Register a signal handler in _dl_start to handle SIGSEGV for dynamiclinker

[wuxi <wu dot andrew dot xi at gmail dot com>]

Hi

I am doing a research project on IA-64 and glibc-2.5. My goal is to do 
instrumentation to enable whole-program (including those functions in 
library, or dynamic linker) taint tracking.

For that purpose, I need to register a special handler to handle SIGSEGV 
at the very start of any program, for I use unmapped area as the bitmap 
for taint tracking (more specifically, 0x0 ~ 0x1000000000000000 on 
Itanium).

My problem is, when I hook signal handler at _dl_start (very start of 
the dynamic linker, according to my study of code) to enable taint 
tracking for dynamic linker, the special signal handler seems not work - 
that is, when I run ld.so, it reports Segment fault directly without 
trapping into my own signal handler. However, I dump the code and 
discover the hook function has been called before such access.

The code to register the signal handler is shown below:

===================================================

 int oldact_saved = shift_sigaction(SIGSEGV, NULL, &oldact);

 act.sa_handler = NULL;
 act.sa_sigaction = B_sigaction;
 act.sa_mask = oldact.sa_mask;
 act.sa_flags = SA_SIGINFO;

 int newact_reged = shift_sigaction(SIGSEGV, &act, NULL);

===================================================

BTW. I use gdb to debug the ld.so,  but gdb cannot stop at the break 
points ...

So, am I missing something here to register a signal handler for dynamic 
linker?

Thanks

yours sincerely

Andrew