cmd/link: disallow pull-only linknames

As mentioned in CL 584598, linkname is a mechanism that, when
abused, can break API integrity and even safety of Go programs.
CL 584598 is a first step to restrict the use of linknames, by
implementing a blocklist. This CL takes a step further, tightening
up the restriction by allowing linkname references ("pull") only
when the definition side explicitly opts into it, by having a
linkname on the definition (possibly to itself). This way, it is at
least clear on the definition side that the symbol, despite being
unexported, is accessed outside of the package. Unexported symbols
without linkname can now be actually private. This is similar to
the symbol visibility rule used by gccgo for years (which defines
unexported non-linknamed symbols as C static symbols).

As there can be pull-only linknames in the wild that may be broken
by this change, we currently only enforce this rule for symbols
defined in the standard library. Push linknames are added in the
standard library to allow things build.

Linkname references to external (non-Go) symbols are still allowed,
as their visibility is controlled by the C symbol visibility rules
and enforced by the C (static or dynamic) linker.

Assembly symbols are treated similar to linknamed symbols.

This is controlled by -checklinkname linker flag, currently not
enabled by default. A follow-up CL will enable it by default.

Change-Id: I07344f5c7a02124dbbef0fbc8fec3b666a4b2b0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/585358
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/testing/newcover.go b/src/testing/newcover.go
index 6199f3bd7b..7a70dcfffa 100644
--- a/src/testing/newcover.go
+++ b/src/testing/newcover.go
@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"internal/goexperiment"
 	"os"
+	_ "unsafe" // for linkname
 )
 
 // cover2 variable stores the current coverage mode and a
@@ -20,6 +21,9 @@ var cover2 struct {
 	snapshotcov func() float64
 }
 
+// registerCover2 is injected in testmain.
+//go:linkname registerCover2
+
 // registerCover2 is invoked during "go test -cover" runs by the test harness
 // code in _testmain.go; it is used to record a 'tear down' function
 // (to be called when the test is complete) and the coverage mode.
@@ -42,6 +46,9 @@ func coverReport2() {
 	}
 }
 
+// testGoCoverDir is used in runtime/coverage tests.
+//go:linkname testGoCoverDir
+
 // testGoCoverDir returns the value passed to the -test.gocoverdir
 // flag by the Go command, if goexperiment.CoverageRedesign is
 // in effect.