[llvm-dev] [ELF] [RFC] Padding between executable sections

[Rui Ueyama via llvm-dev]

Yes, I think we should fill gaps in executable sections with 0xcc (or an
equivalent instruction for each arch) by default. LLD supports linker
script's `=FILLEXP`, but in order to use that you also specify the entire
section layout, which you probably don't want to do.

On Fri, Mar 10, 2017 at 9:22 AM, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> On 03/10/2017 10:19 AM, James Henderson via llvm-dev wrote:
>
> Hi,
>
> I was doing some experiments with LLD and noticed that it pads out
> OutputSections with null bytes in all cases (as far as I could tell).
> However, for executable sections on some targets, 0x00 forms part of an
> executable instruction that is not nop. In particular, for x86_64 targets
> at least, the sequence 0x00 0x00 is an add instruction. This can result in
> confusing disassembly.
>
> For example, on x86_64, given a simple InputSection that is a single "0xc3
> retq" instruction, and given an alignment of 16 bytes, 15 null bytes are
> inserted between the end of that InputSection and the next. In the
> disassembly I then see the retq instruction followed by a series of adds,
> the last of which actually consumes 1 or more bytes of the next section to
> form a valid instruction, which can in turn throw off the disassembly of
> the following instructions at the start of the next section.
>
> What do people think about using a target-specific nop instruction
> sequence to pad executable sections? Another possible option, on x86_64 at
> least (I haven't investigated other targets), is to use the 0xcc interrupt
> instruction.
>
>
> As I understand it, filling with nops has undesirable security
> consequences. As you suggest, using some trap sequence is probably better.
>
>  -Hal
>
>
> Regards,
>
> James
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170310/173fc65c/attachment.html>