* 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.
