[PATCH] Completely honor prctl(PR_SET_KEEPCAPS, 1)

[Dax Kelson]

Originally when a process set*uided all capabilities bits were cleared.  
Then sometime later (wish BK went back 3 years), the behaviour was
modified according to the comment "A process may, via prctl(), elect to
keep its capabilites when it calls setuid() and switches away from
uid==0. Both permitted and effective sets will be retained."

The current behavior/implementation doesn't match the comment. Only 
permitted capabilities are retained.

This patch against 2.4.18-3 (RHL7.3 kernel, should apply against stock)  
fixes it.  Now both permitted and effective capabilities are retained.

--- kernel/sys.c-org	Wed May  8 01:20:37 2002
+++ kernel/sys.c	Wed May  8 01:35:04 2002
@@ -482,7 +482,7 @@
 		cap_clear(current->cap_permitted);
 		cap_clear(current->cap_effective);
 	}
-	if (old_euid == 0 && current->euid != 0) {
+	if ((old_euid == 0 && current->euid != 0) && !current->keep_capabilities) {
 		cap_clear(current->cap_effective);
 	}
 	if (old_euid != 0 && current->euid == 0) {