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. Cheers, Mark
