So I updated a few packages(kernel-image, udev, systemd) for a computer that has been sitting idle for a few years. After the update, I am unable to login to tty with root or any other user. I get the login prompt and when I type in root, it throws me back to the login prompt. At first, I thought it was systemd so I removed systemd. Same problem. Boot into /bin/bash and looked around. For some reason, my syslog was missing so installed rsyslog. Looked at /etc/var/auth.log and noticed some pam related issues(see below). It said unable to dlopen pam_rootok.so, pam_unix.so, and a few others; said they was missing. So I make a sym links to /lib/x86_64-linux-gnu/security/pam* but it just gave me different errors "undefined symbol: pcre_free_study". Removed those symlinks, they didn't help. I reinstalled all the libpam* packages, libselinux, and some other packages but still no go. Looked around /etc/securetty, /etc/pam.d/ but didn't find anything wrong when compared to a i386 distro. Another forum said those pam errors are normal, I don't believe it because my machine didn't produce those errors until I updated and my i386 does not produce those errors. Any ideas? I am close to reinstalling. Guess I can do a upgrade to upgrade all packages.

Jul 22 15:13:12 localhost su[2605]: PAM unable to dlopen(pam_rootok.so): /lib/security/pam_rootok.so: cannot open shared object file: No such file or directory
Jul 22 15:13:12 localhost su[2605]: PAM adding faulty module: pam_rootok.so
Jul 22 15:13:12 localhost su[2605]: PAM unable to dlopen(pam_unix.so): /lib/security/pam_unix.so: cannot open shared object file: No such file or directory
Jul 22 15:13:12 localhost su[2605]: PAM adding faulty module: pam_unix.so
Jul 22 15:13:12 localhost su[2605]: PAM unable to dlopen(pam_gnome_keyring.so): /lib/x86_64-linux-gnu/libselinux.so.1: undefined symbol: pcre_free_study
Jul 22 15:13:12 localhost su[2605]: PAM adding faulty module: pam_gnome_keyring.so
Jul 22 15:13:12 localhost su[2605]: pam_authenticate: Authentication failure
Jul 22 15:13:12 localhost su[2605]: FAILED su for stan by root
Jul 22 15:13:12 localhost su[2605]: - /dev/console root:stan
Jul 22 15:13:36 localhost login[3143]: PAM unable to dlopen(pam_selinux.so): /lib/security/pam_selinux.so: cannot open shared object file: No such file or directory
Jul 22 15:13:36 localhost login[3143]: PAM adding faulty module: pam_selinux.so
Jul 22 15:13:36 localhost login[3143]: PAM unable to dlopen(pam_unix.so): /lib/security/pam_unix.so: cannot open shared object file: No such file or directory
Jul 22 15:13:36 localhost login[3143]: PAM adding faulty module: pam_unix.so
Jul 22 15:13:36 localhost login[3143]: PAM unable to dlopen(pam_gnome_keyring.so): /lib/x86_64-linux-gnu/libselinux.so.1: undefined symbol: pcre_free_study
Jul 22 15:13:36 localhost login[3143]: PAM adding faulty module: pam_gnome_keyring.so
Jul 22 15:13:39 localhost login[3149]: PAM unable to dlopen(pam_selinux.so): /lib/security/pam_selinux.so: cannot open shared object file: No such file or directory
Jul 22 15:13:39 localhost login[3149]: PAM adding faulty module: pam_selinux.so
Jul 22 15:13:39 localhost login[3149]: PAM unable to dlopen(pam_unix.so): /lib/security/pam_unix.so: cannot open shared object file: No such file or directory
Jul 22 15:13:39 localhost login[3149]: PAM adding faulty module: pam_unix.so
Jul 22 15:13:39 localhost login[3149]: PAM unable to dlopen(pam_gnome_keyring.so): /lib/x86_64-linux-gnu/libselinux.so.1: undefined symbol: pcre_free_study
Jul 22 15:13:39 localhost login[3149]: PAM adding faulty module: pam_gnome_keyring.so
