Date: Thu, 26 Apr 2007 12:15:16 +0100 From: Tom Evans <tevans.uk@googlemail.com> To: Kelly Jones <kelly.terry.jones@gmail.com> Cc: techtalk@linuxchix.org, freebsd-questions@freebsd.org, linuxusersgroup@googlegroups.com, nmosug-l@mailman.swcp.com, nmlug@nmlug.org Subject: Re: Using LD_PRELOAD to make date return a specific date Message-ID: <1177586116.3530.4.camel@zoot.mintel.co.uk> In-Reply-To: <26face530704241939x1acd5d6en56b26fe4bdf69d85@mail.gmail.com> References: <26face530704241939x1acd5d6en56b26fe4bdf69d85@mail.gmail.com>
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On Tue, 2007-04-24 at 19:39 -0700, Kelly Jones wrote:
> I recently discovered LD_PRELOAD, a cool environment variable that
> lets a library "intercept" system calls. For example, setting
> LD_PRELOAD to /usr/lib/libtsocks.so lets tsocks intercept socket
> connections and redirect them to a SOCKS proxy.
>=20
> My question: how can I write a library that intercepts the
> gettimeofday() system call (or time() or whatever the 'date' command
> uses) and gets 'date' to return, say, "Thu Jan 1 00:00:00 UTC 1970"?
>=20
> I realize this involves a couple of steps (writing a C "library" for
> one), so any pointers are appreciated. My real intentions are more
> complex (and sinister <G>).
>=20
It's quite straightforward. /bin/date actually uses localtime(), not
gettimeofday(), but the principle is the same.
> $ cat localtime_hack.c=20
#include <sys/types.h>
#include <time.h>
struct tm *
localtime(const time_t *clock)
{
static struct tm tv;
time_t epochal =3D 1;
localtime_r(&epochal, &tv);
return &tv;
}
> $ gcc -Wall -fpic -c -o localtime_hack.o localtime_hack.c
> $ gcc -shared -Wl,-soname,localtime_hack.so.1 -o \ =20
liblocaltime_hack.so.1.0 localtime_hack.o
> $ LD_PRELOAD=3D"`pwd`/liblocaltime_hack.so.1.0" /bin/date
Thu 1 Jan 1970 01:00:01 BST
Cheers
Tom
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