loader, crypto/internal/entropy: fix 32 MiB RAM overflow on microcontrollers

Go 1.26 added a CPU jitter-based SP 800-90B entropy source for FIPS 140-3
compliance (crypto/internal/entropy/v1.0.0). It declares a 32 MiB global
ScratchBuffer ([1<<25]byte) used for memory access timing noise. On systems
with virtual memory this stays in .noptrbss and is lazily paged, but on
baremetal targets it becomes static RAM, causing fatal overflow (e.g.
pybadge with 192 KB SRAM reports 33 MB overflow).

Since FIPS jitter entropy is never used on TinyGo targets (fips140.Enabled
is always false, so drbg.Read falls through to sysrand.Read), provide a
TinyGo overlay that replaces ScratchBuffer with [0]byte and stubs out all
entropy functions with panics.

loader/goroot.go

@@ -268,6 +268,16 @@ func pathsToOverride(goMinor int, needsSyscallPackage bool) map[string]bool {
 		paths["crypto/internal/boring/sig/"] = false
 	}
 
+	if goMinor >= 26 {
+		// Go 1.26 added a CPU jitter entropy source for FIPS 140-3 that
+		// allocates a 32 MiB global buffer (ScratchBuffer [1<<25]byte).
+		// This is fine on systems with virtual memory, but causes RAM
+		// overflow on microcontrollers. Replace with a zero-size stub
+		// since TinyGo targets never use FIPS jitter entropy.
+		paths["crypto/internal/entropy/"] = true
+		paths["crypto/internal/entropy/v1.0.0/"] = false
+	}
+
 	if needsSyscallPackage {
 		paths["syscall/"] = true // include syscall/js
 		paths["internal/syscall/"] = true

src/crypto/internal/entropy/v1.0.0/entropy.go

@@ -0,0 +1,56 @@
+// 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.
+
+// Stripped-down version of the entropy package for TinyGo.
+//
+// The upstream Go 1.26 package allocates a [1<<25]byte (32 MiB) global buffer
+// for CPU jitter-based SP 800-90B entropy collection. This is fine on systems
+// with virtual memory where BSS pages are lazily backed, but it causes a fatal
+// "RAM overflowed" error on microcontrollers and other memory-constrained
+// targets.
+//
+// Because FIPS 140-3 jitter entropy is never used on TinyGo targets (the DRBG
+// falls through to sysrand.Read when fips140.Enabled is false), this overlay
+// replaces the 32 MiB buffer with a zero-size type.
+package entropy
+
+// Version returns the version of the entropy source.
+func Version() string {
+	return "v1.0.0"
+}
+
+// ScratchBuffer is a large buffer in upstream Go (32 MiB). TinyGo replaces it
+// with a zero-size type since the CPU jitter entropy source is not used.
+type ScratchBuffer [0]byte
+
+// Seed returns a 384-bit seed with full entropy.
+// On TinyGo targets this is never called because FIPS mode is not enabled.
+func Seed(memory *ScratchBuffer) ([48]byte, error) {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}
+
+// Samples collects entropy samples. Not supported on TinyGo targets.
+func Samples(samples []uint8, memory *ScratchBuffer) error {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}
+
+// SHA384 computes SHA-384 over 1024 bytes. Not supported on TinyGo targets.
+func SHA384(p *[1024]byte) [48]byte {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}
+
+// TestingOnlySHA384 computes SHA-384 over arbitrary bytes. Not supported on TinyGo targets.
+func TestingOnlySHA384(p []byte) [48]byte {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}
+
+// RepetitionCountTest implements the repetition count test from SP 800-90B.
+func RepetitionCountTest(samples []uint8) error {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}
+
+// AdaptiveProportionTest implements the adaptive proportion test from SP 800-90B.
+func AdaptiveProportionTest(samples []uint8) error {
+	panic("entropy: CPU jitter entropy source is not supported on TinyGo targets")
+}