Re: Problem with syscall_disestablish() - PR kern/50430

[Paul Goyette <paul%vps1.whooppee.com@localhost>]

On Thu, 19 Nov 2015, Masao Uebayashi wrote:

> On Wed, Nov 18, 2015 at 11:07 AM, Paul Goyette <paul%vps1.whooppee.com@localhost> wrote:
> > Based on earlier comments, I've come up with a much-less-intrusive
> > set of changes.  This time around, there are no bit masks and no new
> > members in any system structures.  (I'm pretty sure we won't even
> > need a kernel version bump for this.)
> >
> > Instead, I've modified sy_call() to check the current value of
> > l_sysent before updating with the new syscall pointer.  If the
> > current value is non-NULL then set a new flag bit LP_LOST_SYSENT in
> > the lwp's l_pflag member.  This check is the only per-syscall cost:
> >
> >         if (l->l_sysent != NULL)
> >                 l->l_pflag |= LP_LOST_SYSENT;
>
> My understanding is that l_sysent is *always* overriden by syscalls
> from within signal handlers, right?

It's not a question of "being overwritten".  The real situation is
that the _initial_ syscall returns first (with ERESTART), and l_sysent
is reset to NULL.  Then when the signal handler calls some other
syscall, there is nothing to lose.

Then, when the handler returns, the _original_ syscall is automatically
restarted.

As I said earlier, I have not identified the exact code where all of
this happens.  But empirical evidence (from printf()s inserted in
sy_call() both before and after the dispatch) shows that it works as
I've described.  There was never any non-NULL value in l_sysent at
the point where sy_call() stores a new value.  And the printf() I
had inserted after the syscall dispatch clearly showed a return value
of -3 == ERESTART.


> I don't fully understand these things yet, but ... my gut feeling is
> that l_sysent should be saved on stack in mi_switch(), like oldspl,
> before cpu_switchto(), and restored after that.  In resuming code
> path, check if syscall exists, in case syscall was forcibly
> disestablished.  If it doesn't exist, forcibly kill self lwp ... maybe
> using sigexit()?

I'm expecting that the old l_sysent is being saved on the stack by
the signal code;  I just don't know where (since there doesn't seem
to be any such reference to l_sysent!).

When the syscall gets restarted, it will get auto-loaded again if it
has been disestablished, so nothing is lost.

> (I believe someone from CS department understand such things 100% and
> correct me if wrong. ;)


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| Paul Goyette     | PGP Key fingerprint:     | E-mail addresses:       |
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