rseq: How to test for compat task at signal delivery

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

Hi Andy,

I would like to make the behavior rseq on compat tasks more robust
by ensuring that kernel/rseq.c:rseq_get_rseq_cs() clears the high
bits of rseq_cs->abort_ip, rseq_cs->start_ip and
rseq_cs->post_commit_offset when a 32-bit binary is run on a 64-bit
kernel.

The intent here is that if user-space has garbage rather than zeroes
in its struct rseq_cs fields padding, the behavior will be the same
whether the binary is run on 32-bit or 64 kernels.

I know that internally, the kernel is making a transition from
is_compat_task() to in_compat_syscall().

I'm fine with using in_compat_syscall() when rseq_get_rseq_cs() is
invoked from a system call, but is it OK to call it when it is
invoked from signal delivery ? AFAIU, signals can be delivered
upon return from interrupt as well.

If not, what strategy do you recommend for arch-agnostic code ?

Thanks,

Mathieu

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