Make sysctl user.local a tunable that can be written at run-time

This sysctl value had been provided as a read-only variable that is
compiled into the C library based on the value of _PATH_LOCALBASE in
paths.h.

After this change, the value is compiled into the kernel as an empty
string, which is translated to _PATH_LOCALBASE by the C library.

This empty string can be overridden at boot time or by a privileged
user at run time and will then be returned by sysctl.

When set to an empty string, the value returned by sysctl reverts to
_PATH_LOCALBASE.

This update does not change the behavior on any system that does
not modify the default value of user.localbase.

I consider this change as experimental and would prefer if the run-time
write permission was reconsidered and the sysctl variable defined with
CLFLAG_RDTUN instead to restrict it to be set at boot time.

MFC after:	1 month

1 files changed, 18 insertions, 10 deletions

diff --git a/lib/libc/gen/sysctl.c b/lib/libc/gen/sysctl.c
index ac6140013038..6015d68e8c60 100644
--- a/lib/libc/gen/sysctl.c
+++ b/lib/libc/gen/sysctl.c
@@ -68,14 +68,14 @@ sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
 	if (retval || name[0] != CTL_USER)
 		return (retval);
 
-	if (newp != NULL) {
-		errno = EPERM;
-		return (-1);
-	}
 	if (namelen != 2) {
 		errno = EINVAL;
 		return (-1);
 	}
+	if (newp != NULL && name[1] != USER_LOCALBASE) {
+		errno = EPERM;
+		return (-1);
+	}
 
 	switch (name[1]) {
 	case USER_CS_PATH:
@@ -88,13 +88,21 @@ sysctl(const int *name, u_int namelen, void *oldp, size_t *oldlenp,
 			memmove(oldp, _PATH_STDPATH, sizeof(_PATH_STDPATH));
 		return (0);
 	case USER_LOCALBASE:
-		if (oldp != NULL && orig_oldlen < sizeof(_PATH_LOCALBASE)) {
-			errno = ENOMEM;
-			return (-1);
+		if (oldlenp != NULL) {
+			if (oldp == NULL) {
+				if (*oldlenp == 1)
+					*oldlenp = sizeof(_PATH_LOCALBASE);
+			} else {
+				if (*oldlenp != 1)
+					return (retval);
+				if (orig_oldlen < sizeof(_PATH_LOCALBASE)) {
+					errno = ENOMEM;
+					return (-1);
+				}
+				*oldlenp = sizeof(_PATH_LOCALBASE);
+				memmove(oldp, _PATH_LOCALBASE, sizeof(_PATH_LOCALBASE));
+			}
 		}
-		*oldlenp = sizeof(_PATH_LOCALBASE);
-		if (oldp != NULL)
-			memmove(oldp, _PATH_LOCALBASE, sizeof(_PATH_LOCALBASE));
 		return (0);
 	}