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From: "fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug dynamic-link/30424] New: Calling dlopen from preinit function
 inhibits proper libc initialization
Date: Fri, 05 May 2023 19:59:20 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D30424

            Bug ID: 30424
           Summary: Calling dlopen from preinit function inhibits proper
                    libc initialization
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

Reproducer:

#define _GNU_SOURCE
#include <dlfcn.h>
#include <gnu/lib-names.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

static void
preinit (int argc, char **argv, char **envp)
{
  printf ("environ: %p\n", environ);
  printf ("USER: %s\n", getenv ("USER"));
  if (argc !=3D 1)
    dlopen (LIBC_SO, RTLD_NOW);
}

static void (*const preinit_array []) (int, char **, char **)
     __attribute__ ((section (".preinit_array"), aligned (sizeof (void *)),
                     used)) =3D
{
  &preinit,
};

int
main (void)
{
  printf ("environ: %p\n", environ);
  printf ("USER: %s\n", getenv ("USER"));
}

Running it without any arguments:

environ: (nil)
USER: (null)
environ: 0x7ffe9b2dcc88
USER: fweimer

This is sort-of expected because at preinit time, libc is not fully initial=
ized
yet because the ELF constructors have not run.

Running with an argument triggers the dlopen:

environ: (nil)
USER: (null)
environ: (nil)
USER: (null)

This is unexpected.  The reason is that _dl_open (in ld.so) does not use the
original data from the kernel, but receives the values to use from dlopen (=
in
libc.so). But as libc.so is not fully initialized at preinit time, it cannot
initialize itself. The fact that ELF constructors have run is recorded, so =
the
full initialization is never performed.

I think we should keep passing down at least environ from libc.so to ld.so
because environ can change legitimately. Nowadays, we have __libc_early_ini=
t,
we can use it to get rid of ELF constructors and initialize libc.so fully
before application code runs. This should resolve the issue because
__libc_early_init runs before the preinit functions.

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