[PATCH v10 09/13] x86/um: nommu: signal handling

[Hajime Tazaki]

Hello Benjamin,

On Sat, 28 Jun 2025 00:02:05 +0900,
Benjamin Berg wrote:
> 
> Hi,
> 
> On Fri, 2025-06-27 at 22:50 +0900, Hajime Tazaki wrote:
> > thanks for the comment on the complicated part of the kernel (signal).
> 
> This stuff isn't simple.
> 
> Actually, I am starting to think that the current MMU UML kernel also
> needs a redesign with regard to signal handling and stack use in that
> case. My current impression is that the design right now only permits
> voluntarily scheduling. More specifically, scheduling in response to an
> interrupt is impossible.
> 
> I suppose that works fine, but it also does not seem quite right.

thanks for the info.  it's very useful to understand what's going on.

(snip)

> > > > +void set_mc_userspace_relay_signal(mcontext_t *mc)
> > > > +{
> > > > + mc->gregs[REG_RIP] = (unsigned long) __userspace_relay_signal;
> > > > +}
> > > > +
> > 
> > This is a bit scary code which I tried to handle when SIGSEGV is
> > raised by host for a userspace program running on UML (nommu).
> > 
> > # and I should remember my XXX tag is important to fix....
> > 
> > let me try to explain what happens and what I tried to solve.
> > 
> > The SEGV signal from userspace program is delivered to userspace but
> > if we don't fix the code raising the signal, after (um) rt_sigreturn,
> > it will restart from $rip and raise SIGSEGV again.
> > 
> > # so, yes, we've already relied on host and um's rt_sigreturn to
> >   restore various things.
> > 
> > when a uml userspace crashes with SIGSEGV,
> > 
> > - host kernel raises SIGSEGV (at original $rip)
> > - caught by uml process (hard_handler)
> > - raise a signal to uml userspace process (segv_handler)
> > - handler ends (hard_handler)
> > - (host) run restorer (rt_sigreturn, registered by (libc)sigaction,
> >   not (host) rt_sigaction)
> > - return back to the original $rip
> > - (back to top)
> > 
> > this is the case where endless loop is happened.
> > um's sa_handler isn't called as rt_sigreturn (um) isn't called.
> > and the my original attempt (__userspace_relay_signal) is what I tried.
> > 
> > I agree that it is lazy to call a dummy syscall (indeed, getpid).
> > I'm trying to introduce another routine to jump into userspace and
> > call (um) rt_sigreturn after (host) rt_sigreturn.
> > 
> > > And this is really confusing me. The way I am reading it, the code
> > > tries to do:
> > >    1. Rewrite RIP to jump to __userspace_relay_signal
> > >    2. Trigger a getpid syscall (to do "nothing"?)
> > >    3. Let do_syscall_64 fire the signal from interrupt_end
> > 
> > correct.
> > 
> > > However, then that really confuses me, because:
> > >  * If I am reading it correctly, then this approach will destroy the
> > >    contents of various registers (RIP, RAX and likely more)
> > >  * This would result in an incorrect mcontext in the userspace signal
> > >    handler (which could be relevant if userspace is inspecting it)
> > >  * However, worst, rt_sigreturn will eventually jump back
> > >    into__userspace_relay_signal, which has nothing to return to.
> > >  * Also, relay_signal doesn't use this? What happens for a SIGFPE, how
> > >    is userspace interrupted immediately in that case?
> > 
> > relay_signal shares the same goal of this, indeed.
> > but the issue with `mc->gregs[REG_RIP]` (endless signals) still exists
> > I guess.
> 
> Well, endless signals only exist as long as you exit to the same
> location. My suggestion was to read the user state from the mcontext
> (as SECCOMP mode does it) and executing the signal right away, i.e.:

thanks too;  below is my understanding.

>  * Fetch the current registers from the mcontext

I guess this is already done in sig_handler_common().

>  * Push the signal context onto the userspace stack

(guess) this is already done on handle_signal() => setup_signal_stack_si().

>  * Modify the host mcontext to set registers for the signal handler

this is something which I'm not well understanding.
- do you mean the host handler when you say "for the signal handler" ?
  or the userspace handler ?
- if former (the host one), maybe mcontext is already there so, it
  might not be the one you mentioned.
- if the latter, how the original handler (the host one,
  hard_handler()) works ? even if we can call userspace handler
  instead of the host one, we need to call the host handler (and
  restorer).  do we call both ?
- and by "to set registers", what register do you mean ? for the
  registers inspected by userspace signal handler ?  but if you set a
  register, for instance RIP, as the fault location to the host
  register, it will return to RIP after handler and restart the fault
  again ?

