doas fix PATH inheritance

[Ted Unangst]

espie found a bug. we reset PATH in the parent too soon, and then it's not
possible to change it back via setenv. 

instead of trying to move code around, save a copy of the path before we mess
with it and make it available later. this lets setenv { PATH=$PATH } work.


Index: doas.c
===================================================================
RCS file: /cvs/src/usr.bin/doas/doas.c,v
retrieving revision 1.77
diff -u -p -r1.77 doas.c
--- doas.c      16 Jun 2019 18:16:34 -0000      1.77
+++ doas.c      17 Jun 2019 19:06:14 -0000
@@ -286,6 +286,7 @@ main(int argc, char **argv)
        const char *confpath = NULL;
        char *shargv[] = { NULL, NULL };
        char *sh;
+       const char *p;
        const char *cmd;
        char cmdline[LINE_MAX];
        char mypwbuf[_PW_BUF_LEN], targpwbuf[_PW_BUF_LEN];
@@ -401,6 +402,11 @@ main(int argc, char **argv)
 
                authuser(mypw->pw_name, login_style, rule->options & PERSIST);
        }
+
+       if ((p = getenv("PATH")) != NULL)
+               formerpath = strdup(p);
+       if (formerpath == NULL)
+               formerpath = "";
 
        if (unveil(_PATH_LOGIN_CONF, "r") == -1)
                err(1, "unveil");
Index: doas.h
===================================================================
RCS file: /cvs/src/usr.bin/doas/doas.h,v
retrieving revision 1.14
diff -u -p -r1.14 doas.h
--- doas.h      16 Jun 2019 18:16:34 -0000      1.14
+++ doas.h      17 Jun 2019 19:06:14 -0000
@@ -29,6 +29,8 @@ extern struct rule **rules;
 extern int nrules;
 extern int parse_errors;
 
+extern const char *formerpath;
+
 struct passwd;
 
 char **prepenv(const struct rule *, const struct passwd *,
Index: env.c
===================================================================
RCS file: /cvs/src/usr.bin/doas/env.c,v
retrieving revision 1.8
diff -u -p -r1.8 env.c
--- env.c       17 Jun 2019 16:01:26 -0000      1.8
+++ env.c       17 Jun 2019 19:06:14 -0000
@@ -28,6 +28,8 @@
 
 #include "doas.h"
 
+const char *formerpath;
+
 struct envnode {
        RB_ENTRY(envnode) node;
        const char *key;
@@ -198,8 +200,12 @@ fillenv(struct env *env, const char **en
                /* assign value or inherit from environ */
                if (eq) {
                        val = eq + 1;
-                       if (*val == '$')
-                               val = getenv(val + 1);
+                       if (*val == '$') {
+                               if (strcmp(val + 1, "PATH") == 0)
+                                       val = formerpath;
+                               else
+                                       val = getenv(val + 1);
+                       }
                } else {
                        val = getenv(name);
                }