[Bug dynamic-link/27646] Linker error for non-existing NSS symbols (e.g. _nss_files_getcanonname_r) from within a dlmopen namespace.

["mbhangui at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27646

--- Comment #8 from cprogrammer <mbhangui at gmail dot com> ---
This is still an issue. Though it solves functions like gethostbyname crashing,
getpwent and family still crash with SIGSEGV. Details below on how to simulate
it. Setting LD_DEBUG to unused prevents the crash. Also modifying
/etc/nsswitch.conf to have passwd obtained only from files prevents the crash
(removing sss).

cat <<EOF>foo.c
#include <stdio.h>
#include <pwd.h>
void bar1()
{
  struct passwd *pw;
  pw = getpwent();
  if (pw)
    printf("%s\n", pw->pw_name);
}
EOF
cat <<EOF>test.c
#define _GNU_SOURCE
#include <dlfcn.h>
#include <stdio.h>
int
main(int argc, char **argv)
{
  void *handle;
  int (*func) ();

  if (!(handle = dlmopen(LM_ID_NEWLM, "./libfoo.so", RTLD_NOW))) {
                perror("dlmopen");
                return 1;
  }
  func = dlsym(handle, "bar1");
  (*func)();
  return 0;
}
EOF

$ gcc -fPIC -shared foo.c -o libfoo.so
$ gcc test.c -o test -ldl
$ ./test
Segmentation fault (core dumped)

$ env LD_DEBUG=unused ./test
root

$ rpm -qf /etc/nsswitch.conf
glibc-2.33-18.fc34.x86_64

$ cat /etc/fedora-release
Fedora release 34 (Thirty Four)

$ egrep -v "^#|^$" /etc/nsswitch.conf
passwd:     sss files systemd
group:      sss files systemd
netgroup:   sss files
automount:  sss files
services:   sss files
shadow:     files sss
hosts:      files myhostname mdns4_minimal [NOTFOUND=return] resolve
[!UNAVAIL=return] dns
bootparams: files
ethers:     files
netmasks:   files
networks:   files
protocols:  files
rpc:        files
publickey:  files
aliases:    files

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