fix(kill): close netns FD to prevent leak and lock OS thread

[AyushSriv06]

Description

Adds a missing close call to prevent leaking file descriptors when joining network namespaces.
Enforces thread locking in Kill() to ensure network operations execute safely on the correct OS thread.

Related issues

N/A

How was this tested?

buid and lint

LLM usage

N/A

Checklist

• I have read the contribution guide.
• The linter passes locally (make lint).
• The e2e tests of at least one tool pass locally (make test_ctr, make test_nerdctl, make test_docker, make test_crictl).
• If LLMs were used: I have read the llm policy.

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[cmainas]

Hello @AyushSriv06 ,

thank you for this PR. Please open an issue first, if you find something. We need to verify it first and then open a PR. FOr example in this PR the locking of the thread takes place earlier here

urunc/cmd/urunc/kill.go

Line 48 in d7814ce

runtime.LockOSThread()

and here

urunc/cmd/urunc/delete.go

Line 50 in d7814ce

runtime.LockOSThread()

[AyushSriv06]

Hello @AyushSriv06 ,

thank you for this PR. Please open an issue first, if you find something. We need to verify it first and then open a PR. FOr example in this PR the locking of the thread takes place earlier here

urunc/cmd/urunc/kill.go

Line 48 in d7814ce

runtime.LockOSThread()

and here

urunc/cmd/urunc/delete.go

Line 50 in d7814ce

runtime.LockOSThread()

okay, it was a confusion from my side, should i open a issue for the fd leak?

[cmainas]

Regarding the netns fd, I can not see how this can lead to a leak. The kernel cleans up all the resources after the process exits and the kill path in urunc, enters the network namespace to clean up and exits.

However, for sanity we can close the file descriptor right after we enter the namespace. No need for a defer.