how to port c/c++ applications to legacy linux kernel versions

Question

Ok, this is just a bit of a fun exercise, but it can't be too hard compiling programmes for some older linux systems, or can it?

I have access to a couple of ancient systems all running linux and maybe it'd be interesting to see how they perform under load. Say as an example we want to do some linear algebra using Eigen which is a nice header-only library. Any chance to compile it on the target system?

user@ancient:~ $ uname -a
Linux local 2.2.16 #5 Sat Jul 8 20:36:25 MEST 2000 i586 unknown
user@ancient:~ $ gcc --version
egcs-2.91.66

Maybe not... So let's compile it on a current system. Below are my attempts, mainly failed ones. Any more ideas very welcome.

1. Compile with -m32 -march=i386

   user@ancient:~ $ ./a.out
   BUG IN DYNAMIC LINKER ld.so: dynamic-link.h: 53: elf_get_dynamic_info: Assertion `! "bad dynamic tag"' failed!
   

2. Compile with -m32 -march=i386 -static: Runs on all fairly recent kernel versions but fails if they are slightly older with the well known error message

   user@ancient:~ $ ./a.out
   FATAL: kernel too old
   Segmentation fault
   

   This is a glibc error which has a minimum kernel version it supports, e.g. kernel 2.6.4 on my system:

   $ file a.out
   a.out: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
   statically linked, for GNU/Linux 2.6.4, not stripped
   

3. Compile glibc myself with support for the oldest kernel possible. This post describes it in more detail but essentially it goes like this

   wget ftp://ftp.gnu.org/gnu/glibc/glibc-2.14.tar.bz2
   tar -xjf glibc-2.14.tar.bz2
   cd glibc-2.14
   mkdir build; cd build
   ../configure --prefix=/usr/local/glibc_32  \
                --enable-kernel=2.0.0 \
                --with-cpu=i486 --host=i486-linux-gnu \
                CC="gcc -m32 -march=i486"  CXX="g++ -m32 -march=i486"
   make -j 4
   make intall
   

   Not sure if the --with-cpu and --host options do anything, most important is to force the use of compiler flags -m32 -march=i486 for 32-bit builds (unfortunately -march=i386 bails out with errors after a while) and --enable-kernel=2.0.0 to make the library compatible with older kernels. Incidentially, during configure I got the warning

   WARNING: minimum kernel version reset to 2.0.10
   

   which is still acceptable, I suppose. For a list of things which change with different kernels see ./sysdeps/unix/sysv/linux/kernel-features.h.

   Ok, so let's link against the newly compiled glibc library, slightly messy but here it goes:

   $ export LIBC_PATH=/usr/local/glibc_32
   $ export LIBC_FLAGS=-nostdlib -L${LIBC_PATH} \
                       ${LIBC_PATH}/crt1.o ${LIBC_PATH}/crti.o \
                       -lm -lc -lgcc -lgcc_eh -lstdc++ -lc \
                       ${LIBC_PATH}/crtn.o
   
   $ g++ -m32 -static prog.o ${LIBC_FLAGS} -o prog
   

   Since we're doing a static compile the link order is important and may well require some trial and error, but basically we learn from what options gcc gives to the linker:

   $ g++ -m32 -static -Wl,-v file.o
   

   Note, crtbeginT.o and crtend.o are also linked against which I didn't need for my programmes so I left them out. The output also includes a line like --start-group -lgcc -lgcc_eh -lc --end-group which indicates inter-dependence between the libraries, see this post. I just mentioned -lc twice in the gcc command line which also solves inter-dependence.

   Right, the hard work has paid off and now I get

   $ file ./prog
   ./prog: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV),
   statically linked, for GNU/Linux 2.0.10, not stripped
   

   Brilliant I thought, now try it on the old system:

   user@ancient:~ $ ./prog
   set_thread_area failed when setting up thread-local storage
   Segmentation fault
   

   This, again, is a glibc error message from ./nptl/sysdeps/i386/tls.h. I fail to understand the details and give up.

4. Compile on the new system g++ -c -m32 -march=i386 and link on the old. Wow, that actually works for C and simple C++ programmes (not using C++ objects), at least for the few I've tested. This is not too surprising as all I need from libc is printf (and maybe some maths) of which the interface hasn't changed but the interface to libstdc++ is very different now.

5. Setup a virtual box with an old linux system and gcc version 2.95. Then compile gcc version 4.x.x ... sorry, but too lazy for that right now ...

6. ???

Answer 1

Have found the reason for the error message:

user@ancient $ ./prog
set_thread_area failed when setting up thread-local storage
Segmentation fault

It's because glibc makes a system call to a function which is only available since kernel 2.4.20. In a way it can be seen as a bug of glibc as it wrongly claims to be compatible with kernel 2.0.10 when it requires at least kernel 2.4.20.

The details:

./glibc-2.14/nptl/sysdeps/i386/tls.h
[...]
     /* Install the TLS.  */                                                  \
     asm volatile (TLS_LOAD_EBX                                               \
                   "int $0x80\n\t"                                            \
                   TLS_LOAD_EBX                                               \
                   : "=a" (_result), "=m" (_segdescr.desc.entry_number)       \
                   : "0" (__NR_set_thread_area),                              \
                     TLS_EBX_ARG (&_segdescr.desc), "m" (_segdescr.desc));    \
[...]
     _result == 0 ? NULL                                                      \
     : "set_thread_area failed when setting up thread-local storage\n"; })
[...]

The main thing here is, it calls the assembly function int 0x80 which is a system call to the linux kernel which decides what to do based on the value of eax, which is set to __NR_set_thread_area in this case and is defined in

$ grep __NR_set_thread_area /usr/src/linux-2.4.20/include/asm-i386/unistd.h
#define __NR_set_thread_area    243

but not in any earlier kernel versions.

So the good news is that point "3. Compiling glibc with --enable-kernel=2.0.0" will probably produce executables which run on all linux kernels >= 2.4.20.

The only chance to make this work with older kernels would be to disable tls (thread-local storage) but which is not possible with glibc 2.14, despite the fact it is offered as a configure option.