mountinfo: linux: add a /proc/self/mountinfo fallback

[cyphar]

In very specific cases (that we hit in runc), it is possible to be running in a child pid namespace (where the Go process's pid is 1) but still have a host /proc. While this is not a problem with /proc/thread-self (since the process is in the host pid namespace), the /proc/self/task/ fallback doesn't work because gettid(2) returns the wrong thread-id for the host pid namespace.

There isn't much we can do in this case other than use /proc/self, because the simple workarounds (grep NSpid /proc/self/task/*/status) aren't available in pre-3.17 kernels and any more complicated schemes (such as setting the signal mask to something unique and scanning for it) are unworkable for obvious reasons.

Fixes: 12c61a3 ("mountinfo: linux: use /proc/thread-self/mountinfo")
Signed-off-by: Aleksa Sarai cyphar@cyphar.com

[cyphar]

This should fix the CI issues in opencontainers/runc#4109. Sorry for the noise 😅.

[samuelkarp]

LGTM

(because I didn't know offhand if os.IsNotExist would work with nil errs: https://go.dev/play/p/Hku5gFHXWtX)

[cyphar]

Yup, it seems this does fix the issue. Before:

=== RUN   TestUsernsCheckpoint
    checkpoint_test.go:95: unexpected error: unable to start container process: error during container init: error preparing rootfs: open /proc/self/task/1/mountinfo: no such file or directory
--- FAIL: TestUsernsCheckpoint (0.28s)
=== RUN   TestCheckpoint
    checkpoint_test.go:95: unexpected error: unable to start container process: error during container init: error preparing rootfs: open /proc/self/task/1/mountinfo: no such file or directory
--- FAIL: TestCheckpoint (0.21s)
=== RUN   TestExecPS
    exec_test.go:43: |: unable to start container process: error during container init: error preparing rootfs: open /proc/self/task/1/mountinfo: no such file or directory
--- FAIL: TestExecPS (0.15s)
=== RUN   TestUsernsExecPS
    exec_test.go:43: |: unable to start container process: error during container init: error preparing rootfs: open /proc/self/task/1/mountinfo: no such file or directory
--- FAIL: TestUsernsExecPS (0.15s)

And now in the test PR (opencontainers/runc#4112), all the tests pass. It is unfortunate that in theory we could produce the wrong data in this one case, but it's only for ancient kernels and (in runc's case at least) we are never running with heterogeneous threads when we are pid1 inside the container.

[kolyshkin]

LGTM