On Thu, Sep 05, 2019 at 03:52:59PM +0200, Petr Mladek wrote: > On Thu 2019-09-05 08:15:02, Josh Poimboeuf wrote: > > 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. > > It looks scary. I just wonder why we have never met this problem during > last few years. Who knows what can happen when you disable jump label patching. Sometimes it may be harmless. A panic is probably the worst case. There may be other fail modes which are harder to detect. > My only guess is that most of these functions are either in core > kernel or in code that we do not livepatch. This is definitely not the case. We recently introduced jump label checking in kpatch-build, and it complains a lot. The workaround is to replace such uses with static_key_enabled(). -- Josh
