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<div class="moz-cite-prefix">On 03/10/2017 10:19 AM, James Henderson
via llvm-dev wrote:<br>
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<div>Hi,<br>
<br>
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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>
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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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<br>
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="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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