[PATCH] D89231: [libunwind] [SEH] Don't interact with foreign exceptions

[Martin Storsjö via Phabricator via llvm-commits]

mstorsjo created this revision.
mstorsjo added reviewers: cdavis5x, rnk, compnerd, libunwind.
Herald added projects: LLVM, libunwind.
Herald added a subscriber: llvm-commits.
Herald added 1 blocking reviewer(s): libunwind.
mstorsjo requested review of this revision.

This unfortunately means that we don't execute C++ destructors when unwinding past such frames for a different SEH unwind purpose (e.g. as part of setjmp/longjmp), but that case isn't handled properly at the moment (the original unwind intent is lost and we end up with an unhandled exception). This patch makes sure the foreign unwind terminates as intended.

After executing a handler, _Unwind_Resume doesn't have access to the target frame parameter of the original foreign unwind. We also currently blindly set ExceptionCode to STATUS_GCC_THROW - we could set that correctly by storing the original code in _GCC_specific_handler, but we don't have access to the original target frame value.

This also matches what libgcc's SEH unwinding code does in this case.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D89231

Files:
  libunwind/src/Unwind-seh.cpp


Index: libunwind/src/Unwind-seh.cpp
===================================================================
--- libunwind/src/Unwind-seh.cpp
+++ libunwind/src/Unwind-seh.cpp
@@ -46,18 +46,6 @@
 /// handling.
 #define STATUS_GCC_UNWIND MAKE_GCC_EXCEPTION(1) // 0x21474343
 
-/// Class of foreign exceptions based on unrecognized SEH exceptions.
-static const uint64_t kSEHExceptionClass = 0x434C4E4753454800; // CLNGSEH\0
-
-/// Exception cleanup routine used by \c _GCC_specific_handler to
-/// free foreign exceptions.
-static void seh_exc_cleanup(_Unwind_Reason_Code urc, _Unwind_Exception *exc) {
-  (void)urc;
-  if (exc->exception_class != kSEHExceptionClass)
-    _LIBUNWIND_ABORT("SEH cleanup called on non-SEH exception");
-  free(exc);
-}
-
 static int __unw_init_seh(unw_cursor_t *cursor, CONTEXT *ctx);
 static DISPATCHER_CONTEXT *__unw_seh_get_disp_ctx(unw_cursor_t *cursor);
 static void __unw_seh_set_disp_ctx(unw_cursor_t *cursor,
@@ -108,10 +96,10 @@
     }
   } else {
     // Foreign exception.
-    exc = (_Unwind_Exception *)malloc(sizeof(_Unwind_Exception));
-    exc->exception_class = kSEHExceptionClass;
-    exc->exception_cleanup = seh_exc_cleanup;
-    memset(exc->private_, 0, sizeof(exc->private_));
+    // We can't interact with them (we don't know the original target frame
+    // that we should pass on to RtlUnwindEx in _Unwind_Resume), so just
+    // pass without calling our destructors here.
+    return ExceptionContinueSearch;
   }
   if (!ctx) {
     __unw_init_seh(&cursor, disp->ContextRecord);


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