Date: Mon, 16 Oct 2023 14:09:06 -0700 From: Farid Zakaria <fmzakari@...c.edu> To: Rich Felker <dalias@...c.org> Cc: musl@...ts.openwall.com Subject: Re: Getting access to section data during dynlink.c Thanks for the advice. I'm now looking at the following two options (seeing if they work): 1. I see that an auxiliary vector is the FD or the name of the file given to dynlink. Can I mmap this file again in it's entirety and read the section I care about? 2. Try to create a segment that maps to the sections I care about such that they are loaded. (Try to apply some heuristic to then identify it) Early attempts at (1) have the mmap failing -- i need to debug further. On Mon, Oct 16, 2023 at 7:25 AM Rich Felker <dalias@...c.org> wrote: > > On Sun, Oct 15, 2023 at 06:06:48PM -0700, Farid Zakaria wrote: > > Hi! > > > > I'd like to read some section data during dynlink.c > > Does anyone have any good suggestions on the best way to do so? > > I believe most ELF files ask for the load to start from the start of the > > ELF file. > > > > I see in dynlink.c the kernel sends AT_PHDR as an auxiliary vector -- > > Should I try applying a fixed offset from it to get to the start of the > > ehdr ? > > > > Any advice is appreciated. > > > > Please include me in the CC for the reply. > > I can't recall if I've subscribed. > > Neither the Ehdrs nor sections are "loadable" parts of an executable > ELF file. They may happen to be present in the mapped pages due to > page granularity of mappings, but that doesn't mean they're guaranteed > to be there; the Ehdrs are for the program loader's use, and the > sections are for the use of linker (non-dynamic), debugger, etc. > > In musl we use Ehdrs in a couple places: the dynamic linker finds its > own program headers via assuming they're mapped, but this is rather > reasonable since we built it and it's either going to always-succeed > or always-fail and get caught before deployment if that build-time > assumption somehow isn't met. It's not contingent on properties of a > program encountered at runtime. We also use Ehdrs when loading a > program (invoking ldso as a command) or shared library, but in that > case we are the loaded and have access to them via the file being > loaded. > > Depending on what you want to do, and whether you just need to be > compatible with your own binaries or arbitrary ones, it may suffice to > do some sort of hack like rounding down from the program header > address to the start of the page and hoping the Ehdrs live there. But > it might make sense to look for other ways to do what you're trying to > do, without needing to access non-runtime data structures. > > Rich
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