llvm dev - Aug 2018 - Consistency of Memory Layout?

Hello, first post here!

    I have been in the #llvm IRC a few times wondering the following: If I have
C++ program compiled with an unknown compiler, and I have compiled llvm+clang
with said compiler and linked them as libraries, and I compile some C++ code
with clang+llvm and inject it into said "host" program, how can I
obtain/manipulate data from the "guest" program from the
"host" program? How do I know the memory layout of an int in the guest
program (for example) is the same memory layout as an int from the
"host" program? Is it possible the LLVM and Clang API's are ABI
aware (of how LLVM+Clang will compile) and can provide access to the data,
int's, floats, function pointers, etc?

Thank you
 - TFB
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If you have an opaque binary, you strictly speaking have no guarantees about
what ABI it may be expecting. In practice, though, most binaries are built
according to a standardized ABI, so if the program exports symbols and header
files to link against, you should (barring bugs or missing features) be able to
communicate effectively using a common ABI.

Given your word choice, however, I suspect that you are interested in the case
where the program isn’t cooperatively exporting symbols and header files. At
this point, recovering the ABI may be possible via parsing debug information and
making some educated guesses (if debug symbols are available) or via outright
decompilation to recover the ABI. LLVM and Clang do not have facilities to
expedite this kind of decompilation.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of The
Floating Brain via llvm-dev
Sent: Friday, August 17, 2018 13:04
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Consistency of Memory Layout?

Hello, first post here!

    I have been in the #llvm IRC a few times wondering the following: If I have
C++ program compiled with an unknown compiler, and I have compiled llvm+clang
with said compiler and linked them as libraries, and I compile some C++ code
with clang+llvm and inject it into said "host" program, how can I
obtain/manipulate data from the "guest" program from the
"host" program? How do I know the memory layout of an int in the guest
program (for example) is the same memory layout as an int from the
"host" program? Is it possible the LLVM and Clang API's are ABI
aware (of how LLVM+Clang will compile) and can provide access to the data,
int's, floats, function pointers, etc?

Thank you
 - TFB

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