Fix nil pointer dereference in netd IPTables error handling

[mikewiacek]

Overview

This PR fixes a crash in netd caused by unsafe handling of errors returned from iptables operations.

Previously, code paths in IPTablesRuleConfig.Ensure assumed all errors from AppendUnique and Delete would be of type *iptables.Error.

If the error was not (e.g. exec.ExitError or another Go error type), the type assertion failed, leaving eerr == nil.
The code still attempted to call eerr.Error() or eerr.ExitStatus(), resulting in a panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation]

Root Cause

• The condition mixed !ok with calls to methods on eerr.
• When !ok, eerr is nil, so dereferencing it caused a panic.

Fix

• Refactor error handling to only access ExitStatus() or Error() when the type assertion succeeds.
• If the error is not an *iptables.Error, bubble it up cleanly instead of attempting to interpret it.

Impact

• Prevents netd from crashing on unexpected iptables error types.
• Makes error handling more robust against changes in underlying iptables behavior or Go’s exec error types.

Before

!ok branch still tried to dereference eerr, leading to panic.

After

• Safe separation of the two cases:
  • Handle *iptables.Error with exit status logic.
  • Return all other error types without dereferencing.

Testing

• Verified with synthetic error injections: non-iptables errors no longer crash the daemon.
• Normal iptables error handling remains unchanged.

[google-oss-prow]

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[jingyuanliang]

			if err := r.IPT.Delete(r.Spec.TableName, r.Spec.ChainName, rs...); err != nil {
				if eerr, eok := err.(*iptables.Error); !eok || eerr.ExitStatus() != 2 && !strings.Contains(eerr.Error(), "No chain/target/match") {
					return err
				}
			}

should be enough?

but I'd like to confirm the exact behavior desired... e.g. what's the definition of the exit status values?

[jingyuanliang]

@MrHohn

[mikewiacek]

ping?