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, 6 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/noder/reader.go b/src/cmd/compile/internal/noder/reader.go
index 00710775d6..ebb5043a05 100644
--- a/src/cmd/compile/internal/noder/reader.go
+++ b/src/cmd/compile/internal/noder/reader.go
@@ -1277,6 +1277,7 @@ func (r *reader) linkname(name *ir.Name) {
 		lsym.Set(obj.AttrIndexed, true)
 	} else {
 		linkname := r.String()
+		std := r.Bool()
 		sym := name.Sym()
 		sym.Linkname = linkname
 		if sym.Pkg == types.LocalPkg && linkname != "" {
@@ -1286,7 +1287,11 @@ func (r *reader) linkname(name *ir.Name) {
 			// corresponding packages). So we can tell in which package
 			// the linkname is used (pulled), and the linker can
 			// make a decision for allowing or disallowing it.
-			sym.Linksym().Set(obj.AttrLinkname, true)
+			if std {
+				sym.Linksym().Set(obj.AttrLinknameStd, true)
+			} else {
+				sym.Linksym().Set(obj.AttrLinkname, true)
+			}
 		}
 	}
 }