windows_exporter/tools/go-overlay/entropy_fips140.go.overlay

// 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.

// Patched copy of $GOROOT/src/crypto/internal/fips140/drbg/entropy_fips140.go
// from Go 1.27.2, applied by tools/go-overlay. The only change is that the
// 32 MiB entropy scratch buffer is allocated on first use instead of being a
// package-level variable. On Windows, the package-level variable is part of
// the writable .data section of the executable image, which is charged as
// commit for every process even if FIPS 140-3 mode is never enabled.
// See https://github.com/golang/go/issues/81956.
//
//go:build !wasm

package drbg

import (
	entropy "crypto/internal/entropy/v1.0.0"
	"sync"
)

// scratchMemory returns a scratch buffer that is accessed between samples by the
// entropy source to expose it to memory access timings.
//
// It is allocated once and shared between Seed calls to avoid the cost of
// zeroing a new allocation every time. The entropy source accesses it using
// atomics (and doesn't care about its contents).
var scratchMemory = sync.OnceValue(func() *entropy.ScratchBuffer {
	return new(entropy.ScratchBuffer)
})

func getEntropy() *[SeedSize]byte {
	var retries int
	seed, err := entropy.Seed(scratchMemory())
	for err != nil {
		// The CPU jitter-based SP 800-90B entropy source has a non-negligible
		// chance of failing the startup health tests.
		//
		// Each time it does, it enters a permanent failure state, and we
		// restart it anew. This is not expected to happen more than a few times
		// in a row.
		if retries++; retries > 100 {
			panic("fips140/drbg: failed to obtain initial entropy")
		}
		seed, err = entropy.Seed(scratchMemory())
	}
	return &seed
}