9 files changed, 93 insertions, 2 deletions
diff --git a/src/cmd/link/doc.go b/src/cmd/link/doc.go
index 219499be0a..604675caec 100644
--- a/src/cmd/link/doc.go
+++ b/src/cmd/link/doc.go
@@ -3,7 +3,7 @@
 // license that can be found in the LICENSE file.
 
 /*
-Link, typically invoked as ``go tool link,'' reads the Go archive or object
+Link, typically invoked as ``go tool link'', reads the Go archive or object
 for a package main, along with its dependencies, and combines them
 into an executable binary.
 
diff --git a/src/cmd/link/internal/arm/asm.go b/src/cmd/link/internal/arm/asm.go
index 22bcb518df..611c96ce35 100644
--- a/src/cmd/link/internal/arm/asm.go
+++ b/src/cmd/link/internal/arm/asm.go
@@ -220,7 +220,7 @@ func adddynrel(target *ld.Target, ldr *loader.Loader, syms *ld.ArchSyms, s loade
 		addpltsym(target, ldr, syms, targ)
 		su := ldr.MakeSymbolUpdater(s)
 		su.SetRelocSym(rIdx, syms.PLT)
-		su.SetRelocAdd(rIdx, int64(ldr.SymPlt(targ)))
+		su.SetRelocAdd(rIdx, int64(braddoff(int32(r.Add()), ldr.SymPlt(targ)/4))) // TODO: don't use r.Add for instruction bytes (issue 19811)
 		return true
 
 	case objabi.R_ADDR:
diff --git a/src/cmd/link/internal/benchmark/bench_test.go b/src/cmd/link/internal/benchmark/bench_test.go
index d8ec717c7c..419dc55724 100644
--- a/src/cmd/link/internal/benchmark/bench_test.go
+++ b/src/cmd/link/internal/benchmark/bench_test.go
@@ -1,6 +1,7 @@
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package benchmark
 
 import (
diff --git a/src/cmd/link/internal/ld/elf.go b/src/cmd/link/internal/ld/elf.go
index 957f5081f6..2862f65f9f 100644
--- a/src/cmd/link/internal/ld/elf.go
+++ b/src/cmd/link/internal/ld/elf.go
@@ -2378,6 +2378,7 @@ func elfadddynsym(ldr *loader.Loader, target *Target, syms *ArchSyms, s loader.S
 		if target.Arch.Family == sys.AMD64 && !cgoeDynamic && dil != "" && !seenlib[dil] {
 			du := ldr.MakeSymbolUpdater(syms.Dynamic)
 			Elfwritedynent(target.Arch, du, DT_NEEDED, uint64(dstru.Addstring(dil)))
+			seenlib[dil] = true
 		}
 	} else {
 
diff --git a/src/cmd/link/internal/ld/elf_test.go b/src/cmd/link/internal/ld/elf_test.go
index 8e86beb1ec..37f0e77336 100644
--- a/src/cmd/link/internal/ld/elf_test.go
+++ b/src/cmd/link/internal/ld/elf_test.go
@@ -13,6 +13,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"runtime"
 	"testing"
 )
 
@@ -77,3 +78,57 @@ func main() {
 		t.Fatalf("Unexpected sh info, want greater than 0, got: %d", section.Info)
 	}
 }
+
+func TestNoDuplicateNeededEntries(t *testing.T) {
+	testenv.MustHaveGoBuild(t)
+	testenv.MustHaveCGO(t)
+
+	// run this test on just a small set of platforms (no need to test it
+	// across the board given the nature of the test).
+	pair := runtime.GOOS + "-" + runtime.GOARCH
+	switch pair {
+	case "linux-amd64", "freebsd-amd64", "openbsd-amd64":
+	default:
+		t.Skip("no need for test on " + pair)
+	}
+
+	t.Parallel()
+
+	dir, err := ioutil.TempDir("", "no-dup-needed")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(dir)
+
+	wd, err := os.Getwd()
+	if err != nil {
+		t.Fatalf("Failed to get working directory: %v", err)
+	}
+
+	path := filepath.Join(dir, "x")
+	argv := []string{"build", "-o", path, filepath.Join(wd, "testdata", "issue39256")}
+	out, err := exec.Command(testenv.GoToolPath(t), argv...).CombinedOutput()
+	if err != nil {
+		t.Fatalf("Build failure: %s\n%s\n", err, string(out))
+	}
+
+	f, err := elf.Open(path)
+	if err != nil {
+		t.Fatalf("Failed to open ELF file: %v", err)
+	}
+	libs, err := f.ImportedLibraries()
+	if err != nil {
+		t.Fatalf("Failed to read imported libraries: %v", err)
+	}
+
+	var count int
+	for _, lib := range libs {
+		if lib == "libc.so" {
+			count++
+		}
+	}
+
+	if got, want := count, 1; got != want {
+		t.Errorf("Got %d entries for `libc.so`, want %d", got, want)
+	}
+}
diff --git a/src/cmd/link/internal/ld/errors.go b/src/cmd/link/internal/ld/errors.go
index c5ce097fde..d6e8ff236d 100644
--- a/src/cmd/link/internal/ld/errors.go
+++ b/src/cmd/link/internal/ld/errors.go
@@ -1,6 +1,7 @@
 // Copyright 2020 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
+
 package ld
 
 import (
diff --git a/src/cmd/link/internal/ld/go.go b/src/cmd/link/internal/ld/go.go
index bf5c9ca1ba..b3541c46c0 100644
--- a/src/cmd/link/internal/ld/go.go
+++ b/src/cmd/link/internal/ld/go.go
@@ -183,6 +183,9 @@ func setCgoAttr(ctxt *Link, lookup func(string, int) loader.Sym, file string, pk
 					hostObjSyms[s] = struct{}{}
 				}
 				havedynamic = 1
+				if lib != "" && ctxt.IsDarwin() {
+					machoadddynlib(lib, ctxt.LinkMode)
+				}
 			}
 
 			continue
diff --git a/src/cmd/link/internal/ld/testdata/issue39256/x.go b/src/cmd/link/internal/ld/testdata/issue39256/x.go
new file mode 100644
index 0000000000..d8562ad172
--- /dev/null
+++ b/src/cmd/link/internal/ld/testdata/issue39256/x.go
@@ -0,0 +1,20 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	_ "unsafe"
+)
+
+//go:cgo_import_dynamic libc_getpid getpid "libc.so"
+//go:cgo_import_dynamic libc_kill kill "libc.so"
+//go:cgo_import_dynamic libc_close close "libc.so"
+//go:cgo_import_dynamic libc_open open "libc.so"
+
+func trampoline()
+
+func main() {
+	trampoline()
+}
diff --git a/src/cmd/link/internal/ld/testdata/issue39256/x.s b/src/cmd/link/internal/ld/testdata/issue39256/x.s
new file mode 100644
index 0000000000..41a54b2e04
--- /dev/null
+++ b/src/cmd/link/internal/ld/testdata/issue39256/x.s
@@ -0,0 +1,10 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+TEXT ·trampoline(SB),0,$0
+	CALL	libc_getpid(SB)
+	CALL	libc_kill(SB)
+	CALL	libc_open(SB)
+	CALL	libc_close(SB)
+	RET