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222 lines
8.2 KiB
ReStructuredText
=======
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Bitsets
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=======
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This is a mechanism that allows IR modules to co-operatively build pointer
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sets corresponding to addresses within a given set of globals. One example
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of a use case for this is to allow a C++ program to efficiently verify (at
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each call site) that a vtable pointer is in the set of valid vtable pointers
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for the type of the class or its derived classes.
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To use the mechanism, a client creates a global metadata node named
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``llvm.bitsets``. Each element is a metadata node with three elements:
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1. a metadata object representing an identifier for the bitset
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2. either a global variable or a function
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3. a byte offset into the global (generally zero for functions)
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Each bitset must exclusively contain either global variables or functions.
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.. admonition:: Limitation
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The current implementation only supports functions as members of bitsets on
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the x86-32 and x86-64 architectures.
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An intrinsic, :ref:`llvm.bitset.test <bitset.test>`, is used to test
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whether a given pointer is a member of a bitset.
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Representing Type Information using Bitsets
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===========================================
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This section describes how Clang represents C++ type information associated with
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virtual tables using bitsets.
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Consider the following inheritance hierarchy:
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.. code-block:: c++
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struct A {
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virtual void f();
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};
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struct B : A {
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virtual void f();
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virtual void g();
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};
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struct C {
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virtual void h();
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};
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struct D : A, C {
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virtual void f();
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virtual void h();
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};
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The virtual table objects for A, B, C and D look like this (under the Itanium ABI):
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.. csv-table:: Virtual Table Layout for A, B, C, D
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:header: Class, 0, 1, 2, 3, 4, 5, 6
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A, A::offset-to-top, &A::rtti, &A::f
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B, B::offset-to-top, &B::rtti, &B::f, &B::g
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C, C::offset-to-top, &C::rtti, &C::h
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D, D::offset-to-top, &D::rtti, &D::f, &D::h, D::offset-to-top, &D::rtti, thunk for &D::h
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When an object of type A is constructed, the address of ``&A::f`` in A's
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virtual table object is stored in the object's vtable pointer. In ABI parlance
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this address is known as an `address point`_. Similarly, when an object of type
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B is constructed, the address of ``&B::f`` is stored in the vtable pointer. In
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this way, the vtable in B's virtual table object is compatible with A's vtable.
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D is a little more complicated, due to the use of multiple inheritance. Its
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virtual table object contains two vtables, one compatible with A's vtable and
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the other compatible with C's vtable. Objects of type D contain two virtual
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pointers, one belonging to the A subobject and containing the address of
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the vtable compatible with A's vtable, and the other belonging to the C
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subobject and containing the address of the vtable compatible with C's vtable.
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The full set of compatibility information for the above class hierarchy is
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shown below. The following table shows the name of a class, the offset of an
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address point within that class's vtable and the name of one of the classes
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with which that address point is compatible.
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.. csv-table:: Bitsets for A, B, C, D
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:header: VTable for, Offset, Compatible Class
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A, 16, A
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B, 16, A
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, , B
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C, 16, C
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D, 16, A
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, , D
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, 48, C
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The next step is to encode this compatibility information into the IR. The way
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this is done is to create bitsets named after each of the compatible classes,
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into which we add each of the compatible address points in each vtable.
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For example, these bitset entries encode the compatibility information for
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the above hierarchy:
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::
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!0 = !{!"_ZTS1A", [3 x i8*]* @_ZTV1A, i64 16}
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!1 = !{!"_ZTS1A", [4 x i8*]* @_ZTV1B, i64 16}
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!2 = !{!"_ZTS1B", [4 x i8*]* @_ZTV1B, i64 16}
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!3 = !{!"_ZTS1C", [3 x i8*]* @_ZTV1C, i64 16}
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!4 = !{!"_ZTS1A", [7 x i8*]* @_ZTV1D, i64 16}
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!5 = !{!"_ZTS1D", [7 x i8*]* @_ZTV1D, i64 16}
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!6 = !{!"_ZTS1C", [7 x i8*]* @_ZTV1D, i64 48}
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With these bitsets, we can now use the ``llvm.bitset.test`` intrinsic to test
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whether a given pointer is compatible with a bitset. Working backwards,
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if ``llvm.bitset.test`` returns true for a particular pointer, we can also
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statically determine the identities of the virtual functions that a particular
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virtual call may call. For example, if a program assumes a pointer to be in the
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``!"_ZST1A"`` bitset, we know that the address can be only be one of ``_ZTV1A+16``,
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``_ZTV1B+16`` or ``_ZTV1D+16`` (i.e. the address points of the vtables of A,
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B and D respectively). If we then load an address from that pointer, we know
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that the address can only be one of ``&A::f``, ``&B::f`` or ``&D::f``.
