cmd/asm, cmd/compile, runtime: add -spectre=ret mode

This commit extends the -spectre flag to cmd/asm and adds
a new Spectre mitigation mode "ret", which enables the use
of retpolines.

Retpolines prevent speculation about the target of an indirect
jump or call and are described in more detail here:
https://support.google.com/faqs/answer/7625886

Change-Id: I4f2cb982fa94e44d91e49bd98974fd125619c93a
Reviewed-on: https://go-review.googlesource.com/c/go/+/222661
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/src/runtime/stubs_amd64.go b/src/runtime/stubs_amd64.go
index b4c0df1153..5b79d66762 100644
--- a/src/runtime/stubs_amd64.go
+++ b/src/runtime/stubs_amd64.go
@@ -9,3 +9,20 @@ func stackcheck()
 
 // Called from assembly only; declared for go vet.
 func settls() // argument in DI
+
+// Retpolines, used by -spectre=ret flag in cmd/asm, cmd/compile.
+func retpolineAX()
+func retpolineCX()
+func retpolineDX()
+func retpolineBX()
+func retpolineBP()
+func retpolineSI()
+func retpolineDI()
+func retpolineR8()
+func retpolineR9()
+func retpolineR10()
+func retpolineR11()
+func retpolineR12()
+func retpolineR13()
+func retpolineR14()
+func retpolineR15()