rseq with syscall as the last instruction

[Dmitry Vyukov]

Hi rseq maintainers,

I wonder if rseq can be used in the following scenario (or extended to be used).
I want to pass extra arguments to syscalls using a kind of
side-channel, for example, to say "do fault injection for the next
system call", or "trace the next system call". But what is "next"
system call should be atomic with respect to signals.
Let's say there is shared per-task memory location known to the kernel
where these arguments can be stored:

__thread struct trace_descriptor desk;
prctl(REGISTER_PER_TASK_TRACE_DESCRIPTOR, &desk);

then before a system call I can setup the descriptor to enable tracing:

desk = ...
SYSCALL;

The problem is that if a signal arrives in between we setup desk and
SYSCALL instruction, we will actually trace some unrelated syscall in
the signal handler.
Potentially the kernel could switch/restore 'desk' around syscall
delivery, but it becomes tricky/impossible for signal handlers that do
longjmp or mess with PC in other ways; and also would require
extending ucontext to include the desc information (not sure if it's
feasible).

So instead the idea is to protect this sequence with rseq that will be
restarted on signal delivery:

enter rseq critical section with end right after SYSCALL instruction;
desk = ...
SYSCALL;

Then, the kernel can simply clear 'desc', on syscall delivery.

rseq docs seem to suggest that this can work:

https://lwn.net/Articles/774098/
+Restartable sequences are atomic with respect to preemption (making it
+atomic with respect to other threads running on the same CPU), as well
+as signal delivery (user-space execution contexts nested over the same
+thread). They either complete atomically with respect to preemption on
+the current CPU and signal delivery, or they are aborted.

But the doc also says that the sequence must not do syscalls:

+Restartable sequences must not perform system calls. Doing so may result
+in termination of the process by a segmentation fault.

The question is:
Can this restriction be weakened to allow syscalls as the last instruction?
For flags in this case we would pass
RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT and
RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE, but no
RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL.

I don't see any fundamental reasons why this couldn't work b/c if we
restart only on signals, then once we reach the syscall, rseq critical
section is committed, right?

Do you have any feeling of how hard it would be to support or if there
can be some implementation issues?

Thank you