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

[Mathieu Desnoyers]

----- On Jul 14, 2020, at 4:51 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> +  /* The rseq_len parameter does not allow extending struct rseq.  Fix its
>> +     value to 32 as expected by the Linux kernel.  */
>> +  ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, 32, 0, RSEQ_SIG);
> 
> If the layout of struct rseq can change in the kernel headers, than far
> more significant changes are needed.  glibc cannot change its ABI
> depending on the version of the kernel headers it is compiled against.

What I have in mind for struct rseq is that the current structure layout
is guaranteed as a "minimum" requirement. It can then be extended by the
kernel UAPI in a backward compatible way: if user-space provides enough
room for extension, and the kernel supports the additional features, then
user-space can use those additional features.

Adding extra room in the __rseq_abi can be done though:

- Upgrading glibc with knowledge of the extra layout,
- Preloading an interposition library which defines __rseq_abi with the
  extended layout,
- Defining __rseq_abi with extended layout in the application binary.

What I suspect is your concern here is that two glibc-2.32 builds against
different versions of kernel headers could lead to different __rseq_abi
size, which is unexpected.

Would using the internal struct rseq all the time in glibc (rather than
uapi linux/rseq.h) resolve your concern ? This way, glibc could integrate
extended layouts on version bump (e.g. 2.33).

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com