Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [RFC PATCH glibc 4/8] glibc: Perform rseq(2) registration at C startup and thread creation (v15) | Date | Thu, 19 Mar 2020 20:05:43 +0100 |
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* 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.
> >> >>>>>> 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. >>> >>> It is worth noting that rseq.h is not the only Linux uapi header >>> which uses __attribute__ ((aligned)), so this ABI problem exists today >>> anyway for those compilers. >> >> Yuck. Even with larger-than-16 alignment? > > There are two: > > target_core_user.h > 45:#define ALIGN_SIZE 64 /* Should be enough for most CPUs */ > 58: __u32 cmd_tail __attribute__((__aligned__(ALIGN_SIZE)));
That one is tough to figure out:
struct tcmu_mailbox { __u16 version; __u16 flags; __u32 cmdr_off; __u32 cmdr_size;
__u32 cmd_head;
/* Updated by user. On its own cacheline */ __u32 cmd_tail __attribute__((__aligned__(ALIGN_SIZE)));
} __attribute__((packed));
Apparently, the expectation is that the compiler ignores __attribute__ ((packed) in this context. Ugh.
> netfilter_bridge/ebtables.h:90: char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace)))); > netfilter_bridge/ebtables.h:132: unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace)))); > netfilter_bridge/ebtables.h:145: unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace)))); > netfilter_bridge/ebtables.h:158: unsigned char data[0] __attribute__ ((aligned (__alignof__(struct ebt_replace)))); > netfilter_bridge/ebtables.h:191: unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
I think these values are lower than max_align_t, so uncritical.
>>>> 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.
>>>>> 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.
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