Re: Bug#1003095: /usr/bin/script: hangs when child doesn't read input fast enough

[Karel Zak <kzak@redhat.com>]

On Tue, Jan 04, 2022 at 06:31:24PM +0100, наб wrote:
> Control: tags -1 + upstream
> 
> On Tue, Jan 04, 2022 at 05:24:54PM +0100, Chris Hofstaedtler wrote:
> > * наб <nabijaczleweli@nabijaczleweli.xyz> [220104 00:06]:
> > > (This, at least, responds to ^\, but it also seems to function
> > >  slightly differently. Also, this is a race and you're more
> > >  likely to lose it under strace. The loopy thing seems
> > >  like it's pretty good at hitting it 100% of the time.)
> As an additional note, because it's a race, if you're using bash,
>   script < some-photo.jpeg
> also hangs, because setup takes long enough.
> 
> > 1) is this Debian-specific or already present upstream?
> Debian doesn't patch script.c at all, so this is an upstream bug.
> 
> > 2) did this work with previous versions of util-linux?
> The oldest one I fould from the site at Homepage: in d/control is
> "util-linux-ng 2.13", dated 19.1.2012. It's much closer to the original
> 4.4BSD-Lite implementation and still forks twice. As expected, testing
> reveals it does not have the bug.
> 
> Performing a simple manual bisect across the versions available therein
> reveals that 2.25 is the first broken version. (Though, skimming the
> source, with a slightly different code path (select(2)?), since it still
> double-forks and is not so hard-stuck so as to be immune to ^\.)
> 
> The first version that does get hard-stuck (because it forks once
> and only uses poll) is 2.27.

Resolve the problem with the signal should be simple. Now it
ignores all when it write to the master. Something like:


diff --git a/lib/pty-session.c b/lib/pty-session.c
index 6f038e1c5..84ea33860 100644
--- a/lib/pty-session.c
+++ b/lib/pty-session.c
@@ -292,7 +292,20 @@ static int write_output(char *obuf, ssize_t bytes)
 
 static int write_to_child(struct ul_pty *pty, char *buf, size_t bufsz)
 {
-       return write_all(pty->master, buf, bufsz);
+       sigset_t set, org;
+       int rc;
+
+       sigemptyset(&set);
+       sigemptyset(&org);
+       sigaddset(&set, SIGINT);
+       sigaddset(&set, SIGTERM);
+
+       sigprocmask(SIG_UNBLOCK, &set, &org);
+
+       rc = write_all(pty->master, buf, bufsz);
+
+       sigprocmask(SIG_SETMASK, &org, NULL);
+       return rc;
 }


-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com