SIGCHLD may be inherited blocked

[Nicolas Williams]

So, we just found some ugly behaviour of OpenSSH on Solaris.

Sometimes, it seems, sshd gets started with SIGCHLD blocked, this,
apparently, being the setting of sshd's parent (a shell no doubt);
signal blocking is inherited across exec*(). I don't know exactly which
shell, or what really is at fault, but it happens.

The problem is that the code in collect_children() first blocks SIGCHLD
(SIGCLD) and then resets the signal block mask to whatever it was
before, so if SIGCHLD was blocked to begin with, then it never gets
unblocked in sshd.

The resulting behaviour is that SSHv2 connections may hang.

The Solaris proc tools, specifically /usr/proc/bin/psig, along with
truss/strace, show the bug in action quite nicely.

As much as this behaviour may not be a bug in OpenSSH, it may nonetheless
be desirable to add a couple of calls to sigprocmask() in sshd.c:main()
to make sure that SIGCHLD is not blocked.

While looking at this I noticed that the compatibility shim in
openbsd-compat/sigact.c for sigprocmask() has a bug: the second argument
may be NULL but the shim does not check for this.

A patch to openbsd-compatc/sigact.c:sigprocmask() and sshd.c:main() is
attached.

Thoughts? Should I file a bug report in bugzilla?

Cheers,

Nico
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-------------- next part --------------
Index: 3_0_2p1_w_gssk5_ubsw_prod.2/openbsd-compat/sigact.c
--- 3_0_2p1_w_gssk5_ubsw_prod.2/openbsd-compat/sigact.c Wed, 21 Nov 2001 10:38:46 -0500
+++ 3_0_2p1_w_gssk5_ubsw_prod.2(w)/openbsd-compat/sigact.c Fri, 07 Jun 2002 15:42:50 -0400
@@ -61,6 +61,7 @@
 	sigset_t current = sigsetmask(0);
 
 	if (omask) *omask = current;
+	if (!mask) return 0;
 
 	if (mode==SIG_BLOCK)
 		current |= *mask;
Index: 3_0_2p1_w_gssk5_ubsw_prod.2/sshd.c
--- 3_0_2p1_w_gssk5_ubsw_prod.2/sshd.c Thu, 17 Jan 2002 17:53:49 -0500
+++ 3_0_2p1_w_gssk5_ubsw_prod.2(w)/sshd.c Fri, 07 Jun 2002 15:53:22 -0400
@@ -556,6 +556,11 @@
 	int startups = 0;
 	Key *key;
 	int ret, key_used = 0;
+	sigset_t curr_mask;
+
+	sigprocmask(0, NULL, &curr_mask);
+	sigdelset(&current_mask, SIGCHLD);
+	sigprocmask(SIG_SETMASK, &curr_mask, NULL);
 
 	__progname = get_progname(av[0]);
 	init_rng();
-------------- next part --------------

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