building a .so that is also an executable

Question

So everyone probably knows that glibc's /lib/libc.so.6 can be executed in the shell like a normal executable in which cases it prints its version information and exits. This is done via defining an entry point in the .so. For some cases it could be interesting to use this for other projects too. Unfortunately, the low-level entry point you can set by ld's -e option is a bit too low-level: the dynamic loader is not available so you cannot call any proper library functions. glibc for this reason implements the write() system call via a naked system call in this entry point.

My question now is, can anyone think of a nice way how one could bootstrap a full dynamic linker from that entry point so that one could access functions from other .so's?

Answer 1

Update 2: see Andrew G Morgan's slightly more complicated solution which does work for any GLIBC (that solution is also used in libc.so.6 itself (since forever), which is why you can run it as ./libc.so.6 (it prints version info when invoked that way)).

Update 1: this no longer works with newer GLIBC versions:

./a.out: error while loading shared libraries: ./pie.so: cannot dynamically load position-independent executable 

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Original answer from 2009:

Building your shared library with -pie option appears to give you everything you want:

/* pie.c */ #include <stdio.h> int foo() {   printf("in %s %s:%d\n", __func__, __FILE__, __LINE__);   return 42;  } int main()  {    printf("in %s %s:%d\n", __func__, __FILE__, __LINE__);   return foo();  }   /* main.c */ #include <stdio.h>  extern int foo(void); int main()  {    printf("in %s %s:%d\n", __func__, __FILE__, __LINE__);   return foo();  }   $ gcc -fPIC -pie -o pie.so pie.c -Wl,-E $ gcc main.c ./pie.so   $ ./pie.so in main pie.c:9 in foo pie.c:4 $ ./a.out in main main.c:6 in foo pie.c:4 $ 

P.S. glibc implements write(3) via system call because it doesn't have anywhere else to call (it is the lowest level already). This has nothing to do with being able to execute libc.so.6.