[PATCH] D57482: [RuntimeDyld] Don't try to allocate sections with alignment 0.

[Zachary Turner via Phabricator via llvm-commits]

zturner created this revision.
zturner added reviewers: lhames, labath.
Herald added a subscriber: hiraditya.

ELF sections allow 0 for the alignment, which is specified to be the same as 1.  However many clients do not expect this and will behave poorly in the presence of a 0-aligned section (for
example by trying to modulo something by the section alignment).   We can be more polite by making sure that we always pass a more polite value to clients.

This was discovered when running the LLDB test suite on Linux with a debug build of LLDB / LLVM, and we had several hundred failures do an assertion triggering in MathExtras.h where we tried to call `llvm::alignTo(Size, Align=0, ...)`.

I don't really know this code at all, so I'm not sure if the problem is that the JIT was generating a 0-align section in the first place, but this does at least faithfully match the ELF spec, while also fixing all of the bugs in LLDB's test suite.


https://reviews.llvm.org/D57482

Files:
  llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp


Index: llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
===================================================================
--- llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
+++ llvm/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
@@ -743,6 +743,11 @@
   bool IsReadOnly = isReadOnlyData(Section);
   uint64_t DataSize = Section.getSize();
 
+  // An alignment of 0 (at least with ELF) is identical to an alignment of 1,
+  // while being more "polite".  Other formats do not support 0-aligned sections
+  // anyway, so we should guarantee that the alignment is always at least 1.
+  Alignment = std::max(1u, Alignment);
+
   StringRef Name;
   if (auto EC = Section.getName(Name))
     return errorCodeToError(EC);


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