ld -shared -pie revisited

[Mark Lodato <lodatom at gmail dot com>]

I saw a 2007 discussion* of whether to allow both -shared and -pie
from gcc, where it was decided to use only the last one specified.
But `ld -shared -pie` seems to work fine on x86_64, and in fact allows
one to have an executable shared library - you can both dlopen the
library and run it.  This is awesome!  Am I missing something here?
Will this break in subtle ways?  If so, how do you properly make a
shared library that can be executed (like /lib/ld.so)?  If not, should
gcc x86_64 allow both -shared and -pie to make such libraries, so that
one does not have to invoke the linker manually?

Here's an example:
-----
# My system
$ uname -a
Linux mark-desktop 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17
01:58:03 UTC 2009 x86_64 GNU/Linux
$ gcc --version
gcc (Ubuntu 4.3.3-5ubuntu4) 4.3.3

# Create a sample library.
$ cat <<EOF >library.c
#include <stdio.h>
#include <stdlib.h>

void foo(int x) {
    printf("foo(%d)\n", x);
}

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "USAGE: %s n\n", argv[0]);
        exit(1);
    }
    foo(atoi(argv[1]));
    return 0;
}
EOF

# Compile.
$ gcc -fPIC -fPIE -c library.c

# Link with -pie and -shared.
# The following command was obtained by running
#     gcc '-###' -pic -fPIC -fPIE -rdynamic -o library.so library.o
# and adding '-shared' before '-pie'.
$ /usr/lib/gcc/x86_64-linux-gnu/4.3.3/collect2 \
    --eh-frame-hdr \
    -m elf_x86_64 \
    -shared \
    -pie \
    --export-dynamic \
    -o library.so \
    -z relro \
    /usr/lib/Scrt1.o \
    /usr/lib/crti.o \
    /usr/lib/gcc/x86_64-linux-gnu/4.3.3/crtbeginS.o \
    -L/usr/lib/gcc/x86_64-linux-gnu/4.3.3 \
    -L/usr/lib \
    -L/lib \
    library.o \
    main.o \
    -lgcc \
    --as-needed \
    -lgcc_s \
    --no-as-needed \
    -lc \
    -lgcc \
    --as-needed \
    -lgcc_s \
    --no-as-needed \
    /usr/lib/gcc/x86_64-linux-gnu/4.3.3/crtendS.o \
    /usr/lib/crtn.o

$ ./library.so 12
foo(12)

$ python -c \
    'import ctypes; x = ctypes.cdll.LoadLibrary("./library.so"); x.foo(13)'
foo(13)
-----

* http://sourceware.org/ml/binutils/2007-07/msg00059.html

--
Mark