[PATCH v3 27/28] tcg: When allocating for !splitwx, begin with PROT_NONE

[Richard Henderson]

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.

In this case, instead of changing permissions of N guard pages,
we change permissions of N rwx regions.  The same number of
syscalls are required either way.

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

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/region.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/tcg/region.c b/tcg/region.c
index 604530b902..5e00db4cfb 100644
--- a/tcg/region.c
+++ b/tcg/region.c
@@ -765,12 +765,15 @@ static int alloc_code_gen_buffer(size_t size, int 
splitwx, Error **errp)
         error_free_or_abort(errp);
     }
 
-    prot = PROT_READ | PROT_WRITE | PROT_EXEC;
+    /*
+     * macOS 11.2 has a bug (Apple Feedback FB8994773) in which mprotect
+     * rejects a permission change from RWX -> NONE when reserving the
+     * guard pages later.  We can go the other way with the same number
+     * of syscalls, so always begin with PROT_NONE.
+     */
+    prot = PROT_NONE;
     flags = MAP_PRIVATE | MAP_ANONYMOUS;
-#ifdef CONFIG_TCG_INTERPRETER
-    /* The tcg interpreter does not need execute permission. */
-    prot = PROT_READ | PROT_WRITE;
-#elif defined(CONFIG_DARWIN)
+#ifdef CONFIG_DARWIN
     /* Applicable to both iOS and macOS (Apple Silicon). */
     if (!splitwx) {
         flags |= MAP_JIT;
@@ -901,11 +904,7 @@ void tcg_region_init(size_t tb_size, int splitwx, unsigned 
max_cpus)
             }
         }
         if (have_prot != 0) {
-            /*
-             * 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.
-             */
+            /* Guard pages are nice for bug detection but are not essential. */
             (void)qemu_mprotect_none(end, page_size);
         }
     }
-- 
2.25.1