On Thu, Nov 1, 2018 at 6:18 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: >> > > > > > My one question (and the reason why I went with cmpxchg() in the >> > > > > > first place) would be about the overflow behaviour for >> > > > > > atomic_fetch_inc() and friends. I believe those functions should >> > > > > > be OK on x86, so that when we overflow the counter, it behaves >> > > > > > like an unsigned value and wraps back around. Is that the case >> > > > > > for all architectures? >> > > > > > >> > > > > > i.e. are atomic_t/atomic64_t always guaranteed to behave like >> > > > > > u32/u64 on increment? >> > > > > > >> > > > > > I could not find any documentation that explicitly stated that >> > > > > > they should. >> > > > > >> > > > > Peter, Will, I understand that the atomic_t/atomic64_t ops are >> > > > > required to wrap per 2's-complement. IIUC the refcount code relies >> > > > > on this. >> > > > > >> > > > > Can you confirm? >> > > > >> > > > There is quite a bit of core code that hard assumes 2s-complement. >> > > > Not only for atomics but for any signed integer type. Also see the >> > > > kernel using -fno-strict-overflow which implies -fwrapv, which >> > > > defines signed overflow to behave like 2s-complement (and rids us of >> > > > that particular UB). >> > > >> > > Fair enough, but there have also been bugfixes to explicitly fix unsafe >> > > C standards assumptions for signed integers. See, for instance commit >> > > 5a581b367b5d "jiffies: Avoid undefined behavior from signed overflow" >> > > from Paul McKenney. >> > >> > Yes, I feel Paul has been to too many C/C++ committee meetings and got >> > properly paranoid. Which isn't always a bad thing :-) >> >> Even the C standard defines 2s complement for atomics. > > Ooh good to know. > >> Just not for >> normal arithmetic, where yes, signed overflow is UB. And yes, I do >> know about -fwrapv, but I would like to avoid at least some copy-pasta >> UB from my kernel code to who knows what user-mode environment. :-/ >> >> At least where it is reasonably easy to do so. > > Fair enough I suppose; I just always make sure to include the same > -fknobs for the userspace thing when I lift code. > >> And there is a push to define C++ signed arithmetic as 2s complement, >> but there are still 1s complement systems with C compilers. Just not >> C++ compilers. Legacy... > > *groan*; how about those ancient hardwares keep using ancient compilers > and we all move on to the 70s :-) > >> > But for us using -fno-strict-overflow which actually defines signed >> > overflow, I myself am really not worried. I'm also not sure if KASAN has >> > been taught about this, or if it will still (incorrectly) warn about UB >> > for signed types. >> >> UBSAN gave me a signed-overflow warning a few days ago. Which I have >> fixed, even though 2s complement did the right thing. I am also taking >> advantage of the change to use better naming. > > Oh too many *SANs I suppose; and yes, if you can make the code better, > why not. If there is a warning that we don't want to see at all, then we can disable it. It supposed to be a useful tool, rather than a thing in itself that lives own life. We already I think removed 1 particularly noisy warning and made another optional via a config. But the thing with overflows is that, even if it's defined, it's not necessary the intended behavior. For example, take allocation size calculation done via unsigned size_t. If it overflows it does not help if C defines result or not, it still gives a user controlled write primitive. We've seen similar cases with timeout/deadline calculation in kernel, we really don't want it to just wrap modulo-2, right. Some user-space projects even test with unsigned overflow warnings or implicit truncation warnings, which are formally legal, but frequently bugs.
