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

[Rui Ueyama via llvm-dev]

Filed as http://bugs.llvm.org/show_bug.cgi?id=32227.

On Fri, Mar 10, 2017 at 10:02 AM, Rui Ueyama <ruiu at google.com> wrote:

> 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
>>
>>
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>>
>>
>
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