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Hello, |
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|
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I am trying to make systemtap work with gentoo-kernel (or ideally all |
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dist kernels) and I got a few steps closer with kernel-build.eclass |
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modification I sent this week [0]. However there is still one issue and |
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that is the fact that build-id of the kernel does not match the |
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installed vmlinux file: |
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|
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# stap mba_sc.stp |
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WARNING: Build-id mismatch [man warning::buildid]: |
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"/usr/src/linux-5.17.13-gentoo-dist/vmlinux" pid 0 address |
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0xffffffff8a7b572c, expected c43e775aad5e11755bf5cf1329d2240b519e7518 |
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actual 3a757e0a2b0d777762cd4aaf9cac0c40bc8c398c |
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WARNING: /usr/bin/staprun exited with status: 1 |
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Pass 5: run failed. [man error::pass5] |
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|
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I also noticed that when kernel-build.eclass installs the vmlinux file |
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it also (I presume portage) creates vmlinux.debug using objcopy |
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--only-keep-debug --compress-debug-sections. |
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|
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So now I am in a situation where I have these relevant files on the |
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system: |
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|
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- /usr/src/linux-5.17.13-gentoo-dist/vmlinux |
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- /usr/lib/debug/.build-id/c4/3e775aad5e11755bf5cf1329d2240b519e7518.debug |
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(symlink to the first file) |
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- /usr/lib/debug/usr/src/linux-5.17.13-gentoo-dist/vmlinux.debug and |
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- /boot/vmlinuz-5.17.13-gentoo-dist |
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|
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|
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When I check the build ids (using readelf -n or just "file") of the |
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first three files I get: |
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|
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/usr/src/linux-5.17.13-gentoo-dist/vmlinux: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=c43e775aad5e11755bf5cf1329d2240b519e7518, not stripped |
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|
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/usr/lib/debug/.build-id/c4/3e775aad5e11755bf5cf1329d2240b519e7518.debug: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=c43e775aad5e11755bf5cf1329d2240b519e7518, with debug_info, |
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not stripped |
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|
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/usr/lib/debug/usr/src/linux-5.17.13-gentoo-dist/vmlinux.debug: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=c43e775aad5e11755bf5cf1329d2240b519e7518, with debug_info, |
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not stripped |
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|
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which looks great except: |
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|
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1) the first file does not say it is "with debug_info", |
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|
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2) there is no reason to keep the original vmlinux in place since there |
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is a smaller file that works as a substitute, but I'm not sure what's |
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a clean way to not install it, and most importantly |
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|
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3) the fact that the running kernel has a different build id. |
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|
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The last point is the main issue here. I was trying to find how to |
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check for the build id of the running kernel, but haven't found any way |
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on how to do it with a kernel API, so instead I checked the |
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/boot/vmlinuz-5.17.13-gentoo-dist like this: |
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|
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~/dev/linux/scripts/extract-vmlinux /boot/vmlinuz-5.17.13-gentoo-dist >vmlinux.extracted |
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|
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and for good measure also tried what objcopy does to it: |
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|
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objcopy --only-keep-debug vmlinux.extracted vmlinux.extracted.debug |
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objcopy --only-keep-debug --compress-debug-sections vmlinux.extracted vmlinux.extracted.compressed |
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|
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Now when I check the build id is different from the first files, but |
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unchanged with objcopy and same as systemtap reports for the running |
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kernel: |
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|
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vmlinux.extracted: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=3a757e0a2b0d777762cd4aaf9cac0c40bc8c398c, stripped |
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|
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vmlinux.extracted.compressed: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=3a757e0a2b0d777762cd4aaf9cac0c40bc8c398c, stripped |
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|
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vmlinux.extracted.debug: |
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ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, |
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BuildID[sha1]=3a757e0a2b0d777762cd4aaf9cac0c40bc8c398c, stripped |
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|
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|
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At this point I got stuck, not knowing when and how does the build-id |
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changes and where to extract the debug symbols from. I would also like |
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to clean up the change I did. So I came here with my question(s) and |
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rather lengthy explanations. Does anyone know what would be the best |
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way to deal with this? Or even where to continue looking? I would |
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really like to make systemtap "just work" on Gentoo with the |
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distribution kernels, but I already spent a lot of time on it, so I |
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figured I'll rather ask here since I'm not that proficient with the |
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intricacies of the build system parts. |
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|
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Thanks a lot for any pointers and have a great day, |
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Martin |
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|
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[0] https://github.com/gentoo/gentoo/pull/25789 |
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