iOS: Use PROT_READ, not PROT_NONE, for custom stack memory guard

This allows V4 and other garbage collectors to pass through our
custom stack during the mark-phase without triggering access
violations.

Change-Id: Icafcf4df3537c628c641fe694bb9fe2016519a83
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/plugins/platforms/ios/qioseventdispatcher.mm b/src/plugins/platforms/ios/qioseventdispatcher.mm
index 6148338000..3de7c996f5 100644
--- a/src/plugins/platforms/ios/qioseventdispatcher.mm
+++ b/src/plugins/platforms/ios/qioseventdispatcher.mm
@@ -148,8 +148,10 @@ namespace
             // Add memory guard at the end of the reserved stack, so that any stack
             // overflow during the user's main will trigger an exception at that point,
             // and not when we return and find that the current stack has been smashed.
+            // We allow read though, so that garbage-collection can pass through our
+            // stack in its mark phase without triggering access violations.
             uintptr_t memoryGuardStart = qAlignUp(memoryStart, kPageSize);
-            if (mprotect((void*)memoryGuardStart, kPageSize, PROT_NONE))
+            if (mprotect((void*)memoryGuardStart, kPageSize, PROT_READ))
                 qWarning() << "Failed to add memory guard:" << strerror(errno);
 
             // We don't consider the memory guard part of the usable stack space