Sunday, January 28, 2024

[SOLVED] why isn't shared library fini function called when preloaded to dash?

Issue

I am on the latest Ubuntu Linux.

Here is a shared library with functions that are called when loading and unloading:

shared.c:

#include <fcntl.h>
#include <sys/stat.h>

void init() {
    open("init", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
}

void fini() {
    open("fini", O_RDWR | O_CREAT, S_IRUSR | S_IWUSR);
}

and this is compiled with

gcc -shared -fPIC -Wl,-init=init -Wl,-fini=fini shared.c -o shared.so

and then I do this:

$ rm init fini
$ LD_PRELOAD=$PWD/shared.so  dash /dev/null
$ echo $?
0
$ ls init
init
$ ls fini
ls: cannot access 'fini': No such file or directory

So... loading function is called, but unloading function is not

If I replace dash with bash , both are called.

Using __attribute__((destructor)) does not make a difference.

Why isn't the unloading function called for dash?

Added per Marco Bonelli's request:

$ file $(which dash)
/usr/bin/dash: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, BuildID[sha1]=f7ab02fc1b8ff61b41647c1e16ec9d95ba5de9f0, for GNU/Linux 3.2.0, stripped

$ ldd $(which dash)
        linux-vdso.so.1 (0x00007ffd931c0000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6b19e84000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f6b1a0ec000)

Solution

why isn't shared library fini function called when preloaded to dash?

Because dash calls _exit here which calls exit_group syscall here which terminates the program immediately.

Let's also do a small example:

# Makefile
all:
        gcc -shared -fPIC -Wl,-init=init -Wl,-fini=fini -Wl,-soname,shared.so shared.c -o shared.so
        gcc main.c -o main.out
        LD_PRELOAD=$(PWD)/shared.so ./main.out /dev/null

# main.c
#include <unistd.h>
int main() {
        _exit(0);
}

# shared.c
#include <fcntl.h>
#include <sys/stat.h>
#include <unistd.h>
#include <string.h>
void out(const char *str) {
        int fd = open("/dev/tty", O_WRONLY);
        write(fd, str, strlen(str));
        close(fd);
}
void init() {
        out("\nstart\n\n");
}
void fini() {
        out("\nstop\n\n");
}

On execution does not output stop:

$ make
gcc -shared -fPIC -Wl,-init=init -Wl,-fini=fini -Wl,-soname,shared.so shared.c -o shared.so
gcc main.c -o main.out
LD_PRELOAD=..../shared.so ./main.out /dev/null

start


Answered By - KamilCuk
Answer Checked By - Marilyn (WPSolving Volunteer)
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