[release-branch.go1.15] cmd/link: generate trampoline for inter-dependent packages

Currently, in the trampoline generation pass we expect packages
are laid out in dependency order, so a cross-package jump always
has a known target address so we can check if a trampoline is
needed. With linknames, there can be cycles in the package
dependency graph, making this algorithm no longer work. For them,
as the target address is unkown we conservatively generate a
trampoline. This may generate unnecessary trampolines (if the
packages turn out laid together), but package cycles are extremely
rare so this is fine.

Updates #44639.
Fixes #44748.

Change-Id: I2dc2998edacbda27d726fc79452313a21d07787a
Reviewed-on: https://go-review.googlesource.com/c/go/+/292490
Trust: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
(cherry picked from commit 098504c73ff6ece19566a1ac811ceed73be7c81d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/298030
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

3 files changed, 25 insertions, 15 deletions

diff --git a/src/cmd/link/internal/arm/asm.go b/src/cmd/link/internal/arm/asm.go
index 903e62108e..43643341d2 100644
--- a/src/cmd/link/internal/arm/asm.go
+++ b/src/cmd/link/internal/arm/asm.go
@@ -373,10 +373,16 @@ func trampoline(ctxt *ld.Link, ldr *loader.Loader, ri int, rs, s loader.Sym) {
 	r := relocs.At2(ri)
 	switch r.Type() {
 	case objabi.R_CALLARM:
-		// r.Add is the instruction
-		// low 24-bit encodes the target address
-		t := (ldr.SymValue(rs) + int64(signext24(r.Add()&0xffffff)*4) - (ldr.SymValue(s) + int64(r.Off()))) / 4
-		if t > 0x7fffff || t < -0x800000 || (*ld.FlagDebugTramp > 1 && ldr.SymPkg(s) != ldr.SymPkg(rs)) {
+		var t int64
+		// ldr.SymValue(rs) == 0 indicates a cross-package jump to a function that is not yet
+		// laid out. Conservatively use a trampoline. This should be rare, as we lay out packages
+		// in dependency order.
+		if ldr.SymValue(rs) != 0 {
+			// r.Add is the instruction
+			// low 24-bit encodes the target address
+			t = (ldr.SymValue(rs) + int64(signext24(r.Add()&0xffffff)*4) - (ldr.SymValue(s) + int64(r.Off()))) / 4
+		}
+		if t > 0x7fffff || t < -0x800000 || ldr.SymValue(rs) == 0 || (*ld.FlagDebugTramp > 1 && ldr.SymPkg(s) != ldr.SymPkg(rs)) {
 			// direct call too far, need to insert trampoline.
 			// look up existing trampolines first. if we found one within the range
 			// of direct call, we can reuse it. otherwise create a new one.
@@ -447,7 +453,7 @@ func gentramp(arch *sys.Arch, linkmode ld.LinkMode, ldr *loader.Loader, tramp *l
 	arch.ByteOrder.PutUint32(P[8:], o3)
 	tramp.SetData(P)
 
-	if linkmode == ld.LinkExternal {
+	if linkmode == ld.LinkExternal || ldr.SymValue(target) == 0 {
 		r := loader.Reloc{
 			Off:  8,
 			Type: objabi.R_ADDR,
diff --git a/src/cmd/link/internal/ld/data.go b/src/cmd/link/internal/ld/data.go
index d3f308c8fa..2b55a5f6fc 100644
--- a/src/cmd/link/internal/ld/data.go
+++ b/src/cmd/link/internal/ld/data.go
@@ -105,14 +105,12 @@ func trampoline(ctxt *Link, s loader.Sym) {
 		}
 		rs = ldr.ResolveABIAlias(rs)
 		if ldr.SymValue(rs) == 0 && (ldr.SymType(rs) != sym.SDYNIMPORT && ldr.SymType(rs) != sym.SUNDEFEXT) {
-			if ldr.SymPkg(rs) != ldr.SymPkg(s) {
-				if !isRuntimeDepPkg(ldr.SymPkg(s)) || !isRuntimeDepPkg(ldr.SymPkg(rs)) {
-					ctxt.Errorf(s, "unresolved inter-package jump to %s(%s) from %s", ldr.SymName(rs), ldr.SymPkg(rs), ldr.SymPkg(s))
-				}
-				// runtime and its dependent packages may call to each other.
-				// they are fine, as they will be laid down together.
+			if ldr.SymPkg(rs) == ldr.SymPkg(s) {
+				continue // symbols in the same package are laid out together
+			}
+			if isRuntimeDepPkg(ldr.SymPkg(s)) && isRuntimeDepPkg(ldr.SymPkg(rs)) {
+				continue // runtime packages are laid out together
 			}
-			continue
 		}
 
 		thearch.Trampoline(ctxt, ldr, ri, rs, s)
diff --git a/src/cmd/link/internal/ppc64/asm.go b/src/cmd/link/internal/ppc64/asm.go
index b1c0873d7a..a3ecd43f89 100644
--- a/src/cmd/link/internal/ppc64/asm.go
+++ b/src/cmd/link/internal/ppc64/asm.go
@@ -672,13 +672,19 @@ func trampoline(ctxt *ld.Link, ldr *loader.Loader, ri int, rs, s loader.Sym) {
 
 	relocs := ldr.Relocs(s)
 	r := relocs.At2(ri)
-	t := ldr.SymValue(rs) + r.Add() - (ldr.SymValue(s) + int64(r.Off()))
+	var t int64
+	// ldr.SymValue(rs) == 0 indicates a cross-package jump to a function that is not yet
+	// laid out. Conservatively use a trampoline. This should be rare, as we lay out packages
+	// in dependency order.
+	if ldr.SymValue(rs) != 0 {
+		t = ldr.SymValue(rs) + r.Add() - (ldr.SymValue(s) + int64(r.Off()))
+	}
 	switch r.Type() {
 	case objabi.R_CALLPOWER:
 
 		// If branch offset is too far then create a trampoline.
 
-		if (ctxt.IsExternal() && ldr.SymSect(s) != ldr.SymSect(rs)) || (ctxt.IsInternal() && int64(int32(t<<6)>>6) != t) || (*ld.FlagDebugTramp > 1 && ldr.SymPkg(s) != ldr.SymPkg(rs)) {
+		if (ctxt.IsExternal() && ldr.SymSect(s) != ldr.SymSect(rs)) || (ctxt.IsInternal() && int64(int32(t<<6)>>6) != t) || ldr.SymValue(rs) == 0 || (*ld.FlagDebugTramp > 1 && ldr.SymPkg(s) != ldr.SymPkg(rs)) {
 			var tramp loader.Sym
 			for i := 0; ; i++ {
 
@@ -769,7 +775,7 @@ func gentramp(ctxt *ld.Link, ldr *loader.Loader, tramp *loader.SymbolBuilder, ta
 
 		// With external linking, the target address must be
 		// relocated using LO and HA
-		if ctxt.IsExternal() {
+		if ctxt.IsExternal() || ldr.SymValue(target) == 0 {
 			r := loader.Reloc{
 				Off:  0,
 				Type: objabi.R_ADDRPOWER,