[release-branch.go1.14] cmd/link: fix GC data reading from shared library (attempt 2)

This is a backport of CL 240621. This is not a clean cherry-pick,
as Go 1.15 switches to the new linker while it is still the old
linker here. Backporting is straightforward, though.

When linking against a Go shared library, when a global variable
in the main module has a type defined in the shared library, the
linker needs to pull the GC data from the shared library to build
the GC program for the global variable. Currently, this fails
silently, as the shared library file is closed too early and the
read failed (with no error check), causing a zero GC map emitted
for the variable, which in turn causes the runtime to treat the
variable as pointerless.

For now, fix this by keeping the file open. In the future we may
want to use mmap to read from the shared library instead.

Also add error checking. And fix a (mostly harmless) mistake in
size caluculation.

Also remove an erroneous condition for ARM64. ARM64 has a special
case to get the addend from the relocation on the gcdata field.
But that doesn't actually work. And it's no longer necessary to
have any special case, since the addend is now applied directly
to the gcdata field on ARM64, like on all the other platforms.

Fixes #39955.
Updates #39927.

Change-Id: I01c82422b9f67e872d833336885935bc509bc91b
Reviewed-on: https://go-review.googlesource.com/c/go/+/240621
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
(cherry picked from commit 7799756a50f0a4070d66c67e9615375f852f2c04)
Reviewed-on: https://go-review.googlesource.com/c/go/+/240511
Reviewed-by: Austin Clements <austin@google.com>

5 files changed, 82 insertions, 14 deletions

diff --git a/misc/cgo/testshared/shared_test.go b/misc/cgo/testshared/shared_test.go
index b9ef6dad8e..f6cefa3516 100644
--- a/misc/cgo/testshared/shared_test.go
+++ b/misc/cgo/testshared/shared_test.go
@@ -38,7 +38,15 @@ var testWork = flag.Bool("testwork", false, "if true, log and do not delete the
 
 // run runs a command and calls t.Errorf if it fails.
 func run(t *testing.T, msg string, args ...string) {
+	runWithEnv(t, msg, nil, args...)
+}
+
+// runWithEnv runs a command under the given environment and calls t.Errorf if it fails.
+func runWithEnv(t *testing.T, msg string, env []string, args ...string) {
 	c := exec.Command(args[0], args[1:]...)
+	if len(env) != 0 {
+		c.Env = append(os.Environ(), env...)
+	}
 	if output, err := c.CombinedOutput(); err != nil {
 		t.Errorf("executing %s (%s) failed %s:\n%s", strings.Join(args, " "), msg, err, output)
 	}
@@ -1030,3 +1038,11 @@ func TestGeneratedHash(t *testing.T) {
 	goCmd(nil, "install", "-buildmode=shared", "-linkshared", "./issue30768/issue30768lib")
 	goCmd(nil, "test", "-linkshared", "./issue30768")
 }
+
+// Test that GC data are generated correctly by the linker when it needs a type defined in
+// a shared library. See issue 39927.
+func TestGCData(t *testing.T) {
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "./gcdata/p")
+	goCmd(t, "build", "-linkshared", "./gcdata/main")
+	runWithEnv(t, "running gcdata/main", []string{"GODEBUG=clobberfree=1"}, "./main")
+}
diff --git a/misc/cgo/testshared/testdata/gcdata/main/main.go b/misc/cgo/testshared/testdata/gcdata/main/main.go
new file mode 100644
index 0000000000..394862fd94
--- /dev/null
+++ b/misc/cgo/testshared/testdata/gcdata/main/main.go
@@ -0,0 +1,37 @@
+// 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.
+
+// Test that GC data is generated correctly for global
+// variables with types defined in a shared library.
+// See issue 39927.
+
+// This test run under GODEBUG=clobberfree=1. The check
+// *x[i] == 12345 depends on this debug mode to clobber
+// the value if the object is freed prematurely.
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+	"testshared/gcdata/p"
+)
+
+var x p.T
+
+func main() {
+	for i := range x {
+		x[i] = new(int)
+		*x[i] = 12345
+	}
+	runtime.GC()
+	runtime.GC()
+	runtime.GC()
+	for i := range x {
+		if *x[i] != 12345 {
+			fmt.Printf("x[%d] == %d, want 12345\n", i, *x[i])
+			panic("FAIL")
+		}
+	}
+}
diff --git a/misc/cgo/testshared/testdata/gcdata/p/p.go b/misc/cgo/testshared/testdata/gcdata/p/p.go
new file mode 100644
index 0000000000..1fee75429e
--- /dev/null
+++ b/misc/cgo/testshared/testdata/gcdata/p/p.go
@@ -0,0 +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 p
+
+type T [10]*int
diff --git a/src/cmd/link/internal/ld/decodesym.go b/src/cmd/link/internal/ld/decodesym.go
index 3271c85157..88176a3643 100644
--- a/src/cmd/link/internal/ld/decodesym.go
+++ b/src/cmd/link/internal/ld/decodesym.go
@@ -11,6 +11,7 @@ import (
 	"cmd/link/internal/sym"
 	"debug/elf"
 	"fmt"
+	"log"
 )
 
