altcall: fix __alt_call_maybe_initdata so it's safe for livepatch

Setting alternative call variables as __init is not safe for use with
livepatch, as livepatches can rightfully introduce new alternative calls to
structures marked as __alt_call_maybe_initdata (possibly just indirectly due to
replacing existing functions that use those).  Attempting to resolve those
alternative calls then results in page faults as the variable that holds the
function pointer address has been freed.

When livepatch is supported use the __ro_after_init attribute instead of
__initdata for __alt_call_maybe_initdata.

Fixes: f26bb28594 ('xen: Implement xen/alternative-call.h for use in common code')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: af4cd0a6a6
master date: 2024-04-11 18:51:36 +0100

xen/include/xen/alternative-call.h

@@ -50,7 +50,12 @@
 
 #include <asm/alternative.h>
 
-#define __alt_call_maybe_initdata __initdata
+#ifdef CONFIG_LIVEPATCH
+/* Must keep for livepatches to resolve alternative calls. */
+# define __alt_call_maybe_initdata __ro_after_init
+#else
+# define __alt_call_maybe_initdata __initdata
+#endif
 
 #else