Bug#1108549: procps: w segfaults when run as non-root user

[Andrew Bower]

Hi,

On Mon, Jun 30, 2025 at 11:36:59PM +0100, Andrew Bower wrote:
> Running 'w' as a non-root user on i386 segfaults:
> 
> $ gdb w
> GNU gdb (Debian 16.3-1) 16.3
> [...]
> This GDB was configured as "i686-linux-gnu".
> [...]
> (gdb) run
> Starting program: /usr/bin/w
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
>  23:21:56 up 4 min,  0 users,  load average: 0.74, 1.06, 0.52
> USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU  WHAT
> 
> Program received signal SIGSEGV, Segmentation fault.
> Download failed: Invalid argument.  Continuing without source file ./src/w.c.
> 0x00401979 in main (argc=<optimized out>, argv=<optimized out>) at src/w.c:1130
> warning: 1130   src/w.c: No such file or directory
>
> This reliably reproduces in the failing environment but not when run as root or
> on an amd64 host with kernel 6.12.27-1.

The proximate cause seems to be that sessions is undefined and that
sessions_list is then also used, undefined. This part of the bug is
therefore not architecture-specific even if the crash is not generally
reproduced yet.

I attach a patch that fixes the segfault to advance the discussion.
Personally I think this ought to be fixed for trixie.

However, we are unfortunately not out of the woods: elogind is running
on this host and root is able to see the sessions. So is 'who' as
non-root with the newly libsystemd-ised coreutils:

$ w
 21:26:33 up 10 min,  0 users,  load average: 0.00, 0.10, 0.12
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU  WHAT
Segmentation fault
$ debian/procps/usr/bin/w # with patch
 21:26:42 up 11 min,  0 users,  load average: 0.00, 0.10, 0.12
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU  WHAT
$ sudo w
Please touch the FIDO authenticator.
 21:26:50 up 11 min,  1 user,  load average: 0.64, 0.23, 0.16
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU  WHAT
ajb85    pts/0    -                21:26    0.00s  0.03s  0.12s sudo w
$ sudo debian/procps/usr/bin/w # with patch
 21:26:54 up 11 min,  1 user,  load average: 0.59, 0.23, 0.16
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU  WHAT
ajb85    pts/0    -                21:26    0.00s  0.02s  0.06s sudo debian/procps/usr/bin/w
$ who
ajb85    seat0        2025-07-23 21:17
ajb85    tty1         2025-07-23 21:17

This secondary issue could of course be an elogingd problem so I am
copying this message to the elogind package tracker for awareness.

Thanks!
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