Skip to main content
Redhat Developers  Logo
  • Products

    Featured

    • Red Hat Enterprise Linux
      Red Hat Enterprise Linux Icon
    • Red Hat OpenShift AI
      Red Hat OpenShift AI
    • Red Hat Enterprise Linux AI
      Linux icon inside of a brain
    • Image mode for Red Hat Enterprise Linux
      RHEL image mode
    • Red Hat OpenShift
      Openshift icon
    • Red Hat Ansible Automation Platform
      Ansible icon
    • Red Hat Developer Hub
      Developer Hub
    • View All Red Hat Products
    • Linux

      • Red Hat Enterprise Linux
      • Image mode for Red Hat Enterprise Linux
      • Red Hat Universal Base Images (UBI)
    • Java runtimes & frameworks

      • JBoss Enterprise Application Platform
      • Red Hat build of OpenJDK
    • Kubernetes

      • Red Hat OpenShift
      • Microsoft Azure Red Hat OpenShift
      • Red Hat OpenShift Virtualization
      • Red Hat OpenShift Lightspeed
    • Integration & App Connectivity

      • Red Hat Build of Apache Camel
      • Red Hat Service Interconnect
      • Red Hat Connectivity Link
    • AI/ML

      • Red Hat OpenShift AI
      • Red Hat Enterprise Linux AI
    • Automation

      • Red Hat Ansible Automation Platform
      • Red Hat Ansible Lightspeed
    • Developer tools

      • Red Hat Trusted Software Supply Chain
      • Podman Desktop
      • Red Hat OpenShift Dev Spaces
    • Developer Sandbox

      Developer Sandbox
      Try Red Hat products and technologies without setup or configuration fees for 30 days with this shared Openshift and Kubernetes cluster.
    • Try at no cost
  • Technologies

    Featured

    • AI/ML
      AI/ML Icon
    • Linux
      Linux Icon
    • Kubernetes
      Cloud icon
    • Automation
      Automation Icon showing arrows moving in a circle around a gear
    • View All Technologies
    • Programming Languages & Frameworks

      • Java
      • Python
      • JavaScript
    • System Design & Architecture

      • Red Hat architecture and design patterns
      • Microservices
      • Event-Driven Architecture
      • Databases
    • Developer Productivity

      • Developer productivity
      • Developer Tools
      • GitOps
    • Secure Development & Architectures

      • Security
      • Secure coding
    • Platform Engineering

      • DevOps
      • DevSecOps
      • Ansible automation for applications and services
    • Automated Data Processing

      • AI/ML
      • Data Science
      • Apache Kafka on Kubernetes
      • View All Technologies
    • Start exploring in the Developer Sandbox for free

      sandbox graphic
      Try Red Hat's products and technologies without setup or configuration.
    • Try at no cost
  • Learn

    Featured

    • Kubernetes & Cloud Native
      Openshift icon
    • Linux
      Rhel icon
    • Automation
      Ansible cloud icon
    • Java
      Java icon
    • AI/ML
      AI/ML Icon
    • View All Learning Resources

    E-Books

    • GitOps Cookbook
    • Podman in Action
    • Kubernetes Operators
    • The Path to GitOps
    • View All E-books

    Cheat Sheets

    • Linux Commands
    • Bash Commands
    • Git
    • systemd Commands
    • View All Cheat Sheets

    Documentation

    • API Catalog
    • Product Documentation
    • Legacy Documentation
    • Red Hat Learning

      Learning image
      Boost your technical skills to expert-level with the help of interactive lessons offered by various Red Hat Learning programs.
    • Explore Red Hat Learning
  • Developer Sandbox

    Developer Sandbox

    • Access Red Hat’s products and technologies without setup or configuration, and start developing quicker than ever before with our new, no-cost sandbox environments.
    • Explore Developer Sandbox

    Featured Developer Sandbox activities

    • Get started with your Developer Sandbox
    • OpenShift virtualization and application modernization using the Developer Sandbox
    • Explore all Developer Sandbox activities

    Ready to start developing apps?

    • Try at no cost
  • Blog
  • Events
  • Videos

Implicit function declarations: flex's use of "reallocarray"

April 22, 2019
Arjun Shankar

Share:

    Several months ago, I took over the maintenance of the flex package in Fedora and decided to kick the tires by rebasing the package in Fedora Rawhide. I downloaded and hashed the latest tarball at the time, flex-2.6.4, tweaked the spec file, and fired up a local build. Unfortunately, it failed with a SIGSEGV at build time:

    ./stage1flex -o stage1scan.c ./scan.l
    make[2]: *** [Makefile:1695: stage1scan.c] Segmentation fault (core dumped)
    

    Some debugging with gdb led me to the conclusion that the segmentation fault was the result of a block of memory returned from the reallocarray function being written to during flex initialization.  In this article, I'll describe the issue further and explain changes made to address it.

