os: add clone(CLONE_PIDFD) check to pidfd feature check

clone(CLONE_PIDFD) was added in Linux 5.2 and pidfd_open was added in
Linux 5.3. Thus our feature check for pidfd_open should be sufficient to
ensure that clone(CLONE_PIDFD) works.

Unfortuantely, some alternative Linux implementations may not follow
this strict ordering. For example, QEMU 7.2 (Dec 2022) added pidfd_open,
but clone(CLONE_PIDFD) was only added in QEMU 8.0 (Apr 2023).

Debian bookworm provides QEMU 7.2 by default.

Fixes golang#69259.

Change-Id: Ie3f3dc51f0cd76944871bf98690abf59f68fd7bf
Reviewed-on: https://go-review.googlesource.com/c/go/+/592078
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/os/pidfd_linux.go

@@ -8,6 +8,10 @@
 //  v5.3: pidfd_open syscall, clone3 syscall;
 //  v5.4: P_PIDFD idtype support for waitid syscall;
 //  v5.6: pidfd_getfd syscall.
+//
+// N.B. Alternative Linux implementations may not follow this ordering. e.g.,
+// QEMU user mode 7.2 added pidfd_open, but CLONE_PIDFD was not added until
+// 8.0.
 
 package os
 
@@ -139,9 +143,9 @@ func pidfdWorks() bool {
 
 var checkPidfdOnce = sync.OnceValue(checkPidfd)
 
-// checkPidfd checks whether all required pidfd-related syscalls work.
-// This consists of pidfd_open and pidfd_send_signal syscalls, and waitid
-// syscall with idtype of P_PIDFD.
+// checkPidfd checks whether all required pidfd-related syscalls work. This
+// consists of pidfd_open and pidfd_send_signal syscalls, waitid syscall with
+// idtype of P_PIDFD, and clone(CLONE_PIDFD).
 //
 // Reasons for non-working pidfd syscalls include an older kernel and an
 // execution environment in which the above system calls are restricted by
@@ -172,5 +176,19 @@ func checkPidfd() error {
 		return NewSyscallError("pidfd_send_signal", err)
 	}
 
+	// Verify that clone(CLONE_PIDFD) works.
+	//
+	// This shouldn't be necessary since pidfd_open was added in Linux 5.3,
+	// after CLONE_PIDFD in Linux 5.2, but some alternative Linux
+	// implementations may not adhere to this ordering.
+	if err := checkClonePidfd(); err != nil {
+		return err
+	}
+
 	return nil
 }
+
+// Provided by syscall.
+//
+//go:linkname checkClonePidfd
+func checkClonePidfd() error

src/syscall/exec_linux.go

@@ -7,6 +7,7 @@
 package syscall
 
 import (
+	errpkg "errors"
 	"internal/itoa"
 	"runtime"
 	"unsafe"
@@ -330,6 +331,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 	if clone3 != nil {
 		pid, err1 = rawVforkSyscall(_SYS_clone3, uintptr(unsafe.Pointer(clone3)), unsafe.Sizeof(*clone3), 0)
 	} else {
+		// N.B. Keep in sync with doCheckClonePidfd.
 		flags |= uintptr(SIGCHLD)
 		if runtime.GOARCH == "s390x" {
 			// On Linux/s390, the first two arguments of clone(2) are swapped.
@@ -758,3 +760,82 @@ func forkAndExecFailureCleanup(attr *ProcAttr, sys *SysProcAttr) {
 		*sys.PidFD = -1
 	}
 }
+
+// checkClonePidfd verifies that clone(CLONE_PIDFD) works by actually doing a
+// clone.
+//
+//go:linkname os_checkClonePidfd os.checkClonePidfd
+func os_checkClonePidfd() error {
+	pidfd := int32(-1)
+	pid, errno := doCheckClonePidfd(&pidfd)
+	if errno != 0 {
+		return errno
+	}
+
+	if pidfd == -1 {
+		// Bad: CLONE_PIDFD failed to provide a pidfd. Reap the process
+		// before returning.
+
+		var err error
+		for {
+			var status WaitStatus
+			_, err = Wait4(int(pid), &status, 0, nil)
+			if err != EINTR {
+				break
+			}
+		}
+		if err != nil {
+			return err
+		}
+
+		return errpkg.New("clone(CLONE_PIDFD) failed to return pidfd")
+	}
+
+	// Good: CLONE_PIDFD provided a pidfd. Reap the process and close the
+	// pidfd.
+	defer Close(int(pidfd))
+
+	for {
+		const _P_PIDFD = 3
+		_, _, errno = Syscall6(SYS_WAITID, _P_PIDFD, uintptr(pidfd), 0, WEXITED, 0, 0)
+		if errno != EINTR {
+			break
+		}
+	}
+	if errno != 0 {
+		return errno
+	}
+
+	return nil
+}
+
+// doCheckClonePidfd implements the actual clone call of os_checkClonePidfd and
+// child execution. This is a separate function so we can separate the child's
+// and parent's stack frames if we're using vfork.
+//
+// This is go:noinline because the point is to keep the stack frames of this
+// and os_checkClonePidfd separate.
+//
+//go:noinline
+func doCheckClonePidfd(pidfd *int32) (pid uintptr, errno Errno) {
+	flags := uintptr(CLONE_VFORK|CLONE_VM|CLONE_PIDFD|SIGCHLD)
+	if runtime.GOARCH == "s390x" {
+		// On Linux/s390, the first two arguments of clone(2) are swapped.
+		pid, errno = rawVforkSyscall(SYS_CLONE, 0, flags, uintptr(unsafe.Pointer(pidfd)))
+	} else {
+		pid, errno = rawVforkSyscall(SYS_CLONE, flags, 0, uintptr(unsafe.Pointer(pidfd)))
+	}
+	if errno != 0 || pid != 0 {
+		// If we're in the parent, we must return immediately
+		// so we're not in the same stack frame as the child.
+		// This can at most use the return PC, which the child
+		// will not modify, and the results of
+		// rawVforkSyscall, which must have been written after
+		// the child was replaced.
+		return
+	}
+
+	for {
+		RawSyscall(SYS_EXIT, 0, 0, 0)
+	}
+}