Linux/MIPS and ELF dynamic linker/loader questions...

Greetings.

I am working on a MIPS dynamic linker/loader for uClibc and would appreciate some clarification on the finer points of ELF and the Linux kernel interface. Forgive the cross post.

The first problem I have discovered is that the value of argc passed back to the userspace process from the Linux kernel is always zero. The argv, environment and auxillary vectors come through just fine. I have to loop through the stack manually to count the number of argument vectors in order to get argc:

#if defined(__mips__)
    argc = 0;
    aux_dat = sp + 1;
    while (*aux_dat++)
        argc++;

#endif

This code snippet is the first code to execute in the dynamic linker, so no trashing of argc should have had a chance to happen. Any insight?

The second question has to do with printing string constants to stderr like so:

     static inline _syscall3(unsigned long, _dl_write, int, fd,
            const void *, buf, unsigned long, count);
     #define SEND_STDERR(X) _dl_write(2, X, _dl_strlen(X));

     SEND_STDERR("ELF header =");

The problem as I understand it is that string constants for MIPS are accessed using the GOT (since the dynamic linker is all PIC code). Since I haven't bootstrapped and relocated the dynamic linker yet, the above SEND_STDERR call causes a SEGFAULT as the address is invalid. Also, it appears that the constants are stored in the .rodata section. Is there a quick hack to get a hold of the string constants? Flames, help, etc. appreciated.

-Steve

 Steven J. Hill - Embedded SW Engineer

In glibc I had no problems finding argc, check
sysdeps/mips/elf/start.S:

/* This is the canonical entry point, usually the first thing in the text
segment. The SVR4/Mips ABI (pages 3-31, 3-32) says that when the entry point runs, most registers' values are unspecified, except for:

v0 ($2) Contains a function pointer to be registered with `atexit'.
This is how the dynamic linker arranges to have DT_FINI functions called for shared libraries that have been loaded before this code runs.

sp ($29) The stack contains the arguments and environment:

		0(%esp)			argc
		4(%esp)			argv[0]
		...
		(4*argc)(%esp)		NULL
		(4*(argc+1))(%esp)	envp[0]
		...
					NULL

ra ($31) The return address register is set to zero so that programs
that search backword through stack frames recognize the last stack frame.
*/

Andreas

 Andreas Jaeger
  SuSE Labs [email protected]
   private [email protected]
    http://www.suse.de/~aj