    Here is a simplified snippet of my gdb session:

    (gdb) bt
    #0 check_mul_overflow_size_t (right=1, left=2048, left@entry=0)
    #1 __GI___libc_reallocarray (optr=0x0, nmemb=2048, elem_size=1)
    #2 allocate_array at misc.c:147
    #3 flexinit at main.c:974
    #4 flex_main at main.c:168
    #5 __libc_start_main
    (gdb) fin
    Run till exit from #0 check_mul_overflow_size_t
    __GI___libc_reallocarray
    33              return realloc (optr, bytes);
    (gdb) fin
    Run till exit from #0 __GI___libc_reallocarray
    in allocate_array
    147             mem = reallocarray(NULL, (size_t) size, element_size);
    Value returned is $1 = (void *) 0x5555557c6420
    (gdb) fin
    Run till exit from #0 allocate_array
    in flexinit
    974             action_array = allocate_character_array (action_size);
    Value returned is $2 = (void *) 0x557c6420
    (gdb) n
    975             defs1_offset = prolog_offset = action_offset = action_index = 0;
    (gdb) n
    976             action_array[0] = '\0';
    (gdb) n
    Program received signal SIGSEGV, Segmentation fault.
    

    I didn't notice anything off here right up to the point at which the segfault occurs, but maybe you already did. All I saw was that the returned pointer was non-NULL on line 974, but writing to it on line 976 resulted in a segfault. It began to look like a malloc bug.

    On a whim, I built the same tarball outside of the Fedora build system. This time, the typical ./configure && make command line didn't segfault at build time. So apparently the difference lay in the build options used by rpmbuild. Some trial and error led me to the cause: -pie, the linker flag that produces a position independent executable. Building with -pie caused the segmentation fault.

    Armed with this "reproducer" and advice from my colleagues at Red Hat, I set about doing a git-bisect on the flex sources. HEAD was building cleanly on the upstream master branch at that point even with -pie, so it was just a matter of finding the commit that fixed the build. The commit in question was the fix for the following issue reported against flex upstream:

    #241: "implicit declaration of function reallocarray is invalid in C99"

    So, flex sources didn't declare _GNU_SOURCE, leading to the compiler's seeing no declaration of the reallocarray function. In such cases, the compiler creates an implicit function declaration with the default return type (int) and generates code accordingly. On 64-bit Intel machines, the int type is only 32 bits wide while pointers are 64 bits wide. Going back and looking at the gdb session, it then became clear to me that the pointer gets truncated:

    147             mem = reallocarray(NULL, (size_t) size, element_size);
    Value returned is $1 = (void *) 0x5555557c6420
    (gdb) fin
    Run till exit from #0  allocate_array
    in flexinit
    974             action_array = allocate_character_array (action_size);
    Value returned is $2 = (void *) 0x557c6420
    

    This only happens in position independent executables because the heap gets mapped to a part of the address space where pointers are larger than INT_MAX, exposing the above flex bug. GCC actually warns of the presence of implicit function declarations via the -Wimplicit-function-declaration option. It appears that there was a fairly recent proposal to enable this warning in Fedora builds, but it was eventually shelved. If enabled, the warning would still cause the flex build to fail—but earlier and at a point where the problem was clear.

    At this point, getting the build to compile successfully was a simple matter of backporting the corresponding flex patch that defines _GNU_SOURCE and exposes the reallocarray prototype to the compiler.

    But we didn't just stop there. One of my colleagues, Florian Weimer—a regular contributor to glibc—thought that all this could have been avoided if reallocarray had been exposed by glibc via the more general _DEFAULT_SOURCE feature test macro. The change has now been committed to glibc upstream and is available since glibc-2.29.

    With this change, we hope to avoid similar situations in other components in Fedora and the glibc user community. glibc now provides the reallocarray function prototype unless the user explicitly requires stricter conformance to a given standard.

    Last updated: April 17, 2019

    Recent Posts

    • Build container images in CI/CD with Tekton and Buildpacks

    • How to deploy OpenShift AI & Service Mesh 3 on one cluster

    • JVM tuning for Red Hat Data Grid on Red Hat OpenShift 4

    • Exploring Llama Stack with Python: Tool calling and agents

    • Enhance data security in OpenShift Data Foundation

    Red Hat Developers logo LinkedIn YouTube Twitter Facebook

    Products

    • Red Hat Enterprise Linux
    • Red Hat OpenShift
    • Red Hat Ansible Automation Platform

    Build

    • Developer Sandbox
    • Developer Tools
    • Interactive Tutorials
    • API Catalog

    Quicklinks

    • Learning Resources
    • E-books
    • Cheat Sheets
    • Blog
    • Events
    • Newsletter

    Communicate

    • About us
    • Contact sales
    • Find a partner
    • Report a website issue
    • Site Status Dashboard
    • Report a security problem

    RED HAT DEVELOPER

    Build here. Go anywhere.

    We serve the builders. The problem solvers who create careers with code.

    Join us if you’re a developer, software engineer, web designer, front-end designer, UX designer, computer scientist, architect, tester, product manager, project manager or team lead.

    Sign me up

    Red Hat legal and privacy links

    • About Red Hat
    • Jobs
    • Events
    • Locations
    • Contact Red Hat
    • Red Hat Blog
    • Inclusion at Red Hat
    • Cool Stuff Store
    • Red Hat Summit
    © 2025 Red Hat

    Red Hat legal and privacy links

    • Privacy statement
    • Terms of use
    • All policies and guidelines
    • Digital accessibility

    Report a website issue