[PATCH v3 for-6.0 2/2] tcg: Workaround macOS 11.2 mprotect bug

[Richard Henderson <richard.henderson@linaro.org>]

There's a change in mprotect() behaviour [1] in the latest macOS
on M1 and it's not yet clear if it's going to be fixed by Apple.

As a short-term fix, ignore failures setting up the guard pages.

[1] https://gist.github.com/hikalium/75ae822466ee4da13cbbe486498a191f

Buglink: https://bugs.launchpad.net/qemu/+bug/1914849
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/tcg.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tcg/tcg.c b/tcg/tcg.c
index 88c9e6f8a4..1fbe0b686d 100644
--- a/tcg/tcg.c
+++ b/tcg/tcg.c
@@ -864,11 +864,15 @@ void tcg_region_init(void)
      */
     for (i = 0; i < region.n; i++) {
         void *start, *end;
-        int rc;
 
         tcg_region_bounds(i, &start, &end);
-        rc = qemu_mprotect_none(end, page_size);
-        g_assert(!rc);
+
+        /*
+         * macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect
+         * rejects a permission change from RWX -> NONE.  Guard pages are
+         * nice for bug detection but are not essential; ignore any failure.
+         */
+        (void)qemu_mprotect_none(end, page_size);
     }
 
     tcg_region_trees_init();
-- 
2.25.1