On Sun, Nov 11, 2018 at 02:42:55PM +0100, Ard Biesheuvel wrote: > On 11 November 2018 at 00:20, Peter Zijlstra <pet...@infradead.org> wrote: > > On Fri, Nov 09, 2018 at 02:50:27PM +0100, Ard Biesheuvel wrote: > >> On 9 November 2018 at 08:28, Ingo Molnar <mi...@kernel.org> wrote: > >> >> - I'm not sure about the objtool approach. Objtool is (currently) > >> >> x86-64 only, which means we have to use the "unoptimized" version > >> >> everywhere else. I may experiment with a GCC plugin instead. > >> > > >> > I'd prefer the objtool approach. It's a pretty reliable first-principles > >> > approach while GCC plugin would have to be replicated for Clang and any > >> > other compilers, etc. > >> > > >> > >> I implemented the GCC plugin approach here for arm64 > > > > I'm confused; I though we only needed objtool for variable instruction > > length architectures, because we can't reliably decode our instruction > > stream. Otherwise we can fairly trivially use the DWARF relocation data, > > no? > > How would that work? We could build vmlinux with --emit-relocs, filter > out the static jump/call relocations and resolve the symbol names to > filter the ones associated with calls to trampolines. But then, we > have to build the static_call_sites section and reinject it back into > the image in some way, which is essentially objtool, no?
It's a _much_ simpler tool than objtool, but yes, we need a tool that reads the relocation stuff and (re)injects it in a new section -- we don't need it on a vmlinux level, it can be done per TU. Anyway, a GCC plugin (I still have to have a peek at your thing) sounds like it should work just fine too.
