On Mon, Nov 30, 2020 at 6:41 PM Mark Wielaard <m...@klomp.org> wrote: > > Hi Bill, > > On Mon, Nov 30, 2020 at 10:22:34PM +0000, Bill Messmer wrote: > > > I'm still a bit confused here. And the reason I ask this is because > > I open this particular vmlinux image with an OSS ELF/DWARF > > library... which gives me the *WRONG* names for various DWARF DIEs. > > I stepped through the library... and the reason the names are wrong > > is because the library applies all of the relocations in > > .rela.debug_info to the sections it opens. I thought there might be > > a bug in the library somewhere, so I started down looking at the > > DWARF data with standard Linux tools and in hex dumps... and it > > seemed incorrect to my -- admittedly limited -- understanding... > > > > Yes, I am using llvm-dwarfdump to textualize the DWARF data > > (llvm-dwarfdump-10 --verbose vmlinux) and I would assume(?) this > > applies the relocations as necessary. And I am using readelf to get > > the section data (readelf -S vmlinux) and the relocation data > > (readelf -r vmlinuix); however, the hex data I show is just a flat > > hexdump of the image (hexdump -C vmlinux -n ... -s ...). > > I traced your steps and did the same on a local vmlinux copy and got > the same results as you. That didn't make sense to me. Till I realized > my original assumption, that the vmlinux image, like kernel modules > were partially linked and so ET_REL files that still needed relocation > applied, seemed wrong. The vmlinux file isn't actually ET_REL, but it > is ET_EXEC (see readelf -h vmlinux). In which case other tools don't > apply the relocations. And so your observation is correct. The offset > to the .debug_str table is right in the .debug_info section, the > relocations are meaningless. That is surprising. > > > Either both that library and my understanding are incorrect, there > > is something wrong with that relocation data, or it flat isn't > > supposed to be applied... > > It is the last thing, the aren't supposed to be applied because it is > an ET_EXEC file (which isn't supposed to have .rela.debug sections, > but apparently has). > > > I also tried what you suggested "eu-strip -- reloc-debug-sections vmlinux > > -f stripped" and looked at the resulting output: > > > > "readelf -S stripped" still shows the reloc sections: > > > > [65] .debug_info PROGBITS 0000000000000000 00059e50 > > 000000000c458644 0000000000000000 0 0 1 > > [66] .rela.debug_info RELA 0000000000000000 0c4b2498 > > 000000001288ae68 0000000000000018 I 78 65 8 > > > > And that relocation is still there via "readelf -r stripped": > > Which now also makes sense, because as the --help text says "only > relevant for ET_REL files". > > So you did find a real mystery, for some reason the way the vmlinux > image is created does get relocations correctly applied, but they (or > at least some) are still left behind in the ELF image even though they > are no longer needed (and if you do try to use/apply them, you get > wrong results). We should probably find out if this happened during > the upstream build or during distro packaging.
Check if CONFIG_X86_NEED_RELOCS is enabled for your kernel. -- H.J.
