forked from OSchip/llvm-project
227 lines
8.4 KiB
ReStructuredText
227 lines
8.4 KiB
ReStructuredText
=============
|
|
Type Metadata
|
|
=============
|
|
|
|
Type metadata is a mechanism that allows IR modules to co-operatively build
|
|
pointer sets corresponding to addresses within a given set of globals. LLVM's
|
|
`control flow integrity`_ implementation uses this metadata to efficiently
|
|
check (at each call site) that a given address corresponds to either a
|
|
valid vtable or function pointer for a given class or function type, and its
|
|
whole-program devirtualization pass uses the metadata to identify potential
|
|
callees for a given virtual call.
|
|
|
|
To use the mechanism, a client creates metadata nodes with two elements:
|
|
|
|
1. a byte offset into the global (generally zero for functions)
|
|
2. a metadata object representing an identifier for the type
|
|
|
|
These metadata nodes are associated with globals by using global object
|
|
metadata attachments with the ``!type`` metadata kind.
|
|
|
|
Each type identifier must exclusively identify either global variables
|
|
or functions.
|
|
|
|
.. admonition:: Limitation
|
|
|
|
The current implementation only supports attaching metadata to functions on
|
|
the x86-32 and x86-64 architectures.
|
|
|
|
An intrinsic, :ref:`llvm.type.test <type.test>`, is used to test whether a
|
|
given pointer is associated with a type identifier.
|
|
|
|
.. _control flow integrity: http://clang.llvm.org/docs/ControlFlowIntegrity.html
|
|
|
|
Representing Type Information using Type Metadata
|
|
=================================================
|
|
|
|
This section describes how Clang represents C++ type information associated with
|
|
virtual tables using type metadata.
|
|
|
|
Consider the following inheritance hierarchy:
|
|
|
|
.. code-block:: c++
|
|
|
|
struct A {
|
|
virtual void f();
|
|
};
|
|
|
|
struct B : A {
|
|
virtual void f();
|
|
virtual void g();
|
|
};
|
|
|
|
struct C {
|
|
virtual void h();
|
|
};
|
|
|
|
struct D : A, C {
|
|
virtual void f();
|
|
virtual void h();
|
|
};
|
|
|
|
The virtual table objects for A, B, C and D look like this (under the Itanium ABI):
|
|
|
|
.. csv-table:: Virtual Table Layout for A, B, C, D
|
|
:header: Class, 0, 1, 2, 3, 4, 5, 6
|
|
|
|
A, A::offset-to-top, &A::rtti, &A::f
|
|
B, B::offset-to-top, &B::rtti, &B::f, &B::g
|
|
C, C::offset-to-top, &C::rtti, &C::h
|
|
D, D::offset-to-top, &D::rtti, &D::f, &D::h, D::offset-to-top, &D::rtti, thunk for &D::h
|
|
|
|
When an object of type A is constructed, the address of ``&A::f`` in A's
|
|
virtual table object is stored in the object's vtable pointer. In ABI parlance
|
|
this address is known as an `address point`_. Similarly, when an object of type
|
|
B is constructed, the address of ``&B::f`` is stored in the vtable pointer. In
|
|
this way, the vtable in B's virtual table object is compatible with A's vtable.
|
|
|
|
D is a little more complicated, due to the use of multiple inheritance. Its
|
|
virtual table object contains two vtables, one compatible with A's vtable and
|
|
the other compatible with C's vtable. Objects of type D contain two virtual
|
|
pointers, one belonging to the A subobject and containing the address of
|
|
the vtable compatible with A's vtable, and the other belonging to the C
|
|
subobject and containing the address of the vtable compatible with C's vtable.
|
|
|
|
The full set of compatibility information for the above class hierarchy is
|
|
shown below. The following table shows the name of a class, the offset of an
|
|
address point within that class's vtable and the name of one of the classes
|
|
with which that address point is compatible.
|
|
|
|
.. csv-table:: Type Offsets for A, B, C, D
|
|
:header: VTable for, Offset, Compatible Class
|
|
|
|
A, 16, A
|
|
B, 16, A
|
|
, , B
|
|
C, 16, C
|
|
D, 16, A
|
|
, , D
|
|
, 48, C
|
|
|
|
The next step is to encode this compatibility information into the IR. The way
|
|
this is done is to create type metadata named after each of the compatible
|
|
classes, with which we associate each of the compatible address points in
|
|
each vtable. For example, these type metadata entries encode the compatibility
|
|
information for the above hierarchy:
|
|
|
|
::
|
|
|
|
@_ZTV1A = constant [...], !type !0
|
|
@_ZTV1B = constant [...], !type !0, !type !1
|
|
@_ZTV1C = constant [...], !type !2
|
|
@_ZTV1D = constant [...], !type !0, !type !3, !type !4
|
|
|
|
!0 = !{i64 16, !"_ZTS1A"}
|
|
!1 = !{i64 16, !"_ZTS1B"}
|
|
!2 = !{i64 16, !"_ZTS1C"}
|
|
!3 = !{i64 16, !"_ZTS1D"}
|
|
!4 = !{i64 48, !"_ZTS1C"}
|
|
|
|
With this type metadata, we can now use the ``llvm.type.test`` intrinsic to
|
|
test whether a given pointer is compatible with a type identifier. Working
|
|
backwards, if ``llvm.type.test`` returns true for a particular pointer,
|
|
we can also statically determine the identities of the virtual functions
|
|
that a particular virtual call may call. For example, if a program assumes
|
|
a pointer to be a member of ``!"_ZST1A"``, we know that the address can
|
|
be only be one of ``_ZTV1A+16``, ``_ZTV1B+16`` or ``_ZTV1D+16`` (i.e. the
|
|
address points of the vtables of A, B and D respectively). If we then load
|
|
an address from that pointer, we know that the address can only be one of
|
|
``&A::f``, ``&B::f`` or ``&D::f``.
|
|
|
|
.. _address point: https://itanium-cxx-abi.github.io/cxx-abi/abi.html#vtable-general
|
|
|
|
Testing Addresses For Type Membership
|
|
=====================================
|
|
|
|
If a program tests an address using ``llvm.type.test``, this will cause
|
|
a link-time optimization pass, ``LowerTypeTests``, to replace calls to this
|
|
intrinsic with efficient code to perform type member tests. At a high level,
|
|
the pass will lay out referenced globals in a consecutive memory region in
|
|
the object file, construct bit vectors that map onto that memory region,
|
|
and generate code at each of the ``llvm.type.test`` call sites to test
|
|
pointers against those bit vectors. Because of the layout manipulation, the
|
|
globals' definitions must be available at LTO time. For more information,
|
|
see the `control flow integrity design document`_.
|
|
|
|
A type identifier that identifies functions is transformed into a jump table,
|
|
which is a block of code consisting of one branch instruction for each
|
|
of the functions associated with the type identifier that branches to the
|
|
target function. The pass will redirect any taken function addresses to the
|
|
corresponding jump table entry. In the object file's symbol table, the jump
|
|
table entries take the identities of the original functions, so that addresses
|
|
taken outside the module will pass any verification done inside the module.
|
|
|
|
Jump tables may call external functions, so their definitions need not
|
|
be available at LTO time. Note that if an externally defined function is
|
|
associated with a type identifier, there is no guarantee that its identity
|
|
within the module will be the same as its identity outside of the module,
|
|
as the former will be the jump table entry if a jump table is necessary.
|
|
|
|
The `GlobalLayoutBuilder`_ class is responsible for laying out the globals
|
|
efficiently to minimize the sizes of the underlying bitsets.
|
|
|
|
.. _control flow integrity design document: http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html
|
|
|
|
:Example:
|
|
|
|
::
|
|
|
|
target datalayout = "e-p:32:32"
|
|
|
|
@a = internal global i32 0, !type !0
|
|
@b = internal global i32 0, !type !0, !type !1
|
|
@c = internal global i32 0, !type !1
|
|
@d = internal global [2 x i32] [i32 0, i32 0], !type !2
|
|
|
|
define void @e() !type !3 {
|
|
ret void
|
|
}
|
|
|
|
define void @f() {
|
|
ret void
|
|
}
|
|
|
|
declare void @g() !type !3
|
|
|
|
!0 = !{i32 0, !"typeid1"}
|
|
!1 = !{i32 0, !"typeid2"}
|
|
!2 = !{i32 4, !"typeid2"}
|
|
!3 = !{i32 0, !"typeid3"}
|
|
|
|
declare i1 @llvm.type.test(i8* %ptr, metadata %typeid) nounwind readnone
|
|
|
|
define i1 @foo(i32* %p) {
|
|
%pi8 = bitcast i32* %p to i8*
|
|
%x = call i1 @llvm.type.test(i8* %pi8, metadata !"typeid1")
|
|
ret i1 %x
|
|
}
|
|
|
|
define i1 @bar(i32* %p) {
|
|
%pi8 = bitcast i32* %p to i8*
|
|
%x = call i1 @llvm.type.test(i8* %pi8, metadata !"typeid2")
|
|
ret i1 %x
|
|
}
|
|
|
|
define i1 @baz(void ()* %p) {
|
|
%pi8 = bitcast void ()* %p to i8*
|
|
%x = call i1 @llvm.type.test(i8* %pi8, metadata !"typeid3")
|
|
ret i1 %x
|
|
}
|
|
|
|
define void @main() {
|
|
%a1 = call i1 @foo(i32* @a) ; returns 1
|
|
%b1 = call i1 @foo(i32* @b) ; returns 1
|
|
%c1 = call i1 @foo(i32* @c) ; returns 0
|
|
%a2 = call i1 @bar(i32* @a) ; returns 0
|
|
%b2 = call i1 @bar(i32* @b) ; returns 1
|
|
%c2 = call i1 @bar(i32* @c) ; returns 1
|
|
%d02 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 0)) ; returns 0
|
|
%d12 = call i1 @bar(i32* getelementptr ([2 x i32]* @d, i32 0, i32 1)) ; returns 1
|
|
%e = call i1 @baz(void ()* @e) ; returns 1
|
|
%f = call i1 @baz(void ()* @f) ; returns 0
|
|
%g = call i1 @baz(void ()* @g) ; returns 1
|
|
ret void
|
|
}
|
|
|
|
.. _GlobalLayoutBuilder: http://llvm.org/klaus/llvm/blob/master/include/llvm/Transforms/IPO/LowerTypeTests.h
|