dlsym() returns address of a function in library X when provided the handle of library Y - devel

28 Feb 2020

Consider 3 libraries: `libA`, `libB` and `libC`. Only library `libB` 
implements
and exports the function `public_function()`.

A program `dlopen()`s `libA` then `libB` then `libC`. Then the program 
calls
`dlsym(libA, "public_function")` then `dlsym(libB,
"public_function")`
then `dlsym(libC, "public_function")`.

Expected behavior:
- `dlsym(libA, "public_function")` and `dlsym(libC,
"public_function")`
  return `NULL`, and
- `dlsym(libB, "public_function")` returns a non-`NULL` address.

Erroneous behavior:
- `dlsym(libC, "public_function")` return `NULL`, and
- `dlsym(libA, "public_function")` and `dlsym(libB,
"public_function")`
  returns **the same** non-`NULL` address.

Program to reproduce this issue:

```
// dlsym-test1.c

#include <stdio.h>
#include <dlfcn.h>

typedef void (*public_function_t)();

int main() {
    int i;
    void* lib[3] = {};
    void* lib_name[3] = {"liba.so", "libb.so", "libc.so"};
    public_function_t public_function[3] = {};

    for (i = 0; i < 3; ++i) {
        lib[i] = dlopen(lib_name[i], RTLD_NOW | RTLD_LOCAL);
    }

    for (i = 0; i < 3; ++i) {
        if (lib[i] != NULL) {
            public_function[i] = dlsym(lib[i], "public_function");
        }
    }

    for (i = 0; i < 3; ++i) {
        printf("%s:%#x public_function:%#x.\n",
               lib_name[i], lib[i], public_function[i]);
    }

    for (i = 0; i < 3; ++i) {
        if (public_function[i] != NULL) {
            (public_function[i])();
        }
    }

    return 0;
}
```

```
// libb.c

#include <stdio.h>

void public_function() __attribute__ ((visibility ("default")));

void public_function() {
    puts("libB::public_function()");
}
```

```
#!/bin/bash

CROSS_COMPILE=...

CC=${CROSS_COMPILE}gcc

CFLAGS='-fvisibility=hidden -O0 -g -Wextra -Werror'

rm *.so dlsym-test1

$CC $CFLAGS -fPIC -shared -o liba.so empty.c
$CC $CFLAGS -fPIC -shared -o libb.so libb.c
$CC $CFLAGS -fPIC -shared -o libc.so empty.c

$CC $CFLAGS -fPIE -pie '-Wl,-rpath=$ORIGIN' -o dlsym-test1 dlsym-test1.c
```

```
$ ./dlsym-test1
liba.so:0x80579160 public_function:0xb6eb9584.
libb.so:0x80579348 public_function:0xb6eb9584.
libc.so:0x80579520 public_function:0.
libB::public_function()
libB::public_function()
```

---
Dr. Koutheir Attouchi.