libct: make MaxCPU exclusive

MaxCPU was documented and enforced as the highest CPU/NUMA node ID that
ToCPUSet accepts, i.e. an inclusive bound, while unix.NewCPUSet takes an
exclusive one. As a result, tryResetCPUAffinity's NewCPUSet(MaxCPU) mask
was one ID short of what ToCPUSet can parse.

This is harmless in practice, as sched_setaffinity(2) truncates the mask
to cpumask_size() anyway, and no kernel is configured with anywhere near
64K CPUs. Still, having a single constant mean two different things in
two different files is asking for a real off-by-one later on.

Make MaxCPU exclusive to match NewCPUSet, so that NewCPUSet(MaxCPU) is
correct as written, and ToCPUSet's NewCPUSet(maxID+1) can never exceed
it. This restores the "ret >= max" check and the "max-1" wording used
before MaxCPU was introduced, and shifts the test boundary cases by one.

Fixes: daf934f ("libct: use CPUSetDynamic for affinity and mempolicy masks")
Fixes: c55649b ("libct: reuse configs.MaxCPU")
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

Author: kolyshkin

libcontainer/configs/config.go

@@ -307,9 +307,10 @@ type CPUAffinity struct {
 	Initial, Final unix.CPUSetDynamic
 }
 
-// MaxCPU is the highest CPU/NUMA node ID that [ToCPUSet] accepts.
-// It is an arbitrary sanity limit, used to avoid allocating
-// an unreasonably large mask for a bogus input.
+// MaxCPU is one past the highest CPU/NUMA node ID that [ToCPUSet] accepts,
+// i.e. the number of CPUs/nodes a mask can represent. It is an arbitrary
+// sanity limit, used to avoid allocating an unreasonably large mask for a
+// bogus input. It is exclusive, to match [unix.NewCPUSet].
 const MaxCPU = 64 * 1024
 
 // ToCPUSet parses a string in list format (e.g. "0-3,5,7-9")
@@ -349,8 +350,8 @@ func cpuStrToRanges(str string) (maxID int, ranges []cpuRange, _ error) {
 		if err != nil {
 			return 0, err
 		}
-		if ret > MaxCPU {
-			return 0, fmt.Errorf("values larger than %d are not supported", MaxCPU)
+		if ret >= MaxCPU {
+			return 0, fmt.Errorf("values larger than %d are not supported", MaxCPU-1)
 		}
 		return int(ret), nil
 	}

libcontainer/configs/tocpuset_test.go

@@ -59,9 +59,10 @@ func TestToCPUSet(t *testing.T) {
 		{in: "1024", out: set(1024)},
 		{in: "8191", out: set(8191)},
 		{in: "4096-4098", out: set(4096, 4097, 4098)},
-		{in: "0,65536", out: set(0, 65536)},
+		{in: "0,65534", out: set(0, 65534)},
 		// Maximum allowed value.
-		{in: "65536", out: set(65536)},
+		{in: "65535", out: set(65535)},
+		{in: "65532-65535", out: set(65532, 65533, 65534, 65535)},
 
 		// Error cases.
 		{in: "-", isErr: true},
@@ -74,8 +75,8 @@ func TestToCPUSet(t *testing.T) {
 		// Extra spaces inside a range is not OK.
 		{in: "1 - 2", isErr: true},
 		// Larger than the maximum supported value.
-		{in: "65537", isErr: true},
-		{in: "0-65537", isErr: true},
+		{in: "65536", isErr: true},
+		{in: "0-65536", isErr: true},
 	}
 
 	for _, tc := range testCases {