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tree-wide: use /proc/thread-self for thread-local state

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With the idmap work, we will have a tainted Go thread in our
thread-group that has a different mount namespace to the other threads.
It seems that (due to some bad luck) the Go scheduler tends to make this
thread the thread-group leader in our tests, which results in very
baffling failures where /proc/self/mountinfo produces gibberish results.

In order to avoid this, switch to using /proc/thread-self for everything
that is thread-local. This primarily includes switching all file
descriptor paths (CLONE_FS), all of the places that check the current
cgroup (technically we never will run a single runc thread in a separate
cgroup, but better to be safe than sorry), and the aforementioned
mountinfo code. We don't need to do anything for the following because
the results we need aren't thread-local:

 * Checks that certain namespaces are supported by stat(2)ing
   /proc/self/ns/...

 * /proc/self/exe and /proc/self/cmdline are not thread-local.

 * While threads can be in different cgroups, we do not do this for the
   runc binary (or libcontainer) and thus we do not need to switch to
   the thread-local version of /proc/self/cgroups.

 * All of the CLONE_NEWUSER files are not thread-local because you
   cannot set the usernamespace of a single thread (setns(CLONE_NEWUSER)
   is blocked for multi-threaded programs).

Note that we have to use runtime.LockOSThread when we have an open
handle to a tid-specific procfs file that we are operating on multiple
times. Go can reschedule us such that we are running on a different
thread and then kill the original thread (causing -ENOENT or similarly
confusing errors). This is not strictly necessary for most usages of
/proc/thread-self (such as using /proc/thread-self/fd/$n directly) since
only operating on the actual inodes associated with the tid requires
this locking, but because of the pre-3.17 fallback for CentOS, we have
to do this in most cases.

In addition, CentOS's kernel is too old for /proc/thread-self, which
requires us to emulate it -- however in rootfs_linux.go, we are in the
container pid namespace but /proc is the host's procfs. This leads to
the incredibly frustrating situation where there is no way (on pre-4.1
Linux) to figure out which /proc/self/task/... entry refers to the
current tid. We can just use /proc/self in this case.

Yes this is all pretty ugly. I also wish it wasn't necessary.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
This commit is contained in:
Aleksa Sarai
2023-08-24 12:53:53 +10:00
parent a04d88ec73
commit 8e8b136c49
18 changed files with 319 additions and 114 deletions
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libcontainer/criu_linux.go
+3 -2
View File
@@ -526,6 +526,7 @@ func (c *Container) restoreNetwork(req *criurpc.CriuReq, criuOpts *CriuOpts) {
// restore using CRIU. This function is inspired from the code in
// rootfs_linux.go.
func (c *Container) makeCriuRestoreMountpoints(m *configs.Mount) error {
me := mountEntry{Mount: m}
switch m.Device {
case "cgroup":
// No mount point(s) need to be created:
@@ -540,7 +541,7 @@ func (c *Container) makeCriuRestoreMountpoints(m *configs.Mount) error {
// The prepareBindMount() function checks if source
// exists. So it cannot be used for other filesystem types.
// TODO: pass srcFD? Not sure if criu is impacted by issue #2484.
if err := prepareBindMount(mountEntry{Mount: m}, c.config.Rootfs); err != nil {
if err := prepareBindMount(me, c.config.Rootfs); err != nil {
return err
}
default:
@@ -549,7 +550,7 @@ func (c *Container) makeCriuRestoreMountpoints(m *configs.Mount) error {
if err != nil {
return err
}
if err := checkProcMount(c.config.Rootfs, dest, ""); err != nil {
if err := checkProcMount(c.config.Rootfs, dest, me); err != nil {
return err
}
if err := os.MkdirAll(dest, 0o755); err != nil {