On Thu, Nov 05, 2015 at 09:56:56AM -0600, Josh Poimboeuf wrote: > On Thu, Nov 05, 2015 at 04:18:12PM +0100, Miroslav Benes wrote: > > On Wed, 4 Nov 2015, Josh Poimboeuf wrote: > > > > > On Wed, Nov 04, 2015 at 10:52:52AM +0100, Miroslav Benes wrote: > > > > On Tue, 3 Nov 2015, Josh Poimboeuf wrote: > > > > > > Object entry would be empty for not loaded object. I would not > > > > > > dare to propose to remove such object entries. It would make things worse. > > > > > > > > > > Why would removing an empty object entry make things worse? > > > > > > > > I think it all comes down to a question whether the sysfs entries say what > > > > a patch is capable to patch or what this patch is currently patching in > > > > the system. I am inclined to the former so the removal would make me > > > > nervous. But I am not against the second approach. We are still in testing > > > > mode as far as sysfs is concerned so we can try even harsh changes and see > > > > how it's gonna go. > > > > > > I see your point. This approach only describes what is patched now, but > > > it doesn't describe what *will* be patched. Ideally we could find a way > > > to describe both. > > > > > > Speaking of harsh changes, here's an idea. > > > > Which is the very same you proposed last year when I tried to persuade you > > to get rid off old_addr and stuff. I called it crazy I remember :D. So > > here we are again... > > Ah, I knew I had entertained the idea before, but I forgot we discussed > it. It is indeed a little crazy. But this is live patching after all > ;-) > > > > What if we require the patch author to supply the value of 'n' instead > > > of supplying the symbol address? We could get rid of 'old_addr' as an > > > input in klp_func and and replace it with 'old_sympos' which has the > > > value of 'n'. Or alternatively we could require old_name to be of the > > > format "func,n". > > > > > > That would uniquely identify each patched function, even _before_ the > > > object is loaded. > > > > I find it reasonable and we should try it. I think that old_sympos should > > have this semantics > > > > 0 - default, preserve more or less current behaviour. If the symbol is > > unique there is no problem. If it is not the patching would fail. > > 1, 2, ... - occurrence of the symbol in kallsyms. > > > > The advantage is that if the user does not care and is certain that the > > symbol is unique he doesn't have to do anything. If the symbol is not > > unique he still has means how to solve it. > > > > Does it make sense? > > Sounds good! > > > > It would also fix another big problem we have today, where there's no > > > way to disambiguate duplicate symbols in modules, for both function > > > addresses and for relocs. > > > > True. > > > > > It would simplify the code in other places as well: no special handling > > > for kASLR, no need for klp_verify_vmlinux_symbol() vs > > > klp_find_object_symbol(). > > > > Which would be great. > > > > > A drawback is that it requires the patch author to do a little more due > > > diligence when filling out klp_func. But we already require them to be > > > careful. > > > > Yes, I don't think this should be a problem. > > > > > Thoughts? > > > > Yup, we should try it. I suppose that the order of the symbols in kallsyms > > table is stable for once-built kernel. It is the order of the symbols in > > the object files, isn't it? And since each livepatch module is built > > against the specific kernel there should be no issues with this. > > The order of the symbols in an object's symbol table does appear to be > the same as the order in kallsyms (per-object). So yeah, let's try it. > > -- > Josh > Great! Using this feedback to create the next patch. I'll post something in the next few days. --chris -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
