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environ.7: Document that HOME, LOGNAME, SHELL, USER are set at login time 

Clearly document that HOME, LOGNAME, SHELL and USER are set at
login time by a program like such as login(1).

Document also that using su could result in a mixed environment,
and point to the su(1) manual page.

[mtk: edited commit message]

Signed-off-by: Bastien Roucariès <rouca@debian.org>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Bastien Roucariès 2021-01-29 23:29:40 +00:00 committed by Michael Kerrisk
parent b6cb76fb6a
commit 37147e18b6
1 changed files with 30 additions and 4 deletions
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man7/environ.7
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@ -65,15 +65,15 @@ Common examples are:
.TP
.B USER
The name of the logged-in user (used by some BSD-derived programs).
Set at login time, see section NOTES below.
.TP
.B LOGNAME
The name of the logged-in user (used by some System-V derived programs).
Set at login time, see section NOTES below.
.TP
.B HOME
A user's login directory, set by
.BR login (1)
from the password file
.BR passwd (5).
A user's login directory, set a login time.
Set at login time, see section NOTES below.
.TP
.B LANG
The name of a locale to use for locale categories when not overridden
@ -131,6 +131,7 @@ Set by some shells.
.TP
.B SHELL
The absolute pathname of the user's login shell.
Set at login time, see section NOTES below.
.TP
.B TERM
The terminal type for which output is to be prepared.
@ -277,6 +278,30 @@ The
and
.B PR_SET_MM_ENV_END
operations can be used to control the location of the process's environment.
.PP
The
.B HOME,
.B LOGNAME,
.B SHELL
and
.B USER
variables are only set when an user is changing using
session management interface, typically by program
.B login(1)
from user database (for instance, but not limited, by using
.B password (5)
database).
Particularly,
.BR setuid (2)
family of function
does not set theses variables. Notes that as documented,
going to root by
.BR su (8)
may result in a mixed environment where
.B LOGNAME
and
.B USER
are retained from old user.
.SH BUGS
Clearly there is a security risk here.
Many a system command has been
@ -322,6 +347,7 @@ should consider renaming their option to
.BR login (1),
.BR printenv (1),
.BR sh (1),
.BR su (1),
.BR tcsh (1),
.BR execve (2),
.BR clearenv (3),