On Wed 16-01-19 13:37:22, Dmitry Vyukov wrote: > On Wed, Jan 16, 2019 at 12:56 PM Jan Kara <jack@xxxxxxx> wrote: > > > > On Wed 16-01-19 12:03:27, Dmitry Vyukov wrote: > > > On Wed, Jan 16, 2019 at 11:43 AM Jan Kara <jack@xxxxxxx> wrote: > > > > > > > > On Wed 16-01-19 10:47:56, Dmitry Vyukov wrote: > > > > > On Fri, Jan 11, 2019 at 1:46 PM Tetsuo Handa > > > > > <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > On 2019/01/11 19:48, Dmitry Vyukov wrote: > > > > > > >> How did you arrive to the conclusion that it is harmless? > > > > > > >> There is only one relevant standard covering this, which is the C > > > > > > >> language standard, and it is very clear on this -- this has Undefined > > > > > > >> Behavior, that is the same as, for example, reading/writing random > > > > > > >> pointers. > > > > > > >> > > > > > > >> Check out this on how any race that you might think is benign can be > > > > > > >> badly miscompiled and lead to arbitrary program behavior: > > > > > > >> https://software.intel.com/en-us/blogs/2013/01/06/benign-data-races-what-could-possibly-go-wrong > > > > > > > > > > > > > > Also there is no other practical definition of data race for automatic > > > > > > > data race detectors than: two conflicting non-atomic concurrent > > > > > > > accesses. Which this code is. Which means that if we continue writing > > > > > > > such code we are not getting data race detection and don't detect > > > > > > > thousands of races in kernel code that one may consider more harmful > > > > > > > than this one the easy way. And instead will spent large amounts of > > > > > > > time to fix some of then the hard way, and leave the rest as just too > > > > > > > hard to debug so let the kernel continue crashing from time to time (I > > > > > > > believe a portion of currently open syzbot bugs that developers just > > > > > > > left as "I don't see how this can happen" are due to such races). > > > > > > > > > > > > > > > > > > > I still cannot catch. Read/write of sizeof(long) bytes at naturally > > > > > > aligned address is atomic, isn't it? > > > > > > > > > > Nobody guarantees this. According to C non-atomic conflicting > > > > > reads/writes of sizeof(long) cause undefined behavior of the whole > > > > > program. > > > > > > > > Yes, but to be fair the kernel has always relied on long accesses to be > > > > atomic pretty heavily so that it is now de-facto standard for the kernel > > > > AFAICT. I understand this makes life for static checkers hard but such is > > > > reality. > > > > > > Yes, but nobody never defined what "a long access" means. And if you > > > see a function that accepts a long argument and stores it into a long > > > field, no, it does not qualify. I bet this will come at surprise to > > > lots of developers. > > > > Yes, inlining and other optimizations can screw you. > > > > > Check out this fix and try to extrapolate how this "function stores > > > long into a long leads to a serious security bug" can actually be > > > applied to whole lot of places after inlining (or when somebody just > > > slightly shuffles code in a way that looks totally safe) that also > > > kinda look safe and atomic: > > > https://lore.kernel.org/patchwork/patch/599779/ > > > So where is the boundary between "a long access" that is atomic and > > > the one that is not necessary atomic? > > > > So I tend to rely on "long access being atomic" for opaque values (no > > flags, no counters, ...). Just value that gets fetched from some global > > variable / other data structure, stored, read, and possibly compared for > > equality. I agree the compiler could still screw you if it could infer how > > that value got initially created and try to be clever about it... > > So can you, or somebody else, define a set of rules that we can use to > discriminate each particular case? How can we avoid that "the compiler > could still screw you"? > > Inlining is always enabled, right, so one needs to take into account > everything that's possibly can be inlined. Now or in future. And also > link-time-code generation, if we don't use it we are dropping 10% of > performance on the floor. > Also, ensuring that the code works when it's first submitted is the > smaller part of the problem. It's ensuring that it continues to work > in future what's more important. Try to imagine what amount of burden > this puts onto all developers who touch any kernel code in future. > Basically if you slightly alter local logic in a function that does > not do any loads/stores, you can screw multiple "proofs" that long > accesses are atomic. Or, you just move a function from .c file to .h. > I can bet nobody re-proofs all "long accesses are atomic" around the > changed code during code reviews, so these things break over time. > Or, even if only comparisons are involved (that you mentioned as > "safe") I see how that can actually affect compilation process. Say, > we are in the branch where 2 variables compare equal, now since no > concurrency is involved from compiler point of view, it can, say, > discard one variable and then re-load it from the other variable's > location, and say not the other variable has value that the other one > must never have. I don't have a full scenario, but that's exactly the > point. One will never see all possibilities. > > It all becomes super slippery slope very quickly. And we do want > compiler to generate as fast code as possible and do all these > optimizations. And it's not that there are big objective reasons to > not just mark all concurrent accesses and stop spending large amounts > of time on these "proofs". I guess you've convinced me that somehow marking such accesses is desirable. So is using atomic_long_t and atomic_long_set() / atomic_long_read() for manipulation instead what you suggest? Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
