fix obfuscating linkname directives that where the package name contained a dot

main.go

@@ -727,6 +727,7 @@ func (tf *transformer) handleDirectives(comments []*ast.CommentGroup) {
 			}
 			fields := strings.Fields(comment.Text)
 			if len(fields) != 3 {
+				// TODO: the 2nd argument is optional, handle when it's not present
 				continue
 			}
 			// This directive has two arguments: "go:linkname localName newName"
@@ -737,11 +738,25 @@ func (tf *transformer) handleDirectives(comments []*ast.CommentGroup) {
 			// If the new name is of the form "pkgpath.Name", and
 			// we've obfuscated "Name" in that package, rewrite the
 			// directive to use the obfuscated name.
-			target := strings.Split(fields[2], ".")
-			if len(target) != 2 {
+			newName := fields[2]
+			dotCnt := strings.Count(newName, ".")
+			if dotCnt < 1 {
+				// probably a malformed linkname directive
 				continue
 			}
-			pkgPath, name := target[0], target[1]
+
+			// If the package path has multiple dots, split on the
+			// last one.
+			var pkgPath, name string
+			if dotCnt == 1 {
+				target := strings.Split(newName, ".")
+				pkgPath, name = target[0], target[1]
+			} else {
+				lastDotIdx := strings.LastIndex(newName, ".")
+				target := strings.Split(newName[lastDotIdx-1:], ".")
+				pkgPath, name = target[0], target[1]
+			}
+
 			if pkgPath == "runtime" && strings.HasPrefix(name, "cgo") {
 				continue // ignore cgo-generated linknames
 			}

testdata/scripts/linkname.txt

@@ -17,6 +17,12 @@ module test/main
 
 go 1.16
 
+replace big.chungus/meme => ./big.chungus/meme
+
+require (
+	big.chungus/meme v0.0.0
+)
+
 -- a.go --
 package main
 
@@ -36,6 +42,7 @@ import (
 	_ "strings"
 	_ "unsafe"
 
+	_ "big.chungus/meme"
 	"test/main/imported"
 )
 
@@ -55,11 +62,17 @@ func obfuscatedFunc() string
 //go:linkname renamedFunc madeup.newName
 func renamedFunc() string
 
+// A linkname to an external non-obfuscated func in another
+// module whose package path has a dot in it.
+//go:linkname tagline big.chungus/meme.chungify
+func tagline() string
+
 func main() {
 	println(byteIndex("01234", '3'))
 	println(interfaceEqual("Sephiroth", 7))
 	println(obfuscatedFunc())
 	println(renamedFunc())
+	println(tagline())
 	println(imported.ByteIndex("01234", '3'))
 	linknameCalledInPkg()
 }
@@ -83,10 +96,22 @@ func renamedFunc() string {
 // Different from byteIndex, as we call this from an importer package.
 //go:linkname ByteIndex strings.IndexByte
 func ByteIndex(s string, c byte) int
+-- big.chungus/meme/go.mod --
+module test/main
+
+go 1.16
+
+-- big.chungus/meme/dante.go --
+package meme
+
+func chungify() string {
+	return "featuring Dante from the Devil May Cry series"
+}
 -- main.stderr --
 3
 false
 obfuscated func
 renamed func
+featuring Dante from the Devil May Cry series
 3
 obfuscated func