linux-kernel - Re: [RFC][CFT] signal handling fixes

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Date:   Wed, 28 Jul 2021 11:23:24 +1000 (AEST)
From:   Finn Thain <fthain@...ux-m68k.org>
To:     Al Viro <viro@...iv.linux.org.uk>
cc:     linux-m68k@...ts.linux-m68k.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][CFT] signal handling fixes

On Tue, 27 Jul 2021, Al Viro wrote:

> On Tue, Jul 27, 2021 at 08:21:52PM +1000, Finn Thain wrote:
> > On Sun, 25 Jul 2021, Al Viro wrote:
> > 
> > > 
> > > 	The series is on top of 5.14-rc1; it lives in
> > > git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs.git #untested.m68k
> > > Individual patches in followups...
> > > 
> > > 	_Very_ lightly tested on aranym; no real hardware to test it on.
> > > Any help with review and testing would be very welcome.
> > > 
> > 
> > I can test this branch on a Motorola 68040 machine I have here. Can you 
> > advise how to get decent code coverage? Maybe there's a package out there 
> > with a signal-heavy test suite? Maybe I need a break point in a signal 
> > handler? Or perhaps just send ^C to a process running under strace?
> 
> Generally, SIGINT is not the best insertion vector...
> 

True. I see that 'man 7 signal' says that SIGQUIT will produce a coredump. 
Would that contain anything of interest?

> Set a handler of e.g. SIGALRM with sigaction(), with a couple of other signals
> in sa_mask (e.g. SIGUSR1 and SIGUSR2).  With raise() on those inside the
> SIGALRM handler - then they will become deliverable on return from handler.
> And have SIGUSR1 and SIGUSR2 handlers print siginfo and ucontext contents
> (have them set with SA_SIGINFO in sa_flags, look at the second and third
> arguments of sighandler).
> 
> Use alarm(2) to arrange for SIGALRM and sit in a tight loop - that'll give you
> delivery on return from interrupt.  Alternatively, raise(SIGALRM) will give
> you delivery on return from trap.  And making that a SIGBUS handler instead,
> mmapping a file, truncating it to 0 and dereferencing something in mmapped
> area will give you delivery on return from access error trap.  Division by
> zero (and insertion handler on SIGFPE) ought to give you a type 2 exception
> stack frame (4 bytes of aux data, that makes shifted exception frame bugger
> format and vector fields of the original).
> 
> FWIW, the third argument of handler points to
> struct ucontext {
>         unsigned long     uc_flags;
>         struct ucontext  *uc_link;
>         stack_t           uc_stack;
>         struct mcontext   uc_mcontext;
>         unsigned long     uc_filler[80];
>         sigset_t          uc_sigmask;   /* mask last for extensibility */
> };
> and type/vector is stored in uc_filler[54] (216 bytes into the array), with
> aux data from exception stack frame starting from uc_filler[55].
> 

OK, give me a week or so and I'll see what I can come up with.