On Donnerstag, 11. Oktober 2018 19:37:07 CEST Mark Wielaard wrote: > Hi, > > My apologies for not having looked deeper at this. > It is a bit tricky and I just didnt have enough time to > really sit down and think it all through yet. > > On Thu, Oct 11, 2018 at 05:02:18PM +0000, Ulf Hermann wrote: > > is there any pattern in how the loader maps the ELF sections into > > memory? What sections does it actually map and which of those do we need > > for unwinding? > > Yes, it would be helpful to have some examples of mmap events plus > the associated segment header (eu-readelf -l) of the ELF file. > > Note that the kernel and dynamic loader will use the (PT_LOAD) segments, > not the sections, to map things into memory. Each segment might contain > multiple sections. > > libdwfl then tries to associate the correct sections (and address bias) > with how the ELF file was mapped into memory. > > > I hope that only one of those MMAPs per ELF is actually meaningful and > > we can simply add that one's pgoff as an extra member to Dwfl_Module and > > use it whenever we poke the underlying file. > > One "trick" might be to just substract the pgoff from the load address. > And so report as if the ELF file was being mapped from the start. This > isn't really correct, but it might be interesting to see if that makes > libdwfl able to just associate the whole ELF file with the correct > address map.
I'll try to come up with some minimal code examples we can use to test all of this. But from what I remember, neither of the above suggestions will be sufficient as we can still run into overlapping module errors from elfutils when we always load everything. I.e. I believe we've seen mappings that eventually become partially obsoleted by a future mmap event. At that point, we somehow need to be able to only map parts of a file, not all of it. So just subtracting or honoring pgoff is not enough, I believe we also need to be able to explicitly say how much of a file to map. But to make this discussion easier to follow for others, I'll create some standalone cpp code that takes a `perf script --show-mmap-events | grep PERF_RECORD_MMAP` input file and then runs this through elfutils API to reproduce the issues we are facing. I'll get back to you all once this is done. Cheers -- Milian Wolff m...@milianw.de http://milianw.de
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