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From: Herbert Xu <herbert@gondor.apana.org.au>
To: Harald van Dijk <harald@gigawatt.nl>
Cc: steffen@sdaoden.eu, dash@vger.kernel.org,
	vda.linux@googlemail.com, jilles@stack.nl
Subject: [PATCH] jobs: Block signals during tcsetpgrp
Date: Wed, 6 Jan 2021 15:45:12 +1100	[thread overview]
Message-ID: <20210106044512.GA28191@gondor.apana.org.au> (raw)
In-Reply-To: <9bb6fbb5-20e1-eae1-0144-67a4c7e20496@gigawatt.nl>

Harald van Dijk <harald@gigawatt.nl> wrote:
> On 19/12/2020 22:21, Steffen Nurpmeso wrote:
>> Steffen Nurpmeso wrote in
>>   <20201219172838.1B-WB%steffen@sdaoden.eu>:
>>   |Long story short, after falsely accusing BSD make of not working
>> 
>> After dinner i shortened it a bit more, and attach it again, ok?
>> It is terrible, but now less redundant than before.
>> Sorry for being so terse, that problem crosses my head for about
>> a week, and i was totally mislead and if you bang your head
>> against the wall so many hours bugs or misbehaviours in a handful
>> of other programs is not the expected outcome.
> 
> I think a minimal test case is simply
> 
> all:
>         $(SHELL) -c 'trap "echo TTOU" TTOU; set -m; echo all good'
> 
> unless I accidentally oversimplified.
> 
> The SIGTTOU is caused by setjobctl's xtcsetpgrp(fd, pgrp) call to make 
> its newly started process group the foreground process group when job 
> control is enabled, where xtcsetpgrp is a wrapper for tcsetpgrp. (That's 
> in dash, the other variants may have some small differences.) tcsetpgrp 
> has this little bit in its specification:
> 
>        Attempts to use tcsetpgrp() from a process which is a member of
>        a background process group on a fildes associated with its con‐
>        trolling  terminal  shall  cause the process group to be sent a
>        SIGTTOU signal. