svn commit: r265003 - head/secure/usr.sbin/sshd
Jilles Tjoelker
jilles at stack.nl
Sun Aug 24 13:13:28 UTC 2014
On Sat, Aug 23, 2014 at 09:42:21PM +0300, Konstantin Belousov wrote:
> On Sat, Aug 23, 2014 at 07:52:44PM +0200, Jilles Tjoelker wrote:
> > On Fri, Aug 22, 2014 at 06:31:39PM +0300, Konstantin Belousov wrote:
> > > On Fri, Aug 22, 2014 at 03:43:53PM +0200, Jilles Tjoelker wrote:
> > > > This is good and necessary for SA_SIGINFO (because of the type of the
> > > > SIG_DFL and SIG_IGN constants, and because POSIX says so in the
> > > > description of SA_RESETHAND in the sigaction() page). However, there
> > > > seems no reason to clear the other flags, which have no effect when the
> > > > disposition is SIG_DFL or SIG_IGN but have historically always been
> > > > preserved. (Slight exception: if kernel code erroneously loops on
> > > > ERESTART, it will eat CPU time iff SA_RESTART is set, independent of the
> > > > signal's disposition.)
> > > Well, I already committed the patch with several bugs fixed comparing
> > > with what was mailed, before your feedback arrived.
> > > Do you consider it is important enough to revert the resetting of other
> > > flags ? In particular, your note about the traditional historic
> > > behaviour makes me wonder.
> > I consider it important enough. Clearing the other flags is not
> > POSIX-compliant and might break applications. For example, I can imagine
> > an application modifying a struct sigaction with sa_handler == SIG_DFL
> > from a sigaction() call.
> This feels somewhat strange to me. E.g., I can easily imagine an
> implementation which relies on some code executing in the process
> user context for default action on some signal. Having the flags,
> like SA_ONSTACK or SA_NODEFER to influence the handler is weird.
> Such implementation is not unix, but I think it is quite possible
> that cygwin or interix do core dumping in userspace.
The implementation of SA_ONSTACK and the like could just as well be tied
to a user-specified handler function.
Anyway, this imaginable application is slightly dumb, since it is either
blindly using SA_* flags from its parent or asking the kernel for things
it already knows. The latter might be done for "modularity" reasons.
> > > I do not see why SA_SIGINFO is so special that it must be reset,
> > > while other flags are not. The absence of the cases where the
> > > default/ignored disposition is affected by the flags seems rather
> > > arbitrary.
> > The difference is that SA_SIGINFO changes the disposition field from
> > sa_handler to sa_sigaction, and it is not unambiguously clear how
> > SIG_DFL and SIG_IGN are represented in sa_sigaction. Note that
> > sa_handler and sa_sigaction may or may not share storage, and
> > implementations may or may not support (void (*)(int, siginfo_t *, void
> > *))SIG_DFL.
> > For example, when I wrote system() using posix_spawn() in
> > https://lists.freebsd.org/pipermail/freebsd-hackers/2012-July/040065.html
> > I needed to know whether SIGINT and SIGQUIT were ignored or not. I wrote
> >
> > ] if ((intact.sa_flags & SA_SIGINFO) != 0 ||
> > ] intact.sa_handler != SIG_IGN)
> > ] (void)sigaddset(&defmask, SIGINT);
> > ] if ((quitact.sa_flags & SA_SIGINFO) != 0 ||
> > ] quitact.sa_handler != SIG_IGN)
> > ] (void)sigaddset(&defmask, SIGQUIT);
> >
> > in the assumption that there is always an actual handler if SA_SIGINFO
> > is set. I did not really like this code and eventually system() was
> > changed to use vfork() directly instead of posix_spawn(), but I think it
> > is correct.
> If the implementation provides separate storage for sa_handler and
> sa_sigaction, then isn't it more correct to assume that sa_handler is
> NULL when sa_sigaction is valid ? In other words, simple
> sa.sa_handler != SIG_IGN
> test would be more reasonable.
> I see that the SUSv4 is worded in a way that SA_SIGINFO always
> accompanies sa_sigaction. Are there any implementations which
> separate the storage for sa_handler and sa_sigaction ?
SUSv4 doesn't allow making any such assumption. The check for
(sa.sa_handler != SIG_IGN) will work fine on most implementations but an
application doing it is not POSIX-compliant.
I don't know of implementations that do not put sa_handler and
sa_sigaction in a union.
> > Note that this means that it is sometimes necessary to install a handler
> > function that will never be called. Per POSIX, SA_SIGINFO must be set
> > for sigwaitinfo() to be guaranteed to return siginfo_t data, and the
> > only way to do this is to specify a handler function, even though it
> > will never be called because the signal is masked. (A never-called
> > handler function also needs to be specified when using sigwait-like
> > functions with signals that default to ignore.)
> > The other flags do not affect the representation of the disposition, and
> > can therefore remain set without problem.
> Anyway, below is the patch with reverts the behaviour WRT flags other
> than SA_SIGINFO. I like the compactness of sigact_flag_test() calls,
> so I kept the function, despite it is only used in non-trivial ways
> for SA_SIGINFO flag test in kern_sigaction().
> diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
> index 8810bf3..f73c801 100644
> --- a/sys/kern/kern_sig.c
> +++ b/sys/kern/kern_sig.c
> @@ -625,9 +625,14 @@ static bool
> sigact_flag_test(struct sigaction *act, int flag)
> {
>
> - return ((act->sa_flags & flag) != 0 &&
> - (__sighandler_t *)act->sa_sigaction != SIG_IGN &&
> - (__sighandler_t *)act->sa_sigaction != SIG_DFL);
> + /*
> + * SA_SIGINFO is reset when signal disposition is set to
> + * ignore or default. Other flags are kept according to user
> + * settings.
> + */
> + return ((act->sa_flags & flag) != 0 && (flag != SA_SIGINFO ||
> + ((__sighandler_t *)act->sa_sigaction != SIG_IGN &&
> + (__sighandler_t *)act->sa_sigaction != SIG_DFL)));
> }
>
> /*
> @@ -916,7 +921,6 @@ siginit(p)
> for (i = 1; i <= NSIG; i++) {
> if (sigprop(i) & SA_IGNORE && i != SIGCONT) {
> SIGADDSET(ps->ps_sigignore, i);
> - SIGADDSET(ps->ps_sigintr, i);
> }
> }
> mtx_unlock(&ps->ps_mtx);
> @@ -936,10 +940,6 @@ sigdflt(struct sigacts *ps, int sig)
> SIGADDSET(ps->ps_sigignore, sig);
> ps->ps_sigact[_SIG_IDX(sig)] = SIG_DFL;
> SIGDELSET(ps->ps_siginfo, sig);
> - SIGADDSET(ps->ps_sigintr, sig);
> - SIGDELSET(ps->ps_sigonstack, sig);
> - SIGDELSET(ps->ps_sigreset, sig);
> - SIGDELSET(ps->ps_signodefer, sig);
> }
>
> /*
Looks good to me.
--
Jilles Tjoelker
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