Jul 22 15:13:40 localhost login[3143]: FAILED LOGIN (1) on '/dev/tty6' FOR 'UNKNOWN', Authentication failure
Jul 22 15:13:40 localhost login[3143]: pam_securetty(login:auth): unexpected response from failed conversation function
Jul 22 15:13:40 localhost login[3143]: pam_securetty(login:auth): cannot determine username
Jul 22 15:13:42 localhost login[3149]: FAILED LOGIN (1) on '/dev/tty1' FOR 'root', Authentication failure
Jul 22 15:13:42 localhost login[3154]: PAM unable to dlopen(pam_selinux.so): /lib/security/pam_selinux.so: cannot open shared object file: No such file or directory
Jul 22 15:13:42 localhost login[3154]: PAM adding faulty module: pam_selinux.so
Jul 22 15:13:42 localhost login[3154]: PAM unable to dlopen(pam_unix.so): /lib/security/pam_unix.so: cannot open shared object file: No such file or directory
Jul 22 15:13:42 localhost login[3154]: PAM adding faulty module: pam_unix.so
Jul 22 15:13:42 localhost login[3154]: PAM unable to dlopen(pam_gnome_keyring.so): /lib/x86_64-linux-gnu/libselinux.so.1: undefined symbol: pcre_free_study
Jul 22 15:13:42 localhost login[3154]: PAM adding faulty module: pam_gnome_keyring.so
Jul 22 15:13:43 localhost login[3143]: FAILED LOGIN (2) on '/dev/tty6' FOR 'UNKNOWN', Error in service module
Jul 22 15:13:43 localhost login[3143]: pam_securetty(login:auth): unexpected response from failed conversation function
Jul 22 15:13:43 localhost login[3143]: pam_securetty(login:auth): cannot determine username
Jul 22 15:13:44 localhost login[3154]: FAILED LOGIN (1) on '/dev/tty2' FOR 'stan', Authentication failure
Jul 22 15:13:46 localhost login[3143]: FAILED LOGIN (3) on '/dev/tty6' FOR 'UNKNOWN', Error in service module
Jul 22 15:13:46 localhost login[3143]: pam_securetty(login:auth): unexpected response from failed conversation function
Jul 22 15:13:46 localhost login[3143]: pam_securetty(login:auth): cannot determine username
Jul 22 15:13:49 localhost login[3143]: FAILED LOGIN (4) on '/dev/tty6' FOR 'UNKNOWN', Error in service module
Jul 22 15:13:49 localhost login[3143]: pam_securetty(login:auth): unexpected response from failed conversation function
Jul 22 15:13:49 localhost login[3143]: pam_securetty(login:auth): cannot determine username
Jul 22 15:13:51 localhost login[3149]: FAILED LOGIN (2) on '/dev/tty1' FOR 'root', Authentication failure
Jul 22 15:13:52 localhost login[3143]: FAILED LOGIN (5) on '/dev/tty6' FOR 'UNKNOWN', Error in service module
Jul 22 15:13:52 localhost login[3143]: TOO MANY LOGIN TRIES (5) on '/dev/tty6' FOR 'UNKNOWN'
Jul 22 15:13:52 localhost login[3143]: pam_mail(login:session): cannot determine username
Jul 22 15:13:56 localhost login[3154]: FAILED LOGIN (2) on '/dev/tty2' FOR 'stan', Authentication failure

What's the distro? Guessing with systemd, it's from the Redhat stable.

With most distros, this sort of mess is a cue to reinstall. It's next to impossible to trace as errors do not always point at the actual problem. RPMs are not very friendly to downgrades.

If you want to continue struggling, a kill or cure thing you could try is booting from an install disc, remove pam (rpm -evh --force) and install it (yum install) and anything it needs. Set passwords then for everyone. Back up now before you try this, as you may be locked out.

Debian AMD64 kernel. I've looked at other forums and other have had this problem. But I have yet to be able to fix it. I will reinstall after I try an upgrade. I'll wait to see if anybody else has any ideas for me to try first. I don't need to use the computer right away so it is good to troubleshoot things. I've already learned a few tricks already.

