[RFC PATCH glibc] Linux: Use fixed rseq_len value for rseq registration

[Florian Weimer]

* Mathieu Desnoyers:

> ----- On Jul 14, 2020, at 12:01 PM, Szabolcs Nagy szabolcs.nagy@arm.com wrote:
>
>> The 07/14/2020 11:30, Mathieu Desnoyers via Libc-alpha wrote:
>>> > I think we are looking at this from the wrong perspective.  It's not
>>> > userspace that is setting the size here, it's the kernel based on the
>>> > features it supports.  So the kernel should put the size into the
>>> > auxiliary vector, and the registration should use that size.  But that
>>> > doesn't align well with the use of an ELF TLS symbol.
>> 
>> this is why it is better to use a function that returns
>> a pointer than a tls symbol as public abi.
>> 
>>> We have a few possible ways to do things here:
>>> 
>>> 1) Kernel exports supported size, incompatible with ELF TLS symbol,
>>> 
>>> 2) Userspace dictates supported size, compatible with ELF TLS symbol,
>>>    triggers failure if the kernel supports a smaller size,
>>> 
>>> 3) Userspace lets kernel know how much space is available for struct rseq
>>>    (through user_size field), and the kernel lets user-space know how much
>>>    of that structure is being filled (through kernel_size field). This
>>>    would also be compatible with ELF TLS symbol AFAIU, and would allow
>>>    extending struct rseq.
>>> 
>>> Option (3) would allow us to have the speed gains that come with using a
>>> TLS from the fast-path, while allowing extension of struct rseq.
>>> 
>>> Or is there anything in that scheme that breaks ELF rules or C language
>>> requirements ?
>> 
>> how would users access those extension fields?
>
> if (__rseq_abi.flags & RSEQ_TLS_FLAG_SIZE) {
>   /* Allowed to access user_size and kernel_size. */
>   if (__rseq_abi.kernel_size >= offsetof(struct rseq, myfield) + sizeof(((struct rseq *)NULL)->myfield)) {
>     /* Allowed to access __rseq_abi.myfield. User code should remember this, e.g. in a static variable. */
>   }
> }

Technically, this needs a compiler extension, so that the allowed
accesses to the __rseq_abi.myfield field are not moved before the flags
and size checks.

Thanks,
Florian