When a program opens a file, that file ends up on a file descriptor that's free at the time. By opening a file before the program starts, you're only making one more file descriptor busy, so the file you're interested in might end up on a different descriptor. If you want the program to open a different file, you'll need to modify the open operation when it takes place, or intervene afterwards.
One way to modify the operation is to wedge some code between the program and the system library, by preloading a small piece of code. This assumes that the program is a dynamically linked binary, or a script executed by a dynamically linked binary (i.e. it isn't statically linked). Write the following code to a file override_fopen.c
:
#include <dlfcn.h>
#include <fcntl.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#ifndef FROM
#error "Define FROM to the path to override in double quotes, e.g. -DFROM='\"/bad\"'"
#endif
#ifndef TO
#error "Define TO to the path to use instead in double quotes, e.g. -DFROM='\"/good\"'"
#endif
FILE *fopen(const char *path, const char *mode) {
void *(*original_fopen)(const char *, const char *) = dlsym(RTLD_NEXT, "fopen");
if (!strcmp(path, FROM)) {
path = TO;
}
return original_fopen(path, mode);
}
int open(const char *path, int oflag, ...) {
int (*original_open)(const char *, int, ...) = dlsym(RTLD_NEXT, "open");
int ret;
va_list args;
if (!strcmp(path, FROM)) {
path = TO;
}
va_start(args, oflag);
if (oflag & O_CREAT) {
ret = original_open(path, oflag, (mode_t)va_arg(args, mode_t));
} else {
ret = original_open(path, oflag);
}
va_end(args);
return ret;
}
Compile with the following command (that's for Linux, other Unix variants may require different options). Note the quotes around the path you want to override.
gcc -DFROM='"/some/path"' -DTO='"/dev/null"' -D_GNU_SOURCE -O -Wall -fPIC -shared -o override_fopen.so override_fopen.c -ldl
Run the program as follows (on OSX, use DYLD_PRELOAD
instead of LD_PRELOAD
):
LD_PRELOAD=./override_fopen.so ./myexe
This only works if the program is calling the fopen
or open
library function. If it calls some other function, you'll need to override that one. You can use ltrace
to see what library calls the program makes.