Here is my strace if anybody can understand it:
mprotect(0x7ff082269000, 4096, PROT_READ) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-auth", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1249, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-auth - aut"..., 4096) = 1249
open("/lib/x86_64-linux-gnu/security/pam_unix.so", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300)\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=60160, ...}) = 0
mmap(NULL, 2204672, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081e4a000
mprotect(0x7ff081e57000, 2097152, PROT_NONE) = 0
mmap(0x7ff082057000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0xd000) = 0x7ff082057000
mmap(0x7ff082059000, 46080, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff082059000
close(5) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 5
fstat(5, {st_mode=S_IFREG|0644, st_size=85659, ...}) = 0
mmap(NULL, 85659, PROT_READ, MAP_PRIVATE, 5, 0) = 0x7ff083c7e000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libcrypt.so.1", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300\f\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=35176, ...}) = 0
mmap(NULL, 2318848, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081c13000
mprotect(0x7ff081c1b000, 2093056, PROT_NONE) = 0
mmap(0x7ff081e1a000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x7000) = 0x7ff081e1a000
mmap(0x7ff081e1c000, 184832, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff081e1c000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0pb\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=138616, ...}) = 0
mmap(NULL, 2242672, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff0819ef000
mprotect(0x7ff081a0f000, 2097152, PROT_NONE) = 0
mmap(0x7ff081c0f000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x20000) = 0x7ff081c0f000
mmap(0x7ff081c11000, 6256, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff081c11000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libpcre.so.3", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0 \30\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=245352, ...}) = 0
mmap(NULL, 2340472, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff0817b3000
mprotect(0x7ff0817ef000, 2093056, PROT_NONE) = 0
mmap(0x7ff0819ee000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x3b000) = 0x7ff0819ee000
close(5) = 0
munmap(0x7ff083c7e000, 85659) = 0
munmap(0x7ff081e4a000, 2204672) = 0
munmap(0x7ff081c13000, 2318848) = 0
munmap(0x7ff0819ef000, 2242672) = 0
munmap(0x7ff0817b3000, 2340472) = 0
open("/lib/security/pam_unix.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
socket(PF_FILE, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 5
connect(5, {sa_family=AF_FILE, path="/dev/log"}, 110) = -1 ENOENT (No such file or directory)
close(5) = 0
socket(PF_FILE, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 5
connect(5, {sa_family=AF_FILE, path="/dev/log"}, 110) = -1 ENOENT (No such file or directory)
close(5) = 0
open("/lib/x86_64-linux-gnu/security/pam_deny.so", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\6\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=5896, ...}) = 0
mmap(NULL, 2101304, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081e63000
mprotect(0x7ff081e64000, 2093056, PROT_NONE) = 0
mmap(0x7ff082063000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0) = 0x7ff082063000
close(5) = 0
mprotect(0x7ff082063000, 4096, PROT_READ) = 0
open("/lib/x86_64-linux-gnu/security/pam_permit.so", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\6\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=5984, ...}) = 0
mmap(NULL, 2101320, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081c61000
mprotect(0x7ff081c62000, 2093056, PROT_NONE) = 0
mmap(0x7ff081e61000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0) = 0x7ff081e61000
close(5) = 0
mprotect(0x7ff081e61000, 4096, PROT_READ) = 0
open("/lib/x86_64-linux-gnu/security/pam_cap.so", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib/security/pam_cap.so", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\320\v\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=7744, ...}) = 0
mmap(NULL, 2103040, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081a5f000
mprotect(0x7ff081a61000, 2093056, PROT_NONE) = 0
mmap(0x7ff081c60000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x1000) = 0x7ff081c60000