>  * Jump back to userspace by doing a "return"

this is still also unclear to me.

it would be very helpful if you point the location of the code (at
uml/next tree) on how SECCOMP mode does.  I'm also looking at but
really hard to map what you described and the code (sorry).

all of above runs within hard_handler() in nommu mode on SIGSEGV.
my best guess is this is different from what ptrace/seccomp do.

> Said differently, I really prefer deferring as much logic as possible
> to the host. This is both safer and easier to understand. Plus, it also
> has the advantage of making it simpler to port UML to other
> architectures.

okay.

> 
> > > Honestly, I really think we should take a step back and swap the
> > > current syscall entry/exit code. That would likely also simplify
> > > floating point register handling, which I think is currently
> > > insufficient do deal with the odd special cases caused by different
> > > x86_64 hardware extensions.
> > > 
> > > Basically, I think nommu mode should use the same general approach as
> > > the current SECCOMP mode. Which is to use rt_sigreturn to jump into
> > > userspace and let the host kernel deal with the ugly details of how to
> > > do that.
> > 
> > I looked at how MMU mode (ptrace/seccomp) does handle this case.
> > 
> > In nommu mode, we don't have external process to catch signals so, the
> > nommu mode uses hard_handler() to catch SEGV/FPE of userspace
> > programs.  While mmu mode calls segv_handler not in a context of
> > signal handler.
> > 
> > # correct me if I'm wrong.
> > 
> > thus, mmu mode doesn't have this situation.
> 
> Yes, it does not have this specific issue. But see the top of the mail
> for other issues that are somewhat related.
> 
> > I'm attempting various ways; calling um's rt_sigreturn instead of
> > host's one, which doesn't work as host restore procedures (unblocking
> > masked signals, restoring register states, etc) aren't called.
> > 
> > I'll update here if I found a good direction, but would be great if
> > you see how it should be handled.
> 
> Can we please discuss possible solutions? We can figure out the details
> once it is clear how the interaction with the host should work.

I was wishing to update to you that I'm working on it.  So, your
comments are always helpful to me.  Thanks.

-- Hajime

> I still think that the idea of using the kernel task stack as the
> signal stack is really elegant. Actually, doing that in normal UML may
> be how we can fix the issues mentioned at the top of my mail. And for
> nommu, we can also use the host mcontext to jump back into userspace
> using a simple "return".
> 
> Conceptually it seems so simple.
> 
> Benjamin
> 
> 
> > 
> > -- Hajime
> > 
> > > I believe that this requires a second "userspace" sigaltstack in
> > > addition to the current "IRQ" sigaltstack. Then switching in between
> > > the two (note that the "userspace" one is also used for IRQs if those
> > > happen while userspace is executing).
> > > 
> > > So, in principle I would think something like:
> > >  * to jump into userspace, you would:
> > >     - block all signals
> > >     - set "userspace" sigaltstack
> > >     - setup mcontext for rt_sigreturn
> > >     - setup RSP for rt_sigreturn
> > >     - call rt_sigreturn syscall
> > >  * all signal handlers can (except pure IRQs):
> > >     - check on which stack they are
> > >       -> easy to detect whether we are in kernel mode
> > >     - for IRQs one can probably handle them directly (and return)
> > >     - in user mode:
> > >        + store mcontext location and information needed for rt_sigreturn
> > >        + jump back into kernel task stack
> > >  * kernel task handler to continue would:
> > >     - set sigaltstack to IRQ stack
> > >     - fetch register from mcontext
> > >     - unblock all signals
> > >     - handle syscall/signal in whatever way needed
> > > 
> > > Now that I wrote about it, I am thinking that it might be possible to
> > > just use the kernel task stack for the signal stack. One would probably
> > > need to increase the kernel stack size a bit, but it would also mean
> > > that no special code is needed for "rt_sigreturn" handling. The rest
> > > would remain the same.
> > > 
> > > Thoughts?
> > > 
> > > Benjamin
> > > 
> > > > [SNIP]
> > > 
> > 
>