[lld] r264232 - Use a memcpy to avoid unaligned store UB.

[Pete Cooper via llvm-commits]

Author: pete
Date: Wed Mar 23 20:05:17 2016
New Revision: 264232

URL: http://llvm.org/viewvc/llvm-project?rev=264232&view=rev
Log:
Use a memcpy to avoid unaligned store UB.

On a 32-bit output, we may write LC_MAIN (which contains a uint64_t) to
an unaligned address.  This changes it to use a memcpy instead which is UB safe.

Modified:
    lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp

Modified: lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp?rev=264232&r1=264231&r2=264232&view=diff
==============================================================================
--- lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp (original)
+++ lld/trunk/lib/ReaderWriter/MachO/MachONormalizedFileBinaryWriter.cpp Wed Mar 23 20:05:17 2016
@@ -943,13 +943,16 @@ std::error_code MachOFileLayout::writeLo
     // If main executable, add LC_MAIN.
     if (_file.fileType == llvm::MachO::MH_EXECUTE) {
       // Build LC_MAIN load command.
-      entry_point_command* ep = reinterpret_cast<entry_point_command*>(lc);
-      ep->cmd       = LC_MAIN;
-      ep->cmdsize   = sizeof(entry_point_command);
-      ep->entryoff  = _file.entryAddress - _seg1addr;
-      ep->stacksize = _file.stackSize;
+      // Note, using a temporary here to appease UB as we may not be aligned
+      // enough for a struct containing a uint64_t when emitting a 32-bit binary
+      entry_point_command ep;
+      ep.cmd       = LC_MAIN;
+      ep.cmdsize   = sizeof(entry_point_command);
+      ep.entryoff  = _file.entryAddress - _seg1addr;
+      ep.stacksize = _file.stackSize;
       if (_swap)
-        swapStruct(*ep);
+        swapStruct(ep);
+      memcpy(lc, &ep, sizeof(entry_point_command));
       lc += sizeof(entry_point_command);
     }