Fix runc exec vs go1.26 + older kernel

.github/workflows/test.yml

@@ -25,7 +25,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-24.04, ubuntu-24.04-arm]
-        go-version: [1.24.x, 1.25.x]
+        go-version: [1.24.x, 1.25.x, 1.26.0-rc.2]
         rootless: ["rootless", ""]
         race: ["-race", ""]
         criu: ["", "criu-dev"]
@@ -34,11 +34,15 @@ jobs:
           # (need to compile criu) and don't add much value/coverage.
           - criu: criu-dev
             go-version: 1.24.x
+          - criu: criu-dev
+            go-version: 1.26.0-rc.2
           - criu: criu-dev
             rootless: rootless
-          # Do race detection only on latest Go.
+          # Do race detection only with latest stable Go version.
           - race: -race
             go-version: 1.24.x
+          - race: -race
+            go-version: 1.26.0-rc.2
 
     runs-on: ${{ matrix.os }}
 

libcontainer/cmd_clone.go

@@ -0,0 +1,37 @@
+package libcontainer
+
+import "os/exec"
+
+// cloneCmd creates a copy of exec.Cmd. It is needed because cmd.Start
+// must only be used once, and go1.26 actually enforces that (see
+// https://go-review.googlesource.com/c/go/+/728642). The implementation
+// is similar to
+//
+//	cmd = *c
+//	return &cmd
+//
+// except it does not copy private fields, or fields populated
+// after the call to cmd.Start.
+//
+// NOTE if Go will add exec.Cmd.Clone, we should switch to it.
+func cloneCmd(c *exec.Cmd) *exec.Cmd {
+	cmd := &exec.Cmd{
+		Path:        c.Path,
+		Args:        c.Args,
+		Env:         c.Env,
+		Dir:         c.Dir,
+		Stdin:       c.Stdin,
+		Stdout:      c.Stdout,
+		Stderr:      c.Stderr,
+		ExtraFiles:  c.ExtraFiles,
+		SysProcAttr: c.SysProcAttr,
+		// Don't copy Process, ProcessState, Err since
+		// these fields are populated after the start.
+
+		// Technically, we do not use Cancel or WaitDelay,
+		// but they are here for the sake of completeness.
+		Cancel:    c.Cancel,
+		WaitDelay: c.WaitDelay,
+	}
+	return cmd
+}

libcontainer/container_linux.go

@@ -528,6 +528,12 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 	}
 
 	cmd := exec.Command(exePath, "init")
+	// Theoretically, exec.Command can set cmd.Err. Practically, this
+	// should never happen (Linux, Go <= 1.26, exePath is absolute),
+	// but in the unlikely case it just did, let's fail early.
+	if cmd.Err != nil {
+		return nil, fmt.Errorf("exec.Command: %w", cmd.Err)
+	}
 	cmd.Args[0] = os.Args[0]
 	cmd.Stdin = p.Stdin
 	cmd.Stdout = p.Stdout

libcontainer/process_linux.go

@@ -380,11 +380,14 @@ func (p *setnsProcess) startWithCgroupFD() error {
 		defer fd.Close()
 	}
 
+	cmdCopy := cloneCmd(p.cmd)
 	err = p.startWithCPUAffinity()
 	if err != nil && p.cmd.SysProcAttr.UseCgroupFD {
 		logrus.Debugf("exec with CLONE_INTO_CGROUP failed: %v; retrying without", err)
 		// SysProcAttr.CgroupFD is never used when UseCgroupFD is unset.
-		p.cmd.SysProcAttr.UseCgroupFD = false
+		cmdCopy.SysProcAttr.UseCgroupFD = false
+		// Must not reuse exec.Cmd.
+		p.cmd = cmdCopy
 		err = p.startWithCPUAffinity()
 	}