cmd/compile, cmd/link: add linknamestd directive for std-only linknames

In the standard library, there are a number of linknames, for
sharing symbols within the standard library. They are not supposed
to be accessed externally. But currently there is no good
mechanism to prevent that. In the linker we have a blocklist of
linknames, which forbids linkname references other than explicitly
allowed packages. The blocklist is manually maintained, requiring
periodic manual update.

To move away from that manually maintained blocklist, this CL
introduces a new directive, linknamestd, that marks a linkname
for use within the standard library only. The linker will allow
references within the standard library and forbid others.

For a proof of concept, runtime.coroswitch is removed from the
blocklist, and replaced with linknamestd. An external reference to
it is still disallowed by the linker, as tested with
cmd/link.TestCheckLinkname with testdata/linkname/coro.go.

Change-Id: I0d0f8746b8835d8cdcfc3ff835d22a551da5f038
Reviewed-on: https://go-review.googlesource.com/c/go/+/749942
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 5 insertions, 4 deletions

diff --git a/src/cmd/compile/internal/noder/noder.go b/src/cmd/compile/internal/noder/noder.go
index 973d917784..4e48126e82 100644
--- a/src/cmd/compile/internal/noder/noder.go
+++ b/src/cmd/compile/internal/noder/noder.go
@@ -103,9 +103,10 @@ type noder struct {
 	err        chan syntax.Error
 }
 
-// linkname records a //go:linkname directive.
+// linkname records a //go:linkname or //go:linknamestd directive.
 type linkname struct {
 	pos    syntax.Pos
+	std    bool
 	local  string
 	remote string
 }
@@ -273,10 +274,10 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 			}
 		}
 
-	case strings.HasPrefix(text, "go:linkname "):
+	case strings.HasPrefix(text, "go:linkname "), strings.HasPrefix(text, "go:linknamestd "):
 		f := strings.Fields(text)
 		if !(2 <= len(f) && len(f) <= 3) {
-			p.error(syntax.Error{Pos: pos, Msg: "usage: //go:linkname localname [linkname]"})
+			p.error(syntax.Error{Pos: pos, Msg: fmt.Sprintf("usage: //%s localname [linkname]", f[0])})
 			break
 		}
 		// The second argument is optional. If omitted, we use
@@ -294,7 +295,7 @@ func (p *noder) pragma(pos syntax.Pos, blankLine bool, text string, old syntax.P
 		} else {
 			panic("missing pkgpath")
 		}
-		p.linknames = append(p.linknames, linkname{pos, f[1], target})
+		p.linknames = append(p.linknames, linkname{pos, f[0] == "go:linknamestd", f[1], target})
 
 	case text == "go:embed", strings.HasPrefix(text, "go:embed "):
 		args, err := parseGoEmbed(text[len("go:embed"):])