unix: add Auxv

Fixes golang/go#67839

Change-Id: I3af38d21159f7cac3786b49ac17657d314fbc178
Reviewed-on: https://go-review.googlesource.com/c/sys/+/644295
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

unix/auxv.go

@@ -0,0 +1,36 @@
+// Copyright 2025 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 go1.21 && (aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos)
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+//go:linkname runtime_getAuxv runtime.getAuxv
+func runtime_getAuxv() []uintptr
+
+// Auxv returns the ELF auxiliary vector as a sequence of key/value pairs.
+// The returned slice is always a fresh copy, owned by the caller.
+// It returns an error on non-ELF platforms, or if the auxiliary vector cannot be accessed,
+// which happens in some locked-down environments and build modes.
+func Auxv() ([][2]uintptr, error) {
+	vec := runtime_getAuxv()
+	vecLen := len(vec)
+
+	if vecLen == 0 {
+		return nil, syscall.ENOENT
+	}
+
+	if vecLen%2 != 0 {
+		return nil, syscall.EINVAL
+	}
+
+	result := make([]uintptr, vecLen)
+	copy(result, vec)
+	return unsafe.Slice((*[2]uintptr)(unsafe.Pointer(&result[0])), vecLen/2), nil
+}

unix/auxv_linux_test.go

@@ -0,0 +1,23 @@
+// Copyright 2025 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 go1.21 && linux
+
+package unix_test
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestAuxv(t *testing.T) {
+	vec, err := unix.Auxv()
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if len(vec) == 0 {
+		t.Errorf("got zero auxv entries on linux, expected > 0")
+	}
+}

unix/auxv_unsupported.go

@@ -0,0 +1,13 @@
+// Copyright 2025 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 !go1.21 && (aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris || zos)
+
+package unix
+
+import "syscall"
+
+func Auxv() ([][2]uintptr, error) {
+	return nil, syscall.ENOTSUP
+}