Get from_current working in openbsd

Author: jeremy-rifkin

.github/workflows/ci.yml

@@ -1017,7 +1017,8 @@ jobs:
             -DCMAKE_BUILD_TYPE=Debug \
             -DCPPTRACE_WERROR_BUILD=On \
             -DCPPTRACE_STD_FORMAT=Off \
-            -DCPPTRACE_BUILD_TESTING=On
+            -DCPPTRACE_BUILD_TESTING=On \
+            -DCMAKE_EXE_LINKER_FLAGS="-Wl,-z,norelro"
           ninja
           ./unittest
 

src/from_current.cpp

@@ -294,13 +294,28 @@ namespace detail {
         if(type_info_addr - page_addr + sizeof(void*) > static_cast<unsigned>(page_size)) {
             throw internal_error("pointer crosses page boundaries");
         }
+        #if IS_WINDOWS
         auto old_protections = mprotect_page_and_return_old_protections(
             reinterpret_cast<void*>(page_addr),
             page_size,
             memory_readwrite
         );
         *static_cast<void**>(type_info_pointer) = static_cast<void*>(new_vtable + 2);
         mprotect_page(reinterpret_cast<void*>(page_addr), page_size, old_protections);
+        #else
+        auto old_protections = get_page_protections(reinterpret_cast<void*>(page_addr));
+        // If the page is already writable, skip mprotect entirely.
+        // This isn't just an optimization, it's needed on openbsd where mprotect would fail with EPERM but we can use
+        // -Wl,-z,norelro to make the pages we care about writable
+        bool need_mprotect = !(old_protections & PROT_WRITE);
+        if(need_mprotect) {
+            mprotect_page(reinterpret_cast<void*>(page_addr), page_size, memory_readwrite);
+        }
+        *static_cast<void**>(type_info_pointer) = static_cast<void*>(new_vtable + 2);
+        if(need_mprotect) {
+            mprotect_page(reinterpret_cast<void*>(page_addr), page_size, old_protections);
+        }
+        #endif
     }
 
     bool can_catch(

src/platform/memory_mapping.hpp

@@ -21,6 +21,9 @@ namespace detail {
      constexpr auto memory_readwrite = PROT_READ | PROT_WRITE;
     #endif
     int get_page_size();
+    #if !IS_WINDOWS
+     int get_page_protections(void* page);
+    #endif
     int mprotect_page_and_return_old_protections(void* page, int page_size, int protections);
     void mprotect_page(void* page, int page_size, int protections);
     void* allocate_page(int page_size);