Linking against a shared library silently injects TLS by enabling morestack gcc feature

Wed Jun 3 11:18:35 GMT 2026

Hello,

we habe a problem with the gcc/linker behaviour when wrapping the
function pthread_create in a shared library.

The compiler version is

Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-linux-gnu/14/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 14.2.0-19' --with-bugurl=file:///usr/share/doc/gcc-14/README.Bugs --enable-languages=c,ada,c++,go,d,fortran,objc,obj-c++,m2,rust --prefix=/usr --with-gcc-major-version-only --program-suffix=-14 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/libexec --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-libstdcxx-backtrace --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/reproducible-path/gcc-14-14.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/reproducible-path/gcc-14-14.2.0/debian/tmp-gcn/usr --enable-offload-defaulted --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=3
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 14.2.0 (Debian 14.2.0-19)

Linking a shared library (libmy.so) against another shared library
(libmywrap.so) seems to silently activate a compiler feature called morestack
and by that silently injects TLS (thread local storage).

This injection causes trouble because we use this application in a real
time context. Therefore this morestack feature during runtime must be
avoided.

When we wrap ioctl() (as a random chosen alternative) instead of pthread_create()
the issue is gone. So it is (very likely) somehow related to the __wrap_pthread_create()
symbol provided by libgcc.

The library dependency looks like

app -> libmy.so -> libmywrap.so

In the error case the app binary is linked against libgcc.

Now the question is why? Because we provide already the __wrap_pthread_create function
in the library libmywrap.so. So normally there is no more need to link
in the libgcc library.

$ ldd app
        linux-vdso.so.1 (0x00007fdb5c8d1000)
        libmy.so => [snip]./libmy.so (0x00007fdb5c8c0000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fdb5c87c000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fdb5c688000)
        /lib64/ld-linux-x86-64.so.2 (0x00007fdb5c8d3000)
        libmywrap.so => [snip]./libmywrap.so (0x00007fdb5c683000)

$ nm app
[snip]
0000000000002370 T __splitstack_block_signals
0000000000002720 T __splitstack_block_signals_context
0000000000002200 T __splitstack_find
0000000000002750 T __splitstack_find_context
00000000000023e0 T __splitstack_getcontext
0000000000002500 T __splitstack_makecontext
0000000000002710 T __splitstack_releasecontext
00000000000026a0 T __splitstack_resetcontext
0000000000002480 T __splitstack_setcontext
0000000000001586 t __stack_split_initialize
[snip]

Is it possible to avoid this split-stack feature in this shared library.

Is this behaviour a gcc feature or is it more a bug which has to be
addressed?

A small example code can be found in the attachment to reproduce the
problem.

With best regards
   Dr. Johann Pfefferl
-- 
Siemens AG -- Foundational Technologies
Research & Predevelopment
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Open Source Embedded Systems
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mailto: johann.pfefferl@siemens.com
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