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Resolve to the last mount in the stack during umount.

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This is done in linux with the LOOKUP_MOUNTPOINT flag. We emulate this behavior
in gVisor with getMountAt().

PiperOrigin-RevId: 415642095
This commit is contained in:
Lucas Manning
2021-12-10 17:10:42 -08:00
committed by gVisor bot
parent 9afac716b1
commit 75bfc5e0e9
2 changed files with 37 additions and 4 deletions
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pkg/sentry/vfs/mount.go
+15 -4
View File
@@ -297,10 +297,21 @@ func (vfs *VirtualFilesystem) UmountAt(ctx context.Context, creds *auth.Credenti
if err != nil {
return err
}
defer vd.DecRef(ctx)
if vd.dentry != vd.mount.root {
defer func() {
vd.DecRef(ctx)
}()
// Linux passes the LOOKUP_MOUNPOINT flag to user_path_at in ksys_umount to resolve to the
// toppmost mount in the stack located at the specified path. vfs.GetMountAt() imitiates this
// behavior. See fs/namei.c:user_path_at(...) and fs/namespace.c:ksys_umount(...).
if vd.dentry.isMounted() {
if realmnt := vfs.getMountAt(ctx, vd.mount, vd.dentry); realmnt != nil {
vd.mount.DecRef(ctx)
vd.mount = realmnt
}
} else if vd.dentry != vd.mount.root {
return linuxerr.EINVAL
}
vfs.mountMu.Lock()
if mntns := MountNamespaceFromContext(ctx); mntns != nil {
defer mntns.DecRef(ctx)
@@ -346,8 +357,8 @@ func (vfs *VirtualFilesystem) UmountAt(ctx context.Context, creds *auth.Credenti
for _, vd := range vdsToDecRef {
vd.DecRef(ctx)
}
for _, mnt := range mountsToDecRef {
mnt.DecRef(ctx)
for _, m := range mountsToDecRef {
m.DecRef(ctx)
}
return nil
}
test/syscalls/linux/mount.cc
+22
View File
@@ -220,6 +220,28 @@ TEST(MountTest, UmountDetach) {
OpenAt(mounted_dir.get(), "..", O_DIRECTORY | O_RDONLY));
}
TEST(MountTest, UmountMountsStackedOnDot) {
SKIP_IF(IsRunningWithVFS1());
SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));
// Verify that unmounting at "." properly unmounts the mount at the top of
// mount stack.
auto const dir = ASSERT_NO_ERRNO_AND_VALUE(TempPath::CreateDir());
TEST_CHECK_SUCCESS(chdir(dir.path().c_str()));
const struct stat before = ASSERT_NO_ERRNO_AND_VALUE(Stat("."));
TEST_CHECK_SUCCESS(mount("", dir.path().c_str(), "tmpfs", 0, "mode=0700"));
TEST_CHECK_SUCCESS(mount("", dir.path().c_str(), "tmpfs", 0, "mode=0700"));
// Unmount the second mount at "."
TEST_CHECK_SUCCESS(umount2(".", MNT_DETACH));
// Unmount the first mount at "."; this will fail if umount does not resolve
// "." to the topmost mount.
TEST_CHECK_SUCCESS(umount2(".", MNT_DETACH));
const struct stat after2 = ASSERT_NO_ERRNO_AND_VALUE(Stat("."));
EXPECT_TRUE(before.st_dev == after2.st_dev && before.st_ino == after2.st_ino);
}
TEST(MountTest, ActiveSubmountBusy) {
SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(HaveCapability(CAP_SYS_ADMIN)));