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docs: Document byte arrays.

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llvm-svn: 231994
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Peter Collingbourne 2015-03-12 00:30:41 +00:00
parent 6d5a4b5dcd
commit 6e7908ddb7
1 changed files with 26 additions and 16 deletions
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42
clang/docs/ControlFlowIntegrityDesign.rst
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@ -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 <main+0xcf>
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 <main+0xcf>
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
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