diff --git a/misc/cgo/test/issue7978.go b/misc/cgo/test/issue7978.go
index e4cbf1d926..7fb62e807b 100644
--- a/misc/cgo/test/issue7978.go
+++ b/misc/cgo/test/issue7978.go
@@ -88,9 +88,20 @@ func issue7978wait(store uint32, wait uint32) {
 
 //export issue7978cb
 func issue7978cb() {
+	// Force a stack growth from the callback to put extra
+	// pressure on the runtime. See issue #17785.
+	growStack(64)
 	issue7978wait(3, 4)
 }
 
+func growStack(n int) int {
+	var buf [128]int
+	if n == 0 {
+		return 0
+	}
+	return buf[growStack(n-1)]
+}
+
 func issue7978go() {
 	C.issue7978c((*C.uint32_t)(&issue7978sync))
 	issue7978wait(7, 8)
diff --git a/src/runtime/cgocall.go b/src/runtime/cgocall.go
index dc4a9a9820..007406b426 100644
--- a/src/runtime/cgocall.go
+++ b/src/runtime/cgocall.go
@@ -120,13 +120,31 @@ func cgocall(fn, arg unsafe.Pointer) int32 {
 	// foreign code.
 	//
 	// The call to asmcgocall is guaranteed not to
-	// split the stack and does not allocate memory,
+	// grow the stack and does not allocate memory,
 	// so it is safe to call while "in a system call", outside
 	// the $GOMAXPROCS accounting.
+	//
+	// fn may call back into Go code, in which case we'll exit the
+	// "system call", run the Go code (which may grow the stack),
+	// and then re-enter the "system call" reusing the PC and SP
+	// saved by entersyscall here.
 	entersyscall(0)
 	errno := asmcgocall(fn, arg)
 	exitsyscall(0)
 
+	// From the garbage collector's perspective, time can move
+	// backwards in the sequence above. If there's a callback into
+	// Go code, GC will see this function at the call to
+	// asmcgocall. When the Go call later returns to C, the
+	// syscall PC/SP is rolled back and the GC sees this function
+	// back at the call to entersyscall. Normally, fn and arg
+	// would be live at entersyscall and dead at asmcgocall, so if
+	// time moved backwards, GC would see these arguments as dead
+	// and then live. Prevent these undead arguments from crashing
+	// GC by forcing them to stay live across this time warp.
+	KeepAlive(fn)
+	KeepAlive(arg)
+
 	endcgo(mp)
 	return errno
 }