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nosatalian
Tux's lil' helper
Joined: 09 Apr 2004
Posts: 98
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 Posted: Sun Aug 07, 2005 10:56 pm Post subject: Attn. Kernel Hackers, help me achieve time travel. |
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Can anyone give me some clarification as to how a program gets the system time from the kernel?
Basically, I would like one of my applications to think that the time is different than the actual time, for example, lets say I want to run my application as if it were Jan 20, 1975 at all times.
I don't think this can be done with a chroot can it?
If not, this will require some kernel hacking on my part. Would it be possible to do something like: <psuedo code>
long getCurrentSystemTimeInMs(){
if (application making this call is X)
then return (Jan 20, 1975)
else return actualTime;
}
I guess my main question is, does there exist a mechanism with which a method can know what called it? Obviously I could pass a referrence to the "caller," but I don't want to add extra parameters to this hypothetical getCurrentSystemTime call. |
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widan
Veteran
Joined: 07 Jun 2005
Posts: 1512
Location: Paris, France
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| Posted: Mon Aug 08, 2005 12:40 am Post subject: |
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You can build a dynamic library exporting the (libc) function you want to replace (in your case, most likely gettimeofday(), but you may want to check that with strace first), and then run your application with:
| Code: | | LD_PRELOAD=/path/to/your/lib.so ./application |
No need to hack the kernel for that. If you need to access the real libc function from within your "overlay" library, dlopen libc.so.6 and use dlsym to get the real address of gettimeofday().
An example that works for the "date" command (that uses clock_gettime(), and not gettimeofday(), so check what your app uses) :
| Code: | widan@melanie ~/clock $ cat clock.c
#include <time.h>
int clock_gettime(clockid_t clk_id, struct timespec *tp) {
tp->tv_sec = 0;
tp->tv_nsec = 0;
return 0;
}
widan@melanie ~/clock $ gcc -shared -fPIC -DPIC -o clock.so clock.c
widan@melanie ~/clock $ date
Mon Aug 8 02:37:43 CEST 2005
widan@melanie ~/clock $ LD_PRELOAD=./clock.so date
Thu Jan 1 01:00:00 CET 1970 |
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Kernel & Hardware