close(5) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 5
fstat(5, {st_mode=S_IFREG|0644, st_size=85659, ...}) = 0
mmap(NULL, 85659, PROT_READ, MAP_PRIVATE, 5, 0) = 0x7ff083c7e000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/libcap.so.2", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\200\25\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=16872, ...}) = 0
mmap(NULL, 2112168, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff08185b000
mprotect(0x7ff08185f000, 2093056, PROT_NONE) = 0
mmap(0x7ff081a5e000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x3000) = 0x7ff081a5e000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libattr.so.1", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\24\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=18672, ...}) = 0
mmap(NULL, 2113896, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081656000
mprotect(0x7ff08165a000, 2093056, PROT_NONE) = 0
mmap(0x7ff081859000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x3000) = 0x7ff081859000
close(5) = 0
mprotect(0x7ff081859000, 4096, PROT_READ) = 0
munmap(0x7ff083c7e000, 85659) = 0
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-account", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1208, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-account - "..., 4096) = 1208
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-session", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1201, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-session - "..., 4096) = 1201
open("/lib/x86_64-linux-gnu/security/pam_ck_connector.so", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\340\r\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=10240, ...}) = 0
mmap(NULL, 2105592, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff081453000
mprotect(0x7ff081455000, 2093056, PROT_NONE) = 0
mmap(0x7ff081654000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x1000) = 0x7ff081654000
close(5) = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 5
fstat(5, {st_mode=S_IFREG|0644, st_size=85659, ...}) = 0
mmap(NULL, 85659, PROT_READ, MAP_PRIVATE, 5, 0) = 0x7ff083c7e000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libdbus-1.so.3", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\220\200\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=280552, ...}) = 0
mmap(NULL, 2376608, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff08120e000
mprotect(0x7ff081251000, 2093056, PROT_NONE) = 0
mmap(0x7ff081450000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x42000) = 0x7ff081450000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/usr/lib/x86_64-linux-gnu/libck-connector.so.0", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\240\17\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=14408, ...}) = 0
mmap(NULL, 2109672, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff08100a000
mprotect(0x7ff08100d000, 2093056, PROT_NONE) = 0
mmap(0x7ff08120c000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x2000) = 0x7ff08120c000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20o\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0755, st_size=137439, ...}) = 0
mmap(NULL, 2213008, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff080ded000
mprotect(0x7ff080e05000, 2093056, PROT_NONE) = 0
mmap(0x7ff081004000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x17000) = 0x7ff081004000
mmap(0x7ff081006000, 13456, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7ff081006000
close(5) = 0
access("/etc/ld.so.nohwcap", F_OK) = 0
open("/lib/x86_64-linux-gnu/librt.so.1", O_RDONLY|O_CLOEXEC) = 5
read(5, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0P#\0\0\0\0\0\0"..., 832) = 832
fstat(5, {st_mode=S_IFREG|0644, st_size=31784, ...}) = 0
mmap(NULL, 2128920, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 5, 0) = 0x7ff080be5000
mprotect(0x7ff080bec000, 2093056, PROT_NONE) = 0
mmap(0x7ff080deb000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 5, 0x6000) = 0x7ff080deb000
close(5) = 0
mprotect(0x7ff081004000, 4096, PROT_READ) = 0
mprotect(0x7ff080deb000, 4096, PROT_READ) = 0
mprotect(0x7ff081450000, 8192, PROT_READ) = 0
mprotect(0x7ff08120c000, 4096, PROT_READ) = 0
mprotect(0x7ff081654000, 4096, PROT_READ) = 0
set_tid_address(0x7ff083c949d0) = 4244
set_robust_list(0x7ff083c949e0, 0x18) = 0
rt_sigaction(SIGRTMIN, {0x7ff080df39f0, [], SA_RESTORER|SA_SIGINFO, 0x7ff080dfc8d0}, NULL, 8) = 0
