[PATCH] D75936: Add a Pass to X86 that builds a Condensed CFG for Load Value Injection (LVI) Gadgets [4/6]

[Matthew Riley via Phabricator via cfe-commits]

mattdr added inline comments.


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:318
+    }
+    auto *VertexArray = new Node[VertexSize + 1 /* terminator node */];
+    auto *EdgeArray = new Edge[EdgeSize];
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sconstab wrote:
> mattdr wrote:
> > sconstab wrote:
> > > mattdr wrote:
> > > > As a general rule `new` is a code-smell in modern C++. This should be a `vector`.
> > > @mattdr I do agree with the general rule. I also think that in this case where the structure is immutable, std::vector is wasteful because it needs to keep separate values for the current number of elements and the current capacity. At local scope within a function the unneeded value would likely be optimized away, but then there would be an awkward handoff to transfer the data from the vector to the array members.
> > > 
> > > I would not want to see the array members changed to vectors, unless LLVM provides an encapsulated array structure that does not need to grow and shrink.
> > So, first: I'm glad you removed the unnecessary use of `new[]` here and the corresponding (and error-prone!) use of `delete[]` later. That removes a memory leak LLVM won't have to debug.
> > 
> > You suggest here that something other than `std::vector` would be more efficient. If so, would `std::array` suffice? If not, can you explain why static allocation is impossible but dynamic allocation would be too expensive?
> A statically sized array (e.g., std::array) is insufficient because the size in this case is not compiler determinable; a dynamically sized and dynamically resizable array (e.g., std::vector) is sufficient but overly costly; a dynamically sized and dynamically //unresizable// array is sufficient and has minimal cost.
I'm not sure we allocate enough of these in the course of a compilation for the one extra word in a `std::vector` to matter, but I won't press the point.


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:17
+///    implemented as a bit vector, wherein each bit corresponds to one edge in
+///    the edge array. This implies a lower bound of 64x spacial improvement
+///    over, e.g., an llvm::DenseSet or llvm::SmallSet. It also means that
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"spatial"


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:41
+class ImmutableGraph {
+  using Traits = GraphTraits<ImmutableGraph<NodeValueT, EdgeValueT> *>;
+  template <typename> friend class ImmutableGraphBuilder;
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I think this self-reference to `ImmutableGraph` dropped the `SizeT` parameter.


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:73
+    // The end of this Node's edges is the beginning of the next node's edges.
+    const Edge *edges_end() const { return (this + 1)->Edges; }
+    ArrayRef<Edge> edges() const {
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Seems like you also want to add a comment here that we know we will never be asked for `edges_end` for the last stored node -- that is, we know that `this + 1` always refers to a valid Node (which is presumably a dummy/sentinel)


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:79
+
+protected:
+  ImmutableGraph(std::unique_ptr<Node[]> Nodes, std::unique_ptr<Edge[]> Edges,
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Why "protected" rather than "private"? Usually seeing "protected" makes me think subclassing is expected, but that doesn't seem to be the case here.


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:117
+    NodeSet(const ImmutableGraph &G, bool ContainsAll = false)
+        : G{G}, V{static_cast<unsigned>(G.nodes_size()), ContainsAll} {}
+    bool insert(const Node &N) {
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How do we know that a value of `size_type` (aka `SizeT`) can be cast to `unsigned` without truncation?


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Comment at: llvm/lib/Target/X86/ImmutableGraph.h:298
+  static_assert(
+      std::is_base_of<ImmutableGraph<node_value_type, edge_value_type>,
+                      GraphT>::value,
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this will also break if a non-default `SizeT` is provided. Maybe a good argument to just leave out `SizeT` for now, and it can be added in the future as needed?


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Comment at: llvm/lib/Target/X86/X86LoadValueInjectionLoadHardening.cpp:113
+          NumFences(NumFences), NumGadgets(NumGadgets) {}
+    MachineFunction &getMF() { // FIXME: This function should be cleaner
+      for (const Node &N : nodes())
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Cleaner how?


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Comment at: llvm/lib/Target/X86/X86LoadValueInjectionLoadHardening.cpp:233
+  if (!STI->useLVILoadHardening() || !STI->is64Bit())
+    return false; // FIXME: support 32-bit
+
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If the user requests hardening and we can't do it, it seems better to fail loudly so they don't accidentally deploy an unmitigated binary.


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  https://reviews.llvm.org/D75936/new/

https://reviews.llvm.org/D75936