Re: [RFC PATCH for 5.8 3/4] rseq: Introduce RSEQ_FLAG_RELIABLE_CPU_ID

* Mathieu Desnoyers:

Those are very good points. One possibility we have would be to let
glibc do the rseq registration without the RSEQ_FLAG_RELIABLE_CPU_ID
flag. On kernels with the bug present, the cpu_id field is still good
enough for typical uses of sched_getcpu() which does not appear to
have a very strict correctness requirement on returning the right
cpu number.

Then libraries and applications which require a reliable cpu_id
field could check this on their own by calling rseq with the
RSEQ_FLAG_RELIABLE_CPU_ID flag. This would not make the state more
complex in __rseq_abi, and let each rseq user decide about its own
fate: whether it uses rseq or keeps using an rseq-free fallback.

I am still tempted to allow combining RSEQ_FLAG_REGISTER |
RSEQ_FLAG_RELIABLE_CPU_ID for applications which would not be using
glibc, and want to check this flag on thread registration.

Well, you could add a bug fix level field to the __rseq_abi variable.
Then applications could check if the kernel has the appropriate level
of non-buggyness.  But the same thing could be useful for many other
kernel interfaces, and I haven't seen such a fix level value for them.
What makes rseq so special?

It won't help with the present bug, but maybe we should add an rseq
API sub-call that blocks future rseq registration for the thread.
Then we can add a glibc tunable that flips off rseq reliably if people
do not want to use it for some reason (and switch to the non-rseq
fallback code instead).  But that's going to help with future bugs
only.