[PATCH v2] altcall: fix __alt_call_maybe_initdata so it's safe for livepatch

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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: f26bb285949b ('xen: Implement xen/alternative-call.h for use in common 
code')
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Use #ifdef instead of #ifndef.
---
 xen/include/xen/alternative-call.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/xen/include/xen/alternative-call.h 
b/xen/include/xen/alternative-call.h
index 5c6b9a562b92..10f7d7637e1e 100644
--- a/xen/include/xen/alternative-call.h
+++ b/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
 
-- 
2.44.0