commit 0eabd7431494b5a27451c5d37e6c2303a5b3a281
author Martin Storsjö <martin@martin.st> Mon Aug 17 22:41:58 2020 +0300
committer Copybara-Service <copybara-worker@google.com> Fri Aug 28 11:18:16 2020 -0700
tree 4b0bae6614858e2331401bb6f43e73ff6054b221
parent 6937bcd208fdea8f762198d8fadd8ac9606ec234

[libunwind] Ensure enough alignment for unw_cursor_t for SEH build configurations

When built in SEH mode, UnwindCursor contains a CONTEXT struct,
which is aligned to 16 bytes in most configurations, causing the
whole UnwindCursor object to have 16 byte alignment.

This fixes backtraces using _Unwind_Backtrace on x86_64 mingw,
where an unw_cursor_t allocated on the stack was misaligned before.

This is an ABI break for this struct for this configuration, but very
few callers call libunwind directly (and even fewer directly allocate
an unw_cursor_t anyway).

Differential Revision: https://reviews.llvm.org/D86102

GitOrigin-RevId: 542db87f1ac67087aeb5cf572b1f9744d482da69

include/libunwind.h

diff --git a/include/libunwind.h b/include/libunwind.h
index 23ef47f..6ec649a 100644
--- a/include/libunwind.h
+++ b/include/libunwind.h
@@ -43,6 +43,12 @@
   #define LIBUNWIND_AVAIL
 #endif
 
+#if defined(_WIN32) && defined(__SEH__)
+  #define LIBUNWIND_CURSOR_ALIGNMENT_ATTR __attribute__((__aligned__(16)))
+#else
+  #define LIBUNWIND_CURSOR_ALIGNMENT_ATTR
+#endif
+
 /* error codes */
 enum {
   UNW_ESUCCESS      = 0,     /* no error */
@@ -68,7 +74,7 @@
 
 struct unw_cursor_t {
   uint64_t data[_LIBUNWIND_CURSOR_SIZE];
-};
+} LIBUNWIND_CURSOR_ALIGNMENT_ATTR;
 typedef struct unw_cursor_t unw_cursor_t;
 
 typedef struct unw_addr_space *unw_addr_space_t;

src/UnwindCursor.hpp

diff --git a/src/UnwindCursor.hpp b/src/UnwindCursor.hpp
index 03e21fb..5491336 100644
--- a/src/UnwindCursor.hpp
+++ b/src/UnwindCursor.hpp
@@ -530,6 +530,8 @@
     : _addressSpace(as), _unwindInfoMissing(false) {
   static_assert((check_fit<UnwindCursor<A, R>, unw_cursor_t>::does_fit),
                 "UnwindCursor<> does not fit in unw_cursor_t");
+  static_assert((alignof(UnwindCursor<A, R>) <= alignof(unw_cursor_t)),
+                "UnwindCursor<> requires more alignment than unw_cursor_t");
   memset(&_info, 0, sizeof(_info));
   memset(&_histTable, 0, sizeof(_histTable));
   _dispContext.ContextRecord = &_msContext;
@@ -1182,6 +1184,8 @@
       _isSignalFrame(false) {
   static_assert((check_fit<UnwindCursor<A, R>, unw_cursor_t>::does_fit),
                 "UnwindCursor<> does not fit in unw_cursor_t");
+  static_assert((alignof(UnwindCursor<A, R>) <= alignof(unw_cursor_t)),
+                "UnwindCursor<> requires more alignment than unw_cursor_t");
   memset(&_info, 0, sizeof(_info));
 }