[1.4] libct: switch to (*CPUSet).Fill

Now that we've updated to golang.org/x/sys@v0.37.0, CPUSet has a Fill
helper that does the equivalent to our underflow trick to make setting
all CPUs efficient.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
(cherry picked from commit 93f9a39)
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

Author: cyphar

CHANGELOG.md

@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased 1.4.z]
 
+### Fixed
+ * Switched to `(*CPUSet).Fill` rather than our hacky optimisation when
+   resetting the CPU affinity of runc. (#4926, #4927)
+
 ## [1.4.0-rc.2] - 2025-10-10
 
 > 私の役目は信じるかどうかではない。行うかどうかだ。

libcontainer/process_linux.go

@@ -185,16 +185,8 @@ func tryResetCPUAffinity(pid int) {
 	//
 	// So we can just pass a very large array of set cpumask bits and the
 	// kernel will silently convert that to the correct value very cheaply.
-
-	// Ideally, we would just set the array to 0xFF...FF. Unfortunately, the
-	// size depends on the architecture. It is also a private newtype, so we
-	// can't use (^0) or generics since those require us to be able to name the
-	// type. However, we can just underflow the zero value instead.
-	// TODO: Once <https://golang.org/cl/698015> is merged, switch to that.
-	cpuset := unix.CPUSet{}
-	for i := range cpuset {
-		cpuset[i]-- // underflow to 0xFF..FF
-	}
+	var cpuset unix.CPUSet
+	cpuset.Fill() // set all bits
 	if err := unix.SchedSetaffinity(pid, &cpuset); err != nil {
 		logrus.WithError(
 			os.NewSyscallError("sched_setaffinity", err),