What were you trying to do that didn't work?

In case the user unset SHLVL variable then later replaces the shell through executing a command, ksh will segfault:

# unset -v SHLVL
# exec bash
Segmentation fault (core dumped)

The reason for this is SHLVL variable is getting decremented without any guard making sure it's still in the environment, see ksh-1.0.6/src/cmd/ksh93/bltins/misc.c sources:

 70 /*
 71  * 'exec' special builtin and 'redirect' builtin
 72  */
 :
 77 int    b_exec(int argc,char *argv[], Shbltin_t *context)
 78 {
 :
148                 /* if the main shell is about to be replaced, decrease SHLVL to cancel out a subsequent increase */
149                 if(!sh.realsubshell)
150                         (*SHLVL->nvalue.ip)--;

What is the impact of this issue to you?

Users unsetting SHLVL in the .profile file cannot login anymore.

This is a regression introduced sometimes after ksh-1.0.0~beta.1-3.el9.

Please provide the package NVR for which the bug is seen:

ksh-1.0.6-3.el9

How reproducible is this bug?:

Always, see above.