[llvm-dev] Collect all possible return address and write in a new section

[Dean Michael Berris via llvm-dev]

Adding back llvm-dev
On Wed, Jul 18, 2018 at 10:55 AM bigsky <hahabigsky at gmail.com> wrote:
>
> Hi
>
> I  trace the code of  XRay
>
> I found that "X86AsmPrinter::runOnMachineFunction" would call "emitXRayTable".
>
> So, you use " void AsmPrinter::emitXRayTable() " to write something into a new section, right?
>

Yes.

> In emitXRayTable(),
>
>   for (const auto &Sled : Sleds)
> Sled.emit(WordSizeBytes, OutStreamer.get(), CurrentFnSym);
>
> Sleds is array of  XRayFunctionEntry .
>
> In the  AsmPrinter::XRayFunctionEntry::emit , there is a  Out->EmitSymbolValue(CurrentFnSym, Bytes);
>
>
> I wonder that  whether the "CurrentFnSym" emitted to the new section will be a real adrress?
>
> Will CurrentFnSym be fixed up by linker/loader or someone?
>

It must be fixed up by the linker.

> Thank you
>

Happy to help!

Cheers

>
> 2018-07-17 9:47 GMT+08:00 Dean Michael Berris <dean.berris at gmail.com>:
>>
>> Hi -- can you describe a bit more what you mean by coarse-grained CFI?
>>
>> We do some of what you're describing in the XRay implementation, where
>> we have a section for the instrumentation map and at runtime we're
>> able to tell which functions are instrumented.
>>
>> The way we've had to do this is with attributes at LLVM IR for
>> functions, have a pass that will lower certain instructions (returns
>> and tail exits) into pseudo-instructions, and further lowering to
>> target/platform specific details (sections in ELF/MachO etc.).
>>
>> On Tue, Jul 17, 2018 at 5:53 AM 王大天 via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> >
>> > Hi
>> >
>> > I try to implement a coarse-grained CFI in LLVM
>> >
>> > (CFI = Contorl Flow Integrity)
>> >
>> > I want to collect all address after call instructions
>> >
>> > address after a call equals to a valid return site in coarse-grained CFI
>> >
>> > I want to add a new section
>> >
>> > and write all the possible return address in the new section
>> >
>> > (and then, add the integrity check)
>> >
>> > I have some quetions:
>> >
>> > (1)
>> > Which part of LLVM code should I implement my CFI ?
>> > IR level or SelectionDAG/CodeGen?
>> >
>> > (2)
>> > LLVM MC can let me add a new section, right?
>> >
>> > (3)
>> > How can I get the real address of instruction in compile time?
>> > Or should I modify linker/loader?
>> >
>> > Thank you
>> >
>> >
>> >
>> >
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>> --
>> Dean
>
>


-- 
Dean