<div dir="ltr">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.</div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Mar 10, 2017 at 9:22 AM, Hal Finkel via llvm-dev <span dir="ltr"><<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<div class="m_-5582163343402506840moz-cite-prefix">On 03/10/2017 10:19 AM, James Henderson
via llvm-dev wrote:<br>
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<div>Hi,<br>
<br>
</div>
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.<br>
<br>
</div>
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.<br>
<br>
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.<br>
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As I understand it, filling with nops has undesirable security
consequences. As you suggest, using some trap sequence is probably
better.<br>
<br>
-Hal <br>
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<div>Regards,<br>
<br>
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<div>James<br>
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<pre class="m_-5582163343402506840moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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