<div dir="ltr"><div dir="ltr">Another thing is that 2 RW can avoid an alignment at the cost of an extra PT_LOAD (sizeof(Elf64_Phdr) = 56). You can find more information on <a href="https://reviews.llvm.org/D64906">https://reviews.llvm.org/D64906</a></div><div><br></div><div>GNU ld pads the tail of RELRO to commonpagesize (see <span style="font-size:medium;color:rgb(0,0,0);font-family:monospace">DATA_SEGMENT_RELRO_END(</span><var style="font-size:medium;color:rgb(0,0,0);font-family:monospace">offset</var><span style="font-size:medium;color:rgb(0,0,0);font-family:monospace">, </span><var style="font-size:medium;color:rgb(0,0,0);font-family:monospace">exp</var><span style="font-size:medium;color:rgb(0,0,0);font-family:monospace">)). gold has a similar behavior.</span></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Oct 16, 2019 at 1:23 AM Troy Johnson via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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Thanks, Peter. That patch pointed me to the exact line of code that I needed to know about!</div>
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I don't think it needs to be configurable via the command line unless others run into a problem with this change, too. I happened to be working with some other software (that I can't change) that made some overly strict assumptions about how things were linked
and it did not like having the segments split up.<br>
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-Troy<br>
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<div id="gmail-m_7421459354390845372divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> Peter Smith <<a href="mailto:peter.smith@linaro.org" target="_blank">peter.smith@linaro.org</a>><br>
<b>Sent:</b> Tuesday, October 15, 2019 5:20 AM<br>
<b>To:</b> Troy Johnson <<a href="mailto:troyj@cray.com" target="_blank">troyj@cray.com</a>><br>
<b>Cc:</b> llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>><br>
<b>Subject:</b> Re: [llvm-dev] [LLD] Placing more sections in same segment as data?</font>
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<div dir="ltr">On Mon, 14 Oct 2019 at 19:57, Troy Johnson via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<br>
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I've noticed that lld keeps the data section more isolated than the gold or bfd linkers. For example, readelf -l applied to the "same" executable linked with those three linkers reveals the following under "Section to Segment mapping":<br>
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lld:</div>
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05 .data .got.plt .bss</div>
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gold:</div>
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03 .eh_frame .init_array .fini_array .preinit_array .dynamic .got .got.plt .data .bss</div>
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bfd:</div>
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05 .eh_frame .preinit_array .init_array .fini_array .dynamic .got .got.plt .data .bss</div>
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This separation seems to result in lld creating two LOAD RW segments whenever gold or bfd would create only one. For example, also from readelf -l output:<br>
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<span> LOAD 0x0000000000001a80 0x0000000000203a80 0x0000000000203a80<br>
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<div> 0x00000000000002a8 0x00000000000002a8 RW 0x1000<br>
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<div> LOAD 0x0000000000001d28 0x0000000000204d28 0x0000000000204d28<br>
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<span> 0x0000000000000060 0x000000000000015c RW 0x1000</span><br>
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Could someone explain why the lld behavior is different? Is there a way to make it match the gold/bfd behavior? I've looked through the options list and read some of the lld code, but so far have not been able to find a solution.<br>
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<div>I think this is likely to be <a href="https://reviews.llvm.org/D58892" target="_blank">https://reviews.llvm.org/D58892</a> "Split RW PT_LOAD on the PT_GNU_RELRO boundary", which was made as part of a suggestion in a comment for a related change <a href="https://reviews.llvm.org/D56828" target="_blank">https://reviews.llvm.org/D56828</a> .
The root cause of the justification is that it enables better use of .<a href="http://bss.rel.ro" target="_blank">bss.rel.ro</a> sections. Unfortunately other than turning off RELRO with -znorelo I'm not aware of an easier way to change this behaviour. It may be possible
to write a linker script with a PHDRS command but I've not tried it. If I were to try I'd start by getting the linker script from ld.bfd --verbose, and then add a PHDRS command to it.</div>
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<div>If there is a good case for needing the ld.bfd behaviour I suggest raising a PR, I'm thinking that this could be configurable via a command line option.</div>
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<div>Hope this helps</div>
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<div>Peter</div>
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Thanks,<br>
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<p style="line-height:11.5pt"><b><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(38,202,211)">Troy Johnson, Ph.D.</span></b><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(153,140,135)"> | Manager & Lead, Compiler
Optimization</span></p>
<p style="line-height:11.5pt"><span style="font-size:11pt;font-family:Arial,sans-serif;color:rgb(153,140,135)">Cray, a Hewlett Packard Enterprise company</span><span style="font-family:Arial,sans-serif;color:black"></span></p>
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