[bug#31112] Patching the default PATH of `su`

[Leo Famulari]

On Fri, Apr 06, 2018 at 10:01:57AM +0200, Ludovic Courtès wrote:
> Probably, yes.  It would be good to check how this affects
> mingetty/login, sshd, etc.

I've attached a patch that sets the variables in /etc/login.defs.

The resulting PATH only includes "system" packages, which are found in
/run/current-system and /run/setuid-programs.

It would be better if it also included the user-specific programs in
~/.guix-profile, but I don't think this is possible without patching
`su` to look up usernames.

Shell variables are not expanded by `su`, so using $HOME doesn't work.
And I'm not sure how to use /var/guix/profiles/per-user without making
`su` look up usernames.

Nevertheless, I think it's an improvement, although maybe it's less
confusing for the PATH to be totally wrong than merely missing the
user's packages. WDYT?

With the attached patch, I tested mingetty and agetty's login, as well
as OpenSSH sshd login, and everything seemed to work — this changes
should have no effect in those cases because it affects non-login shells
only.

> Note that libc also has its own default PATH value in <paths.h>:
> 
>   /* Default search path. */
>   #define     _PATH_DEFPATH   "/usr/bin:/bin"
>   /* All standard utilities path. */
>   #define     _PATH_STDPATH \
>           "/usr/bin:/bin:/usr/sbin:/sbin"
> 
> Does ‘su’ rely on this?  In a future rebuild cycle we could change these
> values, but /run/current-system/bin wouldn’t work on foreign distros, so
> it’s not clear there’s much to gain.

AFAICT, `su` doesn't use this. The relevant code is below; it hard-codes
the fall-back PATHs rather than refer to libc.

------
        /*
         * Create the PATH environmental variable and export it.
         */

        cp = getdef_str ((info->pw_uid == 0) ? "ENV_SUPATH" : "ENV_PATH");

        if (NULL == cp) {
                /* not specified, use a minimal default */
                addenv ((info->pw_uid == 0) ? 
"PATH=/sbin:/bin:/usr/sbin:/usr/bin" : "PATH=/bin:/usr/bin", NULL);
        } else if (strchr (cp, '=')) {
                /* specified as name=value (PATH=...) */
                addenv (cp, NULL);
        } else {
                /* only value specified without "PATH=" */
                addenv ("PATH", cp);
}
------

https://github.com/shadow-maint/shadow/blob/15be89f89d553c06d52453721ee8e9a8433cfdfd/libmisc/setupenv.c#L263

0001-system-Provide-a-fall-back-PATH-for-non-login-shells.patch
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