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

[Christian Brauner]

On Tue, Jul 14, 2020 at 03:28:47PM +0200, Florian Weimer via Libc-alpha wrote:
> * 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.
> 
> Indeed.
> 
> > 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).
> 
> It works reliably as long as glibc only ever uses the minimum rseq size.
> And since glibc monopolizes the rseq registration, applications cannot
> register a larger area.  So there is no way to make use of any future
> kernel extensions.

But when you bump ABI in glibc you can switch to a new rseq size, no?

Christian