rt_sigaction(SIGRT_1, {0x7ff080df3a80, [], SA_RESTORER|SA_RESTART|SA_SIGINFO, 0x7ff080dfc8d0}, NULL, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [RTMIN RT_1], NULL, 8) = 0
getrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM_INFINITY}) = 0
munmap(0x7ff083c7e000, 85659) = 0
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
read(3, "", 4096) = 0
close(3) = 0
munmap(0x7ff083cab000, 4096) = 0
open("/etc/pam.d/other", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=520, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083cab000
read(3, "#\n# /etc/pam.d/other - specify t"..., 4096) = 520
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-auth", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1249, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-auth - aut"..., 4096) = 1249
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-account", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1208, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-account - "..., 4096) = 1208
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-password", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1440, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-password -"..., 4096) = 1440
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
stat("/etc/pam.d", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
open("/etc/pam.d/common-session", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=1201, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083caa000
read(4, "#\n# /etc/pam.d/common-session - "..., 4096) = 1201
read(4, "", 4096) = 0
close(4) = 0
munmap(0x7ff083caa000, 4096) = 0
read(3, "", 4096) = 0
close(3) = 0
munmap(0x7ff083cab000, 4096) = 0
open("/etc/passwd", O_RDONLY|O_CLOEXEC) = 3
lseek(3, 0, SEEK_CUR) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=2440, ...}) = 0
mmap(NULL, 2440, PROT_READ, MAP_SHARED, 3, 0) = 0x7ff083cab000
lseek(3, 2440, SEEK_SET) = 2440
munmap(0x7ff083cab000, 2440) = 0
close(3) = 0
rt_sigaction(SIGINT, {SIG_IGN, [INT], SA_RESTORER|SA_RESTART, 0x7ff083307480}, {SIG_DFL, [], 0}, 8) = 0
rt_sigaction(SIGQUIT, {SIG_IGN, [QUIT], SA_RESTORER|SA_RESTART, 0x7ff083307480}, {SIG_IGN, [], 0}, 8) = 0
socket(PF_NETLINK, SOCK_RAW, 9) = 3
fcntl(3, F_SETFD, FD_CLOEXEC) = 0
readlink("/proc/self/exe", "/bin/su", 4096) = 7
sendto(3, "p\0\0\0L\4\5\0\1\0\0\0\0\0\0\0op=PAM:authentic"..., 112, 0, {sa_family=AF_NETLINK, pid=0, groups=00000000}, 12) = 112
poll([{fd=3, events=POLLIN}], 1, 500) = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "$\0\0\0\2\0\0\0\1\0\0\0\224\20\0\0\0\0\0\0p\0\0\0L\4\5\0\1\0\0\0"..., 8988, MSG_PEEK|MSG_DONTWAIT, {sa_family=AF_NETLINK, pid=0, groups=00000000}, [12]) = 36
recvfrom(3, "$\0\0\0\2\0\0\0\1\0\0\0\224\20\0\0\0\0\0\0p\0\0\0L\4\5\0\1\0\0\0"..., 8988, MSG_DONTWAIT, {sa_family=AF_NETLINK, pid=0, groups=00000000}, [12]) = 36
close(3) = 0
socket(PF_FILE, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 3
connect(3, {sa_family=AF_FILE, path="/dev/log"}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
write(2, "su: Authentication failure\n", 27) = 27
munmap(0x7ff08246e000, 2109672) = 0
munmap(0x7ff08226b000, 2105528) = 0
munmap(0x7ff082065000, 2118024) = 0
munmap(0x7ff081e63000, 2101304) = 0
munmap(0x7ff081c61000, 2101320) = 0
munmap(0x7ff081a5f000, 2103040) = 0
munmap(0x7ff08185b000, 2112168) = 0
munmap(0x7ff081656000, 2113896) = 0
munmap(0x7ff081453000, 2105592) = 0
munmap(0x7ff08100a000, 2109672) = 0
munmap(0x7ff08120e000, 2376608) = 0
socket(PF_FILE, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 3
connect(3, {sa_family=AF_FILE, path="/dev/log"}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
open("/etc/login.defs", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=10478, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7ff083cab000
read(3, "#\n# /etc/login.defs - Configurat"..., 4096) = 4096
read(3, " issuing \n# the \"mesg y\" command"..., 4096) = 4096
read(3, "It supports passwords of unlimit"..., 4096) = 2286
read(3, "", 4096) = 0
close(3) = 0
munmap(0x7ff083cab000, 4096) = 0
socket(PF_FILE, SOCK_DGRAM|SOCK_CLOEXEC, 0) = 3
connect(3, {sa_family=AF_FILE, path="/dev/log"}, 110) = -1 ENOENT (No such file or directory)
close(3) = 0
exit_group(1) = ?

Solved. aptitiude upgrade and now it is logging in ok. Blame it on Debian's poor dependency system.

Lovely. Exactly what did you do? It will help others who search.