On Thu, Sep 05, 2019 at 08:08:32AM -0500, Josh Poimboeuf wrote: > On Thu, Sep 05, 2019 at 01:09:55PM +0200, Petr Mladek wrote: > > > I don't have a number, but it's very common to patch a function which > > > uses jump labels or alternatives. > > > > Really? My impression is that both alternatives and jump_labels > > are used in hot paths. I would expect them mostly in core code > > that is always loaded. > > > > Alternatives are often used in assembly that we are not able > > to livepatch anyway. > > > > Or are they spread widely via some macros or inlined functions? > > Jump labels are used everywhere. Looking at vmlinux.o in my kernel: > > Relocation section [19621] '.rela__jump_table' for section [19620] '__jump_table' at offset 0x197873c8 contains 11913 entries: > > Each jump label entry has 3 entries, so 11913/3 = 3971 jump labels. > > $ readelf -s vmlinux.o |grep FUNC |wc -l > 46902 > > 3971/46902 = ~8.5% > > ~8.5% of functions use jump labels. Obviously some functions may use more than one jump label so this isn't exactly bulletproof math. But it gives a rough idea of how widespread they are. > > > > > + How often new problematic features appear? > > > > > > I'm not exactly sure what you mean, but it seems that anytime we add a > > > new feature, we have to try to wrap our heads around how it interacts > > > with the weirdness of late module patching. > > > > I agree that we need to think about it and it makes complications. > > Anyway, I think that these are never the biggest problems. > > > > I would be more concerned about arch-specific features that might need > > special handling in the livepatch code. Everyone talks only about > > alternatives and jump_labels that were added long time ago. > > Jump labels have been around for many years, but we somehow missed > implementing klp.arch for them. As I said this resulted in panics. > > There may be other similar cases lurking, both in x86 and other arches. > It's not a comforting thought! > > And each case requires special klp code in addition to the real code. > > -- > Josh -- Josh
