4 files changed, 61 insertions, 1 deletions
diff --git a/doc/go1.15.html b/doc/go1.15.html
index 977c2815ac..af0b3ba6ac 100644
--- a/doc/go1.15.html
+++ b/doc/go1.15.html
@@ -351,6 +351,28 @@ TODO
     </p>
 </dl><!-- sync -->
 
+<dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
+  <dd>
+    <p><!-- CL 231638 -->
+      On Unix systems, functions that use
+      <a href="/pkg/syscall/#SysProcAttr"><code>SysProcAttr</code></a>
+      will now reject attempts to set both the <code>Setctty</code>
+      and <code>Foreground</code> fields, as they both use
+      the <code>Ctty</code> field but do so in incompatible ways.
+      We expect that few existing programs set both fields.
+    </p>
+    <p>
+      Setting the <code>Setctty</code> field now requires that the
+      <code>Ctty</code> field be set to a file descriptor number in the
+      child process, as determined by the <code>ProcAttr.Files</code> field.
+      Using a child descriptor always worked, but there were certain
+      cases where using a parent file descriptor also happened to work.
+      Some programs that set <code>Setctty</code> will need to change
+      the value of <code>Ctty</code> to use a child descriptor number.
+    </p>
+  </dd>
+</dl>
+
 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
   <dd>
     <p><!-- CL 226877, golang.org/issue/35998 -->
diff --git a/src/os/signal/signal_cgo_test.go b/src/os/signal/signal_cgo_test.go
index 3c23090489..849a96ec0e 100644
--- a/src/os/signal/signal_cgo_test.go
+++ b/src/os/signal/signal_cgo_test.go
@@ -98,7 +98,7 @@ func TestTerminalSignal(t *testing.T) {
 	cmd.SysProcAttr = &syscall.SysProcAttr{
 		Setsid:  true,
 		Setctty: true,
-		Ctty:    int(slave.Fd()),
+		Ctty:    0,
 	}
 
 	if err := cmd.Start(); err != nil {
diff --git a/src/syscall/exec_unix.go b/src/syscall/exec_unix.go
index b3798b6e04..0345af44f9 100644
--- a/src/syscall/exec_unix.go
+++ b/src/syscall/exec_unix.go
@@ -9,6 +9,7 @@
 package syscall
 
 import (
+	errorspkg "errors"
 	"internal/bytealg"
 	"runtime"
 	"sync"
@@ -187,6 +188,15 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 		}
 	}
 
+	// Both Setctty and Foreground use the Ctty field,
+	// but they give it slightly different meanings.
+	if sys.Setctty && sys.Foreground {
+		return 0, errorspkg.New("both Setctty and Foreground set in SysProcAttr")
+	}
+	if sys.Setctty && sys.Ctty >= len(attr.Files) {
+		return 0, errorspkg.New("Setctty set but Ctty not valid in child")
+	}
+
 	// Acquire the fork lock so that no other threads
 	// create new fds that are not yet close-on-exec
 	// before we fork.
diff --git a/src/syscall/exec_unix_test.go b/src/syscall/exec_unix_test.go
index 33614f5221..4eb3c5c6c8 100644
--- a/src/syscall/exec_unix_test.go
+++ b/src/syscall/exec_unix_test.go
@@ -213,3 +213,31 @@ func TestForeground(t *testing.T) {
 
 	signal.Reset()
 }
+
+// Test a couple of cases that SysProcAttr can't handle. Issue 29458.
+func TestInvalidExec(t *testing.T) {
+	t.Parallel()
+	t.Run("SetCtty-Foreground", func(t *testing.T) {
+		t.Parallel()
+		cmd := create(t)
+		cmd.proc.SysProcAttr = &syscall.SysProcAttr{
+			Setctty:    true,
+			Foreground: true,
+			Ctty:       0,
+		}
+		if err := cmd.proc.Start(); err == nil {
+			t.Error("expected error setting both SetCtty and Foreground")
+		}
+	})
+	t.Run("invalid-Ctty", func(t *testing.T) {
+		t.Parallel()
+		cmd := create(t)
+		cmd.proc.SysProcAttr = &syscall.SysProcAttr{
+			Setctty: true,
+			Ctty:    3,
+		}
+		if err := cmd.proc.Start(); err == nil {
+			t.Error("expected error with invalid Ctty value")
+		}
+	})
+}