syscall: check recorded version of ppid instead of 1

Handles the case where the parent is pid 1 (common in docker
containers).

Attempted and failed to write a test for this.

Fixes #9263.

Change-Id: I5c6036446c99e66259a4fab1660b6a594f875020
Reviewed-on: https://go-review.googlesource.com/1372Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/syscall/exec_linux.go

@@ -58,6 +58,9 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		p      [2]int
 	)
 
+	// Record parent PID so child can test if it has died.
+	ppid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
+
 	// Guard against side effects of shuffling fds below.
 	// Make sure that nextfd is beyond any currently open files so
 	// that we can't run the risk of overwriting any of them.
@@ -138,7 +141,7 @@ func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr
 		// duplicate signal in rare cases, but it won't matter when
 		// using SIGKILL.
 		r1, _, _ = RawSyscall(SYS_GETPPID, 0, 0, 0)
-		if r1 == 1 {
+		if r1 != ppid {
 			pid, _, _ := RawSyscall(SYS_GETPID, 0, 0, 0)
 			_, _, err1 := RawSyscall(SYS_KILL, pid, uintptr(sys.Pdeathsig), 0)
 			if err1 != 0 {