Bug: binary STL files are not being recognized in s390x/Ubuntu Noble (big endian)

[j-rivero]

Describe the bug
Using the s390x architecture (big endian) on Ubuntu Noble, when using the function aiImportFileExWithProperties(..) to open an STL binary file, assimp returns the message: Failed to determine STL storage representation for ... which appears when assimp can not determine if the STL file is a binary file or an ascii file in the code:

    if (IsBinarySTL(mBuffer, mFileSize)) {
        bMatClr = LoadBinaryFile();
    } else if (IsAsciiSTL(mBuffer, mFileSize)) {
        LoadASCIIFile(mScene->mRootNode);
    } else {
        throw DeadlyImportError("Failed to determine STL storage representation for ", pFile, ".");
    }

which is stored in the STLLoader.cpp. Note that this same test does not fail in the other supported architectures: amd64 or arm64 to name some of them.

If I tried to printout the values used by the STLLoader.cpp file code linked before, I get in the relevant Dart test:

p25: Running main() from /usr/src/googletest/googletest/src/gtest_main.cc
25: [==========] Running 1 test from 1 test suite.
25: [----------] Global test environment set-up.
25: [----------] 1 test from MjcfParserTest
25: [ RUN      ] MjcfParserTest.RoboticsFetch
25: Warning [Body.cpp:443] [MjcfParser] Unsupported number of <geom> in inferring <inertial>. We use only the first <geom> for now.
25: 1. File is: dart://sample/mjcf/openai/robotics/stls/fetch/base_link_collision.stl
25:  -------------- fileSize: 
25: 236084
25:  ---- faceCount: 
25: 1880227840
25:  --- expectedBinaryFileSize: 
25: 3817078868
25:  -------------- fileSize: 
25: 236084
25:  ---- faceCount: 
25: 1880227840
25:  --- expectedBinaryFileSize: 
25: 3817078868

Values are not the expected.

To Reproduce

1. I've been debugging a problem on the Dart library and using the following Dockerfile.

FROM --platform=s390x ubuntu:noble AS s390_dart_base
ENV LANG C
ENV LC_ALL C
ARG DEBIAN_FRONTEND=noninteractive

RUN apt-get update && \
    apt-get install -y vim ubuntu-dev-tools cmake build-essential git software-properties-common 

WORKDIR /root
RUN sed -i -e 's/Types: deb/Types: deb deb-src/g' /etc/apt/sources.list.d/*.sources \
    && cat /etc/apt/sources.list.d/ubuntu.sources \
    && apt-get update
RUN ls *.deb && dpkg -i libfcl*.deb
RUN apt-get build-dep dart -y
RUN apt-get update \
    && apt-get source dart
RUN cd dart-6.1* \
    && dpkg-buildpackage -j6 || true

2. run test with make -j1 test ARGS\+=--verbose ARGS\+="-R test_MjcfParser"

An easier approach could be just take the STL files from the Dart project and create a minimal test program. The base_link_collision.stl was the one that first failed on my test.

Expected behavior
The file is detected as a binary STL file.
xplain your problem.

Platform (please complete the following information):

• OS: Ubuntu Noble / s390 architecture

Additional context
Note that this is not the only problem related to assimp on s390, see #4788