----- On Mar 19, 2020, at 3:05 PM, Florian Weimer fw@xxxxxxxxxxxxx wrote: > * Mathieu Desnoyers: > >>> Inside glibc, you can assume __attribute__ support. >> >> OK, so the _Static_assert () could sit in sys/rseq.h > > It requires a C11 compiler. In this case, you could use _Alignas. How would _Alignas replace: +_Static_assert (__alignof__ (struct rseq_cs) >= 4 * sizeof(uint64_t), + "alignment"); +_Static_assert (__alignof__ (struct rseq) >= 4 * sizeof(uint64_t), + "alignment"); ? Moreover, I notice that sys/cdefs.h implements a fallback for _Static_assert for cases where it is not supported by the compiler. So I do not think it strictly depends on C11 if I include sys/cdefs.h from sys/rseq.h. >>>>>>> The struct rseq/struct rseq_cs definitions >>>>>>> are broken, they should not try to change the alignment. >>>>>> >>>>>> AFAIU, this means we should ideally not have used __attribute__((aligned)) >>>>>> in the uapi headers in the first place. Why is it broken ? >>>>> >>>>> Compilers which are not sufficiently GCC-compatible define >>>>> __attribute__(X) as the empty expansion, so you silently get a >>>>> different ABI. >>>> [...] >>>>> There is really no need to specify 32-byte alignment here. Is not >>>>> even the size of a standard cache line. It can result in crashes if >>>>> these structs are heap-allocated using malloc, when optimizing for >>>>> AVX2. >>>> >>>> Why would it be valid to allocate those with malloc ? Isn't it the >>>> purpose of posix_memalign() ? >>> >>> It would not be valid, but I don't think we have diagnostics for C >>> like we have them for C++'s operator new. >> >> We could at least make an effort to let people know that alignment is >> required here when allocating struct rseq and struct rseq_cs on the >> heap by adding some comments to that effect in linux/rseq.h ? > > We could use different types on the glibc side, then no special > programmer action will be needed. Can't this lead to problems when mixing up compile units which have been compiled with linux/rseq.h with compile units compiled against sys/rseq.h ? Let me take a step back and try to understand. So far, there appears to be two scenarios where having a 64-byte alignment attribute on struct rseq and struct rseq_cs can cause problems: 1) A user-space programmer uses malloc() to dynamically allocate struct rseq or struct rseq_cs, which does not satisfy any of the alignment requirement of the structure. Combining this with compiler expectations that the structure needs to be aligned on 64-byte (e.g. -mavx2) breaks things. For this first scenario, I am proposing that we document that the programmer should have used posix_memalign(), which provides the required alignment guarantees. 2) A user-space programmer mixes code compiled with compilers honouring the aligned attribute with other compile units compiled with compilers which discard those GCC extension attributes silently, embeds those into a structure, and get different struct layouts. The _Static_assert in sys/rseq.h should detect the case where a compiler is not honouring the aligned attribute, right ? > >>>>>> However, now that it is in the wild, it's a bit late to change that. >>>>> >>>>> I had forgotten about the alignment crashes. I think we should >>>>> seriously consider changing the types. 8-( >>>> >>>> I don't think this is an option at this stage given that it is part >>>> of the Linux kernel UAPI. I am not convinced that it is valid at all >>>> to allocate struct rseq or struct rseq_cs with malloc(), because it >>>> does not guarantee any alignment. >>> >>> The kernel ABI doesn't change. The kernel cannot use the alignment >>> information anyway. Userspace struct layout may change in subtle >>> ways, though. >> >> Considering the amount of pain this can cause in user-space, and because >> it can break userspace, this is not a UAPI change I am willing to consider. >> I'm not sure why we are even discussing the possibility of breaking a Linux >> UAPI considering that those are set in stone. > > Again, the kernel interface is NOT affected. Only if the struct is > used in a non-top-level fashion across an ABI boundary in userspace. > I think making the change now is better than dealing with the breakage > in rseq users when they are built with -mavx2. What I am missing is what are the issues that persist once we add proper documentation of alignment requirements for heap allocation and a static assert to fail early when compiled with a compiler dismissing the aligned attribute ? As you point out, changing the currently public linux/rseq.h UAPI header to remove those attributes ends up breaking user-space in scenarios of non-top-level use across ABI boundary. This is not kernel-vs-userspace ABI, but an ABI exposed by the kernel which ends up being used to coordinate user-space objects within a program. Breaking that does not appear to be any more acceptable. As I recall, the hard requirement for Linux ABIs is to do not break userspace, period. There is not mention of kernel-vs-userspace or userspace-vs-userspace. So if the end result of this change is to break user-space, it should not be changed. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
