Thread: C guru help needed URGENT

If you're getting "no symbols", then I think the LD_PRELOAD is a non-starter.

Code:

$ gcc foo.c
$ ls -l ./a.out 
-rwxrwxr-x 1 sc sc 8435 2012-09-19 11:33 ./a.out
$ gcc -static foo.c
$ ls -l ./a.out 
-rwxrwxr-x 1 sc sc 825738 2012-09-19 11:33 ./a.out
$ strip a.out 
$ ls -l ./a.out 
-rwxrwxr-x 1 sc sc 747096 2012-09-19 11:33 ./a.out
$ nm a.out 
nm: a.out: no symbols

Statically linked executables have the entire library built into them - which is why the default dynamically linked versions are so much smaller (case 1).

A statically linked executable which has been 'stripped' of symbols (case 3) will have no symbols.

Which leads me to ask "how do you know it's gettimeofday" that is the issue here?

Two things
a)
This is in strace of the program so i know its using gettime

Code:

write(1, "\r\n", 2)                     = 2
poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 0) = 0 (Timeout)
rt_sigprocmask(SIG_BLOCK, [ALRM], NULL, 8) = 0
gettimeofday({1348050630, 92763}, NULL) = 0
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 0) = 0 (Timeout)
rt_sigprocmask(SIG_BLOCK, [ALRM], NULL, 8) = 0
gettimeofday({1348050630, 93232}, NULL) = 0
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 0) = 0 (Timeout)
rt_sigprocmask(SIG_BLOCK, [ALRM], NULL, 8) = 0
gettimeofday({1348050630, 93700}, NULL) = 0
rt_sigprocmask(SIG_UNBLOCK, [ALRM], NULL, 8) = 0
poll([{fd=4, events=POLLIN}, {fd=0, events=POLLIN}], 2, 0) = 0 (Timeout)

b) i ran ld for the test program , it looks like it compiles etc fine but
ld shows not using the new library i created with new gettime

machine:~$ ldd main
linux-gate.so.1 => (0xb7788000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb75f2000)
/lib/ld-linux.so.2 (0xb7789000)




ProTip: how to break your linux box
add untested lib.so to /etc/lib.sl.preload
everything stops working , had fun
but learnt somthing
its saying something about undefined __wrap_gettimeofday




EDIT:
machine:~$ ld libltimer.so
ld: warning: cannot find entry symbol _start; not setting start address
libltimer.so: undefined reference to `__real_gettimeofday'

guys we almost have it ..please help
now i removed salems file,
have an empty gettime returning 0 after sleeping usleep(30000);
its going to sleep so i know its this getting getting called
so salem , looks like the wrap thing doesnt work
can you suggest something else

> Actually i get
> nm: i86-image: no symbols
And if you also do
ldd i86-image
do you also get "not a dynamic executable"

If you do, then none of this LD_PRELOAD or wrapping is going to work.
The executable is a closed box containing it's own pre-compiled copy of gettimeofday(), and everything else.

I found this
c - How to 'link' object file to executable/compiled binary? - Stack Overflow
The Dyninst looks like a possible way forward, if somewhat over-engineered for a single-shot patch.

I've been looking at post #33 again.

From the gettimeofday() results, it seems to be looping at around 500uS, which means this bit of code is effectively looping at around 2000 times a second.

I'm curious as to what the code does when it gets a poll timeout (which seems to be what it does most of the time).

Calling poll() with a zero timeout suggests that it is monitoring something else, and doesn't want to block on a file descriptor. Which is fair enough, IF there is something else to do.
But there are no other system calls (apart from repeatedly calling gettimeofday), so what is it up to?

> OK i have the program reading my version ( or your version ) of gettimeday but does not seem to be doing anything tho ?
Good job.

But perhaps you could try patching poll as well (or instead), with something like

Code:

int __wrap_poll(struct pollfd *fds, nfds_t nfds, int timeout) {
    if ( timeout == 0 ) timeout = 5;
    return __real_poll(fds,nfds,timeout);
}

Dammit spoke too soon
Core dumped
well it ran for a whole 10 mins longer than before thats a good sign