinks to real devices, but
that must already be the case because they are in /dev in the first
place. An alternative implementation would be to compare the inode and
host device (st_dev) of the entry, if different /dev entries could have
different labels. I don't think that's possible. But multiple /dev
entries for the same device is definitely possible.

Pick-to: 6.6
Change-Id: I9d43e5b91eb142d6945cfffd1787552af3d09676
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/io/qstorageinfo_linux.cpp

@@ -67,33 +67,57 @@ static inline dev_t deviceIdForPath(const QString &device)
     return st.st_dev;
 }
 
-static inline QString retrieveLabel(const QByteArray &device)
+static inline quint64 retrieveDeviceId(const QByteArray &device, quint64 deviceId = 0)
+{
+    // major = 0 implies an anonymous block device, so we need to stat() the
+    // actual device to get its dev_t. This is required for btrfs (and possibly
+    // others), which always uses them for all the subvolumes (including the
+    // root):
+    // https://codebrowser.dev/linux/linux/fs/btrfs/disk-io.c.html#btrfs_init_fs_root
+    // https://codebrowser.dev/linux/linux/fs/super.c.html#get_anon_bdev
+    // For everything else, we trust the parameter.
+    if (major(deviceId) != 0)
+        return deviceId;
+
+    // don't even try to stat() a relative path or "/"
+    if (device.size() < 2 || !device.startsWith('/'))
+        return 0;
+
+    QT_STATBUF st;
+    if (QT_STAT(device, &st) < 0)
+        return 0;
+    if (!S_ISBLK(st.st_mode))
+        return 0;
+    return st.st_rdev;
+}
+
+static inline QString retrieveLabel(const QByteArray &device, quint64 deviceId)
 {
     static const char pathDiskByLabel[] = "/dev/disk/by-label";
 
-    QFileInfo devinfo(QFile::decodeName(device));
-    QString devicePath = devinfo.canonicalFilePath();
-    if (devicePath.isEmpty())
+    deviceId = retrieveDeviceId(device, deviceId);
+    if (!deviceId)
         return QString();
 
     auto filter = QDir::AllEntries | QDir::System | QDir::Hidden | QDir::NoDotAndDotDot;
     QDirIterator it(QLatin1StringView(pathDiskByLabel), filter);
     while (it.hasNext()) {
         QFileInfo fileInfo = it.nextFileInfo();
-        if (fileInfo.symLinkTarget() == devicePath)
-            return decodeFsEncString(fileInfo.fileName());
+        QString name = fileInfo.fileName();
+        if (retrieveDeviceId(QFile::encodeName(fileInfo.filePath())) == deviceId)
+            return decodeFsEncString(std::move(name));
     }
     return QString();
 }
 
 void QStorageInfoPrivate::doStat()
 {
-    initRootPath();
-    if (rootPath.isEmpty())
+    quint64 deviceId = initRootPath();
+    if (!deviceId)
         return;
 
     retrieveVolumeInfo();
-    name = retrieveLabel(device);
+    name = retrieveLabel(device, deviceId);
 }
 
 void QStorageInfoPrivate::retrieveVolumeInfo()
@@ -132,16 +156,25 @@ static std::vector<MountInfo> parseMountInfo(FilterMountInfo filter = FilterMoun
     return doParseMountInfo(mountinfo, filter);
 }
 
-void QStorageInfoPrivate::initRootPath()
+quint64 QStorageInfoPrivate::initRootPath()
 {
     rootPath = QFileInfo(rootPath).canonicalFilePath();
     if (rootPath.isEmpty())
-        return;
+        return 0;
 
     std::vector<MountInfo> infos = parseMountInfo();
     if (infos.empty()) {
         rootPath = u'/';
-        return;
+
+        // Need to return something non-zero here for this unlikely condition.
+        // Linux currently uses 20 bits for the minor portion[1] in a 32-bit
+        // integer; glibc, MUSL, and 64-bit Bionic use a 64-bit userspace
+        // dev_t, so this value will not match a real device from the kernel.
+        // 32-bit Bionic still has a 32-bit dev_t, but its makedev() macro[2]
+        // returns 64-bit content too.
+        // [1] https://codebrowser.dev/linux/linux/include/linux/kdev_t.h.html#_M/MINORBITS
+        // [2] https://android.googlesource.com/platform/bionic/+/ndk-r19/libc/include/sys/sysmacros.h#39
+        return makedev(0, -1);
     }
 
     // We iterate over the /proc/self/mountinfo list backwards because then any
@@ -162,8 +195,9 @@ void QStorageInfoPrivate::initRootPath()
         device = std::move(it->device);
         fileSystemType = std::move(it->fsType);
         subvolume = std::move(it->fsRoot);
-        return;
+        return it->stDev;
     }
+    return 0;
 }
 
 QList<QStorageInfo> QStorageInfoPrivate::mountedVolumes()
@@ -180,7 +214,7 @@ QList<QStorageInfo> QStorageInfoPrivate::mountedVolumes()
             continue;
         if (info.stDev != deviceIdForPath(d.rootPath))
             continue;       // probably something mounted over this mountpoint
-        d.name = retrieveLabel(d.device);
+        d.name = retrieveLabel(d.device, info.stDev);
         volumes.emplace_back(QStorageInfo(*new QStorageInfoPrivate(std::move(d))));
     }
     return volumes;

src/corelib/io/qstorageinfo_p.h

@@ -30,7 +30,6 @@ class QStorageInfoPrivate : public QSharedData
 public:
     QStorageInfoPrivate() = default;
 
-    void initRootPath();
     void doStat();
 
     static QList<QStorageInfo> mountedVolumes();
@@ -46,18 +45,20 @@ public:
 
 protected:
 #if defined(Q_OS_WIN)
+    void initRootPath();
     void retrieveVolumeInfo();
     void retrieveDiskFreeSpace();
     bool queryStorageProperty();
     void queryFileFsSectorSizeInformation();
 #elif defined(Q_OS_DARWIN)
+    void initRootPath();
     void retrievePosixInfo();
     void retrieveUrlProperties(bool initRootPath = false);
     void retrieveLabel();
-#elif defined(Q_OS_UNIX)
+#elif defined(Q_OS_LINUX)
+    quint64 initRootPath();
     void retrieveVolumeInfo();
 
-#  ifdef Q_OS_LINUX
 public:
     struct MountInfo {
         QString mountPoint;
@@ -73,7 +74,9 @@ public:
           fileSystemType(std::move(info.fsType))
     {
     }
-#  endif
+#elif defined(Q_OS_UNIX)
+    void initRootPath();
+    void retrieveVolumeInfo();
 #endif
 
 public:

tests/auto/corelib/io/qstorageinfo/tst_qstorageinfo.cpp

@@ -184,6 +184,23 @@ void tst_QStorageInfo::storageList()
         QVERIFY(!storage.device().isEmpty());
         QVERIFY(!storage.fileSystemType().isEmpty());
 #endif
+
+        QStorageInfo other(storage.rootPath());
+        QVERIFY(other.isValid());
+        QCOMPARE(other.rootPath(), storage.rootPath());
+        QCOMPARE(other.device(), storage.device());
+        QCOMPARE(other.subvolume(), storage.subvolume());
+        QCOMPARE(other.fileSystemType(), storage.fileSystemType());
+        QCOMPARE(other.name(), storage.name());
+        QCOMPARE(other.displayName(), storage.displayName());
+
+        QCOMPARE(other.bytesTotal(), storage.bytesTotal());
+        QCOMPARE(other.blockSize(), storage.blockSize());
+        // not comparing free space because it may have changed
+
+        QCOMPARE(other.isRoot(), storage.isRoot());
+        QCOMPARE(other.isReadOnly(), storage.isReadOnly());
+        QCOMPARE(other.isReady(), storage.isReady());
 }
 
 static bool checkFilesystemGoodForWriting(QTemporaryFile &file, QStorageInfo &storage)