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>

3 files changed, 10 insertions, 0 deletions

diff --git a/src/cmd/internal/goobj/objfile.go b/src/cmd/internal/goobj/objfile.go
index cca3c840e0..679f4357e2 100644
--- a/src/cmd/internal/goobj/objfile.go
+++ b/src/cmd/internal/goobj/objfile.go
@@ -306,6 +306,7 @@ const (
 	SymFlagDict
 	SymFlagPkgInit
 	SymFlagLinkname
+	SymFlagLinknameStd
 	SymFlagABIWrapper
 	SymFlagWasmExport
 )
@@ -340,6 +341,7 @@ func (s *Sym) IsItab() bool        { return s.Flag2()&SymFlagItab != 0 }
 func (s *Sym) IsDict() bool        { return s.Flag2()&SymFlagDict != 0 }
 func (s *Sym) IsPkgInit() bool     { return s.Flag2()&SymFlagPkgInit != 0 }
 func (s *Sym) IsLinkname() bool    { return s.Flag2()&SymFlagLinkname != 0 }
+func (s *Sym) IsLinknameStd() bool { return s.Flag2()&SymFlagLinknameStd != 0 }
 func (s *Sym) ABIWrapper() bool    { return s.Flag2()&SymFlagABIWrapper != 0 }
 func (s *Sym) WasmExport() bool    { return s.Flag2()&SymFlagWasmExport != 0 }
 
diff --git a/src/cmd/internal/obj/link.go b/src/cmd/internal/obj/link.go
index 088a7c57aa..69c9e7583b 100644
--- a/src/cmd/internal/obj/link.go
+++ b/src/cmd/internal/obj/link.go
@@ -977,6 +977,9 @@ const (
 	// Linkname indicates this is a go:linkname'd symbol.
 	AttrLinkname
 
+	// LinknameStd indicates this is a go:linknamestd'd symbol.
+	AttrLinknameStd
+
 	// attrABIBase is the value at which the ABI is encoded in
 	// Attribute. This must be last; all bits after this are
 	// assumed to be an ABI value.
@@ -1007,6 +1010,7 @@ func (a *Attribute) ABIWrapper() bool         { return a.load()&AttrABIWrapper !
 func (a *Attribute) IsPcdata() bool           { return a.load()&AttrPcdata != 0 }
 func (a *Attribute) IsPkgInit() bool          { return a.load()&AttrPkgInit != 0 }
 func (a *Attribute) IsLinkname() bool         { return a.load()&AttrLinkname != 0 }
+func (a *Attribute) IsLinknameStd() bool      { return a.load()&AttrLinknameStd != 0 }
 
 func (a *Attribute) Set(flag Attribute, value bool) {
 	for {
@@ -1057,6 +1061,7 @@ var textAttrStrings = [...]struct {
 	{bit: AttrABIWrapper, s: "ABIWRAPPER"},
 	{bit: AttrPkgInit, s: "PKGINIT"},
 	{bit: AttrLinkname, s: "LINKNAME"},
+	{bit: AttrLinknameStd, s: "LINKNAMESTD"},
 }
 
 // String formats a for printing in as part of a TEXT prog.
diff --git a/src/cmd/internal/obj/objfile.go b/src/cmd/internal/obj/objfile.go
index ea1483de01..de5f172b11 100644
--- a/src/cmd/internal/obj/objfile.go
+++ b/src/cmd/internal/obj/objfile.go
@@ -360,6 +360,9 @@ func (w *writer) Sym(s *LSym) {
 		// The runtime linknames main.main.
 		flag2 |= goobj.SymFlagLinkname
 	}
+	if s.IsLinknameStd() {
+		flag2 |= goobj.SymFlagLinknameStd
+	}
 	if s.ABIWrapper() {
 		flag2 |= goobj.SymFlagABIWrapper
 	}