buffer.toString() fails when buffer.length >= 32 and null-terminated on big-endian

[kapouer]

Version

v24.18.0, v26.5.0

Platform

Linux zelenka 6.12.96+deb13-s390x #1 SMP Debian 6.12.96-1 (2026-07-20) s390x GNU/Linux

Subsystem

StringBytes

What steps will reproduce the bug?

Run these commands

node -p 'const b=Buffer.alloc(32); b.write("\u3000",0,"utf8");b.toString().charCodeAt(0);'
node -p 'Buffer.from("\u3000").toString().charCodeAt(0);'

How often does it reproduce? Is there a required condition?

Always reproduced on big-endian.

What is the expected behavior? Why is that the expected behavior?

both commands should return 12288

What do you see instead?

On big-endian architecture (e.g. s390x) the first call returns 0 and the last 12288.

Additional information

Tested with the executables provided by nodejs.org.
v22.23.1 is not affected.
Also, this gives the correct result on big-endian:

node -p 'const b=Buffer.alloc(31); b.write("\u3000",0,"utf8");b.toString().charCodeAt(0);'

So the suspected commits are the ones adding the special cases for buflen >= 32 here:

node/src/string_bytes.cc

Line 751 in 6813080

if (buflen >= 32) {

and here

node/src/string_bytes.cc

Line 625 in 6813080

if (buflen >= 32) {