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.. _address point: https://mentorembedded.github.io/cxx-abi/abi.html#vtable-general
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Testing Bitset Addresses
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========================
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If a program tests an address using ``llvm.bitset.test``, this will cause
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a link-time optimization pass, ``LowerBitSets``, to replace calls to this
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intrinsic with efficient code to perform bitset tests. At a high level,
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the pass will lay out referenced globals in a consecutive memory region in
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the object file, construct bit vectors that map onto that memory region,
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and generate code at each of the ``llvm.bitset.test`` call sites to test
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pointers against those bit vectors. Because of the layout manipulation, the
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globals' definitions must be available at LTO time. For more information,
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see the `control flow integrity design document`_.
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A bit set containing functions is transformed into a jump table, which is a
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block of code consisting of one branch instruction for each of the functions
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in the bit set that branches to the target function. The pass will redirect
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any taken function addresses to the corresponding jump table entry. In the
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object file's symbol table, the jump table entries take the identities of
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the original functions, so that addresses taken outside the module will pass
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any verification done inside the module.
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Jump tables may call external functions, so their definitions need not
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be available at LTO time. Note that if an externally defined function is a
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member of a bitset, there is no guarantee that its identity within the module
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will be the same as its identity outside of the module, as the former will
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be the jump table entry if a jump table is necessary.
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The `GlobalLayoutBuilder`_ class is responsible for laying out the globals
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efficiently to minimize the sizes of the underlying bitsets.
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.. _control flow integrity design document: http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html
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:Example:
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::
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target datalayout = "e-p:32:32"
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@a = internal global i32 0
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@b = internal global i32 0
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@c = internal global i32 0
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@d = internal global [2 x i32] [i32 0, i32 0]
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define void @e() {
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ret void
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}
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define void @f() {
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ret void
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}
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declare void @g()
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!llvm.bitsets = !{!0, !1, !2, !3, !4, !5, !6}
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!0 = !{!"bitset1", i32* @a, i32 0}
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!1 = !{!"bitset1", i32* @b, i32 0}
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!2 = !{!"bitset2", i32* @b, i32 0}
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!3 = !{!"bitset2", i32* @c, i32 0}
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!4 = !{!"bitset2", i32* @d, i32 4}
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!5 = !{!"bitset3", void ()* @e, i32 0}
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!6 = !{!"bitset3", void ()* @g, i32 0}
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declare i1 @llvm.bitset.test(i8* %ptr, metadata %bitset) nounwind readnone
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define i1 @foo(i32* %p) {
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%pi8 = bitcast i32* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset1")
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ret i1 %x
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}
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define i1 @bar(i32* %p) {
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%pi8 = bitcast i32* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset2")
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ret i1 %x
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}
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define i1 @baz(void ()* %p) {
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%pi8 = bitcast void ()* %p to i8*
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%x = call i1 @llvm.bitset.test(i8* %pi8, metadata !"bitset3")
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ret i1 %x
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}
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define void @main() {
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%a1 = call i1 @foo(i32* @a) ; returns 1
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%b1 = call i1 @foo(i32* @b) ; returns 1
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%c1 = call i1 @foo(i32* @c) ; returns 0
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%a2 = call i1 @bar(i32* @a) ; returns 0
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%b2 = call i1 @bar(i32* @b) ; returns 1
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%c2 = call i1 @bar(i32* @c) ; returns 1
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%d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0
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%d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
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%e = call i1 @baz(void ()* @e) ; returns 1
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%f = call i1 @baz(void ()* @f) ; returns 0
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%g = call i1 @baz(void ()* @g) ; returns 1
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ret void
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}
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.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerBitSets.h
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