From f3782300d2ebd4a70c594f1eb12a7d79de4b9602 Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Wed, 17 Jan 2024 11:43:28 -0800
Subject: [PATCH] QStorageInfo/Linux: fix getting information on unmounted
 btrfs subvols

Amends 1cd6c6c69e9813c791f8bebb6c0c9214ce765060.

Btrfs can have subvolumes and each one of them is assigned a device ID
when the filesystem is loaded into the kernel. But subvolumes don't all
have to be a mountpoint of their own: if we insist on matching device
IDs, as initRootPath() was doing, we'd fail at finding the mount point.

Fixes: QTBUG-121140
Change-Id: I76ffba14ece04f24b43efffd17ab39f503000dd7
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit 987abb92538f8657d861611b1ced4e7eaa660adf)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/corelib/io/qstorageinfo_linux.cpp | 28 ++++++++++++++++++++++-----
 1 file changed, 23 insertions(+), 5 deletions(-)

diff --git a/src/corelib/io/qstorageinfo_linux.cpp b/src/corelib/io/qstorageinfo_linux.cpp
index 1b2b0adcb04..7d8a5a293d3 100644
--- a/src/corelib/io/qstorageinfo_linux.cpp
+++ b/src/corelib/io/qstorageinfo_linux.cpp
@@ -10,6 +10,8 @@
 #include <private/qcore_unix_p.h>
 #include <private/qtools_p.h>
 
+#include <q20memory.h>
+
 #include <linux/mount.h>
 #include <sys/statfs.h>
 
@@ -199,15 +201,31 @@ quint64 QStorageInfoPrivate::initRootPath()
     //   # mount | tail -2
     //   tmpfs on /tmp/foo/bar type tmpfs (rw,relatime,inode64)
     //   tmpfs on /tmp/foo type tmpfs (rw,relatime,inode64)
-    // But just in case there's a mount --move, we ensure the device ID does
-    // match.
+    //
+    // We try to match the device ID in case there's a mount --move.
+    // We can't *rely* on it because some filesystems like btrfs will assign
+    // device IDs to subvolumes that aren't listed in /proc/self/mountinfo.
+
     const QString oldRootPath = std::exchange(rootPath, QString());
     const dev_t rootPathDevId = deviceIdForPath(oldRootPath);
+    MountInfo *best = nullptr;
     for (auto it = infos.rbegin(); it != infos.rend(); ++it) {
-        if (rootPathDevId != it->stDev || !isParentOf(it->mountPoint, oldRootPath))
+        if (!isParentOf(it->mountPoint, oldRootPath))
             continue;
-        auto stDev = it->stDev;
-        setFromMountInfo(std::move(*it));
+        if (rootPathDevId == it->stDev) {
+            // device ID matches; this is definitely the best option
+            best = q20::to_address(it);
+            break;
+        }
+        if (!best) {
+            // if we can't find a device ID match, this parent path is probably
+            // the correct one
+            best = q20::to_address(it);
+        }
+    }
+    if (best) {
+        auto stDev = best->stDev;
+        setFromMountInfo(std::move(*best));
         return stDev;
     }
     return 0;