From 6e7908ddb7538ff9a6f49aecaa6b603f98cb423e Mon Sep 17 00:00:00 2001 From: Peter Collingbourne Date: Thu, 12 Mar 2015 00:30:41 +0000 Subject: [PATCH] docs: Document byte arrays. llvm-svn: 231994 --- clang/docs/ControlFlowIntegrityDesign.rst | 42 +++++++++++++++++++------------ 1 file changed, 26 insertions(+), 16 deletions(-) diff --git a/clang/docs/ControlFlowIntegrityDesign.rst b/clang/docs/ControlFlowIntegrityDesign.rst index 86a08b6..89aa038 100644 --- a/clang/docs/ControlFlowIntegrityDesign.rst +++ b/clang/docs/ControlFlowIntegrityDesign.rst @@ -53,6 +53,19 @@ The bit vector for static types A, B and C will look like this: B, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0 C, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0 +Bit vectors are represented in the object file as byte arrays. By loading +from indexed offsets into the byte array and applying a mask, a program can +test bits from the bit set with a relatively short instruction sequence. Bit +vectors may overlap so long as they use different bits. For the full details, +see the `ByteArrayBuilder`_ class. + +In this case, assuming A is laid out at offset 0 in bit 0, B at offset 0 in +bit 1 and C at offset 0 in bit 2, the byte array would look like this: + +.. code-block:: c++ + + char bits[] = { 0, 0, 1, 0, 0, 0, 3, 0, 0, 0, 0, 5, 0, 0 }; + To emit a virtual call, the compiler will assemble code that checks that the object's virtual table pointer is in-bounds and aligned and that the relevant bit is set in the bit vector. @@ -61,23 +74,19 @@ For example on x86 a typical virtual call may look like this: .. code-block:: none - 159a: 48 8b 03 mov (%rbx),%rax - 159d: 48 8d 15 6c 33 00 00 lea 0x336c(%rip),%rdx - 15a4: 48 89 c1 mov %rax,%rcx - 15a7: 48 29 d1 sub %rdx,%rcx - 15aa: 48 c1 c1 3d rol $0x3d,%rcx - 15ae: 48 83 f9 51 cmp $0x51,%rcx - 15b2: 77 3b ja 15ef - 15b4: 48 89 ca mov %rcx,%rdx - 15b7: 48 c1 ea 05 shr $0x5,%rdx - 15bb: 48 8d 35 b8 07 00 00 lea 0x7b8(%rip),%rsi - 15c2: 8b 14 96 mov (%rsi,%rdx,4),%edx - 15c5: 0f a3 ca bt %ecx,%edx - 15c8: 73 25 jae 15ef - 15ca: 48 89 df mov %rbx,%rdi - 15cd: ff 10 callq *(%rax) + ca7fbb: 48 8b 0f mov (%rdi),%rcx + ca7fbe: 48 8d 15 c3 42 fb 07 lea 0x7fb42c3(%rip),%rdx + ca7fc5: 48 89 c8 mov %rcx,%rax + ca7fc8: 48 29 d0 sub %rdx,%rax + ca7fcb: 48 c1 c0 3d rol $0x3d,%rax + ca7fcf: 48 3d 7f 01 00 00 cmp $0x17f,%rax + ca7fd5: 0f 87 36 05 00 00 ja ca8511 + ca7fdb: 48 8d 15 c0 0b f7 06 lea 0x6f70bc0(%rip),%rdx + ca7fe2: f6 04 10 10 testb $0x10,(%rax,%rdx,1) + ca7fe6: 0f 84 25 05 00 00 je ca8511 + ca7fec: ff 91 98 00 00 00 callq *0x98(%rcx) [...] - 15ef: 0f 0b ud2 + ca8511: 0f 0b ud2 The compiler relies on co-operation from the linker in order to assemble the bit vectors for the whole program. It currently does this using LLVM's @@ -85,6 +94,7 @@ the bit vectors for the whole program. It currently does this using LLVM's .. _address point: https://mentorembedded.github.io/cxx-abi/abi.html#vtable-general .. _bit sets: http://llvm.org/docs/BitSets.html +.. _ByteArrayBuilder: http://llvm.org/docs/doxygen/html/structllvm_1_1ByteArrayBuilder.html Optimizations ------------- -- 2.7.4