What are link maps in libdl and why they crash my app?

Question

Recently I tried to build a complex C++ application. App was built. But when I tried to launch it it crashed with illegal memory access. Actually it was null pointer with offset. I started to investigate the reason. Crash happens while dynamically loading a library (libXcursor.so) with dlopen. So the reason was null pointer link_map->l_versions which wasn't checked in the process of relocation inside elf_machine_rela function (particularry in RESOLVE_MAP macro). This happens here: map->l_versions[ndx]

          ElfW(Half) ndx = version[ELFW(R_SYM) (r->r_info)] & 0x7fff;
          elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)],
                   &map->l_versions[ndx],
                   (void *) (l_addr + r->r_offset), skip_ifunc);

Aforementioned link_map is a dependency description of a library in loading process. This dependency was not completely initialized as I can see. But what is more strange is that this is not actually a dependency: it's a libgthread library DSO. And libXcursor.so indeed does not depend on it.

The mentioned link_map of libgthread is passed for relocation from dl_open_worker:

    _dl_relocate_object (l, l->l_scope, reloc_mode, 0);

Beeing collected to the list of dependecies this way:

  l = new;
  do
    {
      if (! l->l_real->l_relocated)
    maps[nmaps++] = l;
      l = l->l_next;
    }
  while (l != NULL);

Here new is the found link_map of libXcursor.so itself. And this piece of code is very strange IMHO. Because at the same time other deps are actually initialized with this code:

  /* Load that object's dependencies.  */
  _dl_map_object_deps (new, NULL, 0, 0,
               mode & (__RTLD_DLOPEN | RTLD_DEEPBIND | __RTLD_AUDIT));

  /* So far, so good.  Now check the versions.  */
  for (unsigned int i = 0; i < new->l_searchlist.r_nlist; ++i)
    if (new->l_searchlist.r_list[i]->l_real->l_versions == NULL)
      (void) _dl_check_map_versions (new->l_searchlist.r_list[i]->l_real,
                     0, 0);

So question is why is other list of dependencies passed and no additional checks are made in such an important place (system loader). And probably where to fix the crash.

Used libdl's version is /lib64/libdl-2.27.so from glibc-2.27-38.fc28.x86_64

Seems like problem was fixed in here, here, there and there. It does not reproduces. Small app can't reproduce original crash with version 2.27 either.

To have any hope of getting an answer, try to create stackoverflow.com/help/mcve — Employed Russian
– Employed Russian, Commented Feb 10, 2024 at 14:22
I'll try to launch app in upgraded environment. Seems like something was changed in glibc ver 2.32 regarding dependency checks. But that requires installing chroot with newer system at least. --- As for minimal example: I'll try to reproduce the same in a small app. The sequence is clear: load smth like libgdk with deps on libgthread and libXcursor, then load libXcursor again. — Anton Astafiev
– Anton Astafiev, Commented Feb 10, 2024 at 15:47

Anton Astafiev · Accepted Answer · 2024-02-22 22:42:27Z

So as a result of trying with new glibc we have the following.

Seems like problem was fixed in here, here, there and there. It is not reproduced.

A little further analysis showed that the culprit of the crash was library libqt5_shim.so. The app in question is chromium of latest version. So libqt5_shim.so was its freshly build dependency which in turn depends on Qt5Core, Qt5Gui and Qt5Widgets. For some reason it was build with expectation of having version 5.15 of Qt. But the installed system Qt has just version 5.11. So the loading process looked like this

    file=libQt5Core.so.5 [0];  needed by /mnt/t/develop/chromium/chromium/src/out/default/libqt5_shim.s
    file=libQt5Core.so.5 [0];  generating link map
    file=libpcre2-16.so.0 [0];  needed by /lib64/libQt5Core.so.5 [0]
    ...
    file=libgthread-2.0.so.0 [0];  needed by /lib64/libQt5Core.so.5 [0]
    /lib64/libQt5Core.so.5: error: version lookup error: version `Qt_5.15' not found (required by /mnt/t/develop/chromium/chromium/src/out/default/libqt5_shim.so) (fatal)
    file=/mnt/t/develop/chromium/chromium/src/out/default/libqt5_shim.so [0];  destroying link map
    file=/lib64/libQt5Gui.so.5 [0];  destroying link map
    file=/lib64/libQt5Core.so.5 [0];  destroying link map
    .... ;  destroying link map
    file=/lib64/libgthread-2.0.so.0 [0];  destroying link map
    .... further destroying
    file=/mnt/t/develop/chromium/chromium/src/out/default/libqt6_shim.so [0];  dynamically loaded by /mnt/t/develop/chromium/chromium/src/out/default/libui_qt.so [0]
    file=/mnt/t/develop/chromium/chromium/src/out/default/libqt6_shim.so [0];  generating link map

After that I suppose there were some dangling references in the dl linker map. Further work was incorrect. libdl was finding incorrect dependencies. As for reproducable example, it also crashes if Qt5Core is of inappropriate version but for another reason: while doing init of libpthread it catches SEGFAULT. Here is the code

#include <dlfcn.h>
#include <iostream>

int main() {
    void* res = dlopen("libqt5_shim.so", RTLD_LAZY);
    std::cout << "Open libQt5Core res=" << res << std::endl;
//  res = dlopen("libgdk-3.so.0", RTLD_LAZY);
//  std::cout << "Open libgdk res=" << res << std::endl;
    res = dlopen("libXcursor.so.1", RTLD_LAZY);
    std::cout << "Open libXcursor res=" << res << std::endl;
}

Deleting the system Qt5Core partially solves the problem for a while. Both crahses don't reproduce with glibc-2.32 from chrooted Fedora 33 installation.

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