On Sun, May 07, 2017 at 11:08:19AM -0700, [email protected] wrote: > On May 7, 2017 10:59:16 AM PDT, Ingo Molnar <[email protected]> wrote: > > > >* Josh Poimboeuf <[email protected]> wrote: > > > >> One instance of the structure would exist for each time the stack > >> pointer changes, e.g. for every function entry, push/pop, and rsp > >> add/subtract. The data could be assembled and sorted offline, > >possibly > >> derived from DWARF, or more likely, generated by objtool. After > >doing > >> some rough calculations, I think the section size would be comparable > >to > >> the sizes of the DWARF .eh_frame sections it would replace. > > > >That's something I've been thinking about as well: if objtool generates > >the > >unwinder data structures then the kernel is not directly exposed to > >tooling bugs > >anymore. > > > >A fair chunk of the fragility of DWARF comes from the fact that it's > >generated by > >a tool chain that we cannot fix as part of the kernel project. If GCC > >generates > >crap debuginfo, and GDB happens to work with it but the kernel not, > >we'll have to > >work it around in the kernel. If GCC starts bloating debuginfo in the > >future we > >are screwed as well, etc. > > > >If objtool generates debuginfo then it's _our_ responsibility to have > >sane > >unwinder info and we obviously manage its structure and size as well. > >Win-win. > > > >The unwinder itself should still do sanity checks, etc. (like all good > >debugging > >infrastructure code) - but the nature of the kernel's exposure to tool > >chain > >details changes in a very fundamental way. > > > >So yes, I think this is a very good idea, assuming it works in > >practice! ;-) > > > >Thanks, > > > > Ingo > > Can objtool verify the unwinder at each address in the kernel, or is that an > AI-complete problem?
It can't verify the *unwinder*, but it can verify the data which is fed to the unwinder (either DWARF or the structs I proposed above). For each function, it follows every possible code path, and it can keep track of the stack pointer while doing so. -- Josh
