<div dir="ltr"><br><div class="gmail_extra"><br><div class="gmail_quote">On Thu, Feb 2, 2017 at 4:35 PM, Dmitry Golovin <span dir="ltr"><<a href="mailto:dima@golovin.in" target="_blank">dima@golovin.in</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div>I have just checked it, the startup.elf and realmode.elf are fine. Only few changes are required for mainline kernel and one commit has to be reverted from lld and a few patches have to be applied.</div><div> </div><div>The only step when I have used BFD is linking vmlinux. I have manually set LD variable in vmlinux_link() function. The vmlinux produced by lld doesn't work yet. I will compare it to the one produced by GNU ld and try to figure out what is wrong (maybe you can suggest some useful objdump flags?)</div></blockquote><div><br></div><div>With objdump I would recommend looking at program headers. In particular at PT_LOAD's and the dynamic symbol table. Anything in the dynamic table is also worth scrutinizing. One thing to keep an eye out for is addresses/offsets that look "weird"; e.g. maybe the LLD version thinks a symbol has address 0 or some insane value, vs BFD/gold which has a more sane value.</div><div><br></div><div>Also, set up your system so that you rebuild/reinstall the bootloader too so that you can add printf's in there to hone in on where the boot is going wrong. The following workflow might be useful:</div><div><br></div><div>Step 1: add a printf to the bootloader to try to hone in on the exact place where things are going wrong</div><div>Step 2: rebuild/reinstall/reboot the new bootloader with the LLD-linked kernel</div><div>Step 3: boot and observe the print's (or maybe things crashed before reaching your print, which is just as useful to know)</div><div>Step 4: think about what you observed in Step 3, then go to Step 1, using these results to inform the next set of print's to add</div><div><br></div><div>With appropriate scripts (and a nice qemu setup), one iteration of this may take 10 minutes (say). You may have to repeat it (say) 20 times to pinpoint the exact place where things are going wrong (e.g. "the bootloader is crashing in the memcpy for the second PT_LOAD" or "the boot is failing because the bootloader is reading from a bogus address that it got from this part of the binary"). That is 200 minutes which isn't too bad.</div><div><br></div><div>One thing to keep in mind is that this is not like debugging a race condition or other nasty nondeterministic bug. This should be quite deterministic so you just have to be systematic and keep narrowing down until you find where things go wrong. It just requires determination.</div><div><br></div><div>Once narrowed-down, you should hopefully have a clear indication of where to look in the binary and compare with gold/bfd and hopefully the discrepancy is pretty clear. Then you "just" need to figure out why LLD produces this result and what to change to avoid the problem.<br></div><div><br></div><div><br></div><div>One amazing tool if you are working with object files is "010 Editor" <a href="https://www.sweetscape.com/010editor/">https://www.sweetscape.com/010editor/</a> with a "binary template" for ELF files. I think there is an ELF "binary template" for 010 Editor floating around the net, but the best one is Michael's one that he has evolved over the years (ask him for it). If you haven't done so already, I recommend that you sit down at Michael's desk one day and work with him to debug one of these nasty "what is wrong with this binary and why?" issues so you can see him do his thing; he's amazingly good at it.</div><div><br></div><div><br></div><div>Also, if you need a quick refresher about this x86 boot stuff (to be somewhat oriented about the environment in which all this stuff is happening), you may want to skim:</div><div><a href="http://duartes.org/gustavo/blog/post/how-computers-boot-up/">http://duartes.org/gustavo/blog/post/how-computers-boot-up/</a><br></div><div><a href="http://duartes.org/gustavo/blog/post/kernel-boot-process/">http://duartes.org/gustavo/blog/post/kernel-boot-process/</a><br></div><div><a href="http://duartes.org/gustavo/blog/post/memory-translation-and-segmentation/">http://duartes.org/gustavo/blog/post/memory-translation-and-segmentation/</a><br></div><div><a href="http://duartes.org/gustavo/blog/post/cpu-rings-privilege-and-protection/">http://duartes.org/gustavo/blog/post/cpu-rings-privilege-and-protection/</a><br></div><div><br></div><div>-- Sean Silva</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div> </div><div>Regards,</div><div>Dmitry</div><div><br></div><div><br></div><div>03.02.2017, 02:23, "Sean Silva" <<a href="mailto:chisophugis@gmail.com" target="_blank">chisophugis@gmail.com</a>>:</div><div class="gmail-HOEnZb"><div class="gmail-h5"><blockquote type="cite"><div dir="ltr"><br><div><br><div>On Thu, Feb 2, 2017 at 12:38 AM, George Rimar <span dir="ltr"><<a href="mailto:grimar@accesssoftek.com" target="_blank">grimar@accesssoftek.com</a>></span> wrote:<br><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p><span style="color:rgb(33,33,33);font-size:12pt">>As far as the setup, I would recommend setting up qemu for actually running the LLD-linked kernel and custom bootloader etc. because then you can have a single >script that rebuilds the bootloader and
kernel and copies the files to the VM. This reduces iteration time significantly.</span><br>
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<div>>Davide is the one that set that up and could probably provide more details, but qemu docs might be good enough that you can set things up without much effort </div>
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</span><div>By the way, yesterday I configured "smallest possible kernel", linked it with BFD and launched under QEMU.<br>
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<div>It is very small and takes a few seconds to build it from scratch for me, used next article:<br>
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<div><a href="http://mgalgs.github.io/2015/05/16/how-to-build-a-custom-linux-kernel-for-qemu-2015-edition.html" target="_blank">http://mgalgs.github.io/2015/0<wbr>5/16/how-to-build-a-custom-lin<wbr>ux-kernel-for-qemu-2015-editio<wbr>n.html</a><br>
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<div>Now I am going to link it with LLD and check if it boots or now. </div>
<div>I think that should be fastest way - boot that little core and then enable features<br>
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<div>one by one or group by group and fix other things on the road.<br></div></div></div></div></div></div></div></blockquote><div><br></div><div>My experience with linker bugs is that usually when things are mis-linked, they are in the "core". E.g. startup code. So linking a small kernel may not avoid as many bugs as you expect. For example, for FreeBSD, I don't think we hit any issues in anything that could have been configured out.</div><div><br></div><div>-- Sean Silva</div><div> </div><blockquote style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr" style="font-size:12pt;color:rgb(0,0,0);background-color:rgb(255,255,255);font-family:calibri,arial,helvetica,sans-serif"><div style="color:rgb(33,33,33)"><div><div dir="ltr"><div><div><div>
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<div>Previously I also worked on a patches for kernel but did not try to minimize it and used some default configuration,<br>
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<div>what probably was good for finding mutliple issues from all sides, but not ideal way to fix/test startup and things.<span><font color="#888888"><br>
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<div>George.<br>
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