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

[Hal Finkel via llvm-dev]

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
>
>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

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