 // Decoding the type.* symbols.	 This has to be in sync with
@@ -93,7 +94,7 @@ func decodetypeHasUncommon(arch *sys.Arch, p []byte) bool {
 func findShlibSection(ctxt *Link, path string, addr uint64) *elf.Section {
 	for _, shlib := range ctxt.Shlibs {
 		if shlib.Path == path {
-			for _, sect := range shlib.File.Sections {
+			for _, sect := range shlib.File.Sections[1:] { // skip the NULL section
 				if sect.Addr <= addr && addr <= sect.Addr+sect.Size {
 					return sect
 				}
@@ -112,9 +113,15 @@ func decodetypeGcprog(ctxt *Link, s *sym.Symbol) []byte {
 			// A gcprog is a 4-byte uint32 indicating length, followed by
 			// the actual program.
 			progsize := make([]byte, 4)
-			sect.ReadAt(progsize, int64(addr-sect.Addr))
+			_, err := sect.ReadAt(progsize, int64(addr-sect.Addr))
+			if err != nil {
+				log.Fatal(err)
+			}
 			progbytes := make([]byte, ctxt.Arch.ByteOrder.Uint32(progsize))
-			sect.ReadAt(progbytes, int64(addr-sect.Addr+4))
+			_, err = sect.ReadAt(progbytes, int64(addr-sect.Addr+4))
+			if err != nil {
+				log.Fatal(err)
+			}
 			return append(progsize, progbytes...)
 		}
 		Exitf("cannot find gcprog for %s", s.Name)
@@ -124,14 +131,6 @@ func decodetypeGcprog(ctxt *Link, s *sym.Symbol) []byte {
 }
 
 func decodetypeGcprogShlib(ctxt *Link, s *sym.Symbol) uint64 {
-	if ctxt.Arch.Family == sys.ARM64 {
-		for _, shlib := range ctxt.Shlibs {
-			if shlib.Path == s.File {
-				return shlib.gcdataAddresses[s]
-			}
-		}
-		return 0
-	}
 	return decodeInuxi(ctxt.Arch, s.P[2*int32(ctxt.Arch.PtrSize)+8+1*int32(ctxt.Arch.PtrSize):], ctxt.Arch.PtrSize)
 }
 
@@ -141,8 +140,15 @@ func decodetypeGcmask(ctxt *Link, s *sym.Symbol) []byte {
 		ptrdata := decodetypePtrdata(ctxt.Arch, s.P)
 		sect := findShlibSection(ctxt, s.File, addr)
 		if sect != nil {
-			r := make([]byte, ptrdata/int64(ctxt.Arch.PtrSize))
-			sect.ReadAt(r, int64(addr-sect.Addr))
+			bits := ptrdata / int64(ctxt.Arch.PtrSize)
+			r := make([]byte, (bits+7)/8)
+			// ldshlibsyms avoids closing the ELF file so sect.ReadAt works.
+			// If we remove this read (and the ones in decodetypeGcprog), we
+			// can close the file.
+			_, err := sect.ReadAt(r, int64(addr-sect.Addr))
+			if err != nil {
+				log.Fatal(err)
+			}
 			return r
 		}
 		Exitf("cannot find gcmask for %s", s.Name)
diff --git a/src/cmd/link/internal/ld/lib.go b/src/cmd/link/internal/ld/lib.go
index 45cc87287c..cd63963d7f 100644
--- a/src/cmd/link/internal/ld/lib.go
+++ b/src/cmd/link/internal/ld/lib.go
@@ -2000,7 +2000,9 @@ func ldshlibsyms(ctxt *Link, shlib string) {
 		Errorf(nil, "cannot open shared library: %s", libpath)
 		return
 	}
-	defer f.Close()
+	// Keep the file open as decodetypeGcprog needs to read from it.
+	// TODO: fix. Maybe mmap the file.
+	//defer f.Close()
 
 	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
 	if err != nil {