runtime: use asmcgocall_no_g in libInit

libInit runs before rt0_go, which is where TLS setup occurs. Thus the
contents of the TLS may not be defined, so the g lookup in asmcgocall is
not safe.

Concretely, android-386 c-shared builds crash without this change
because asmcgocall reads an invalid non-zero g from the TLS.

Move libInit to a file limited to GOARCH that support c-archive or
c-shared so that only those require asmcgocall_no_g. In addition,
loong64 and s390x need asmcgocall_no_g implementations.

Fixes #78480.

Cq-Include-Trybots: luci.golang.try:gotip-linux-loong64,gotip-linux-s390x
Change-Id: I175e6d020339af89c9b576535d79c1e76a6a6964
Reviewed-on: https://go-review.googlesource.com/c/go/+/761541
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/runtime/libinit.go b/src/runtime/libinit.go
new file mode 100644
index 0000000000..7e298a2eca
--- /dev/null
+++ b/src/runtime/libinit.go
@@ -0,0 +1,34 @@
+// Copyright 2026 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.
+
+//go:build 386 || amd64 || arm || arm64 || loong64 || ppc64 || ppc64le || riscv64 || s390x
+
+package runtime
+
+import (
+	"internal/abi"
+	"unsafe"
+)
+
+// libInit is common startup code for most architectures when
+// using -buildmode=c-archive or -buildmode=c-shared.
+//
+// May run with m.p==nil, so write barriers are not allowed.
+//
+//go:nowritebarrierrec
+//go:nosplit
+func libInit() {
+	// Synchronous initialization.
+	libpreinit()
+
+	// Asynchronous initialization.
+	// Prefer creating a thread via cgo if it is available.
+	if _cgo_sys_thread_create != nil {
+		// No g because the TLS is not set up until later in rt0_go.
+		asmcgocall_no_g(_cgo_sys_thread_create, unsafe.Pointer(abi.FuncPCABIInternal(rt0_lib_go)))
+	} else {
+		const stackSize = 0x800000 // 8192KB
+		newosproc0(stackSize, unsafe.Pointer(abi.FuncPCABIInternal(rt0_lib_go)))
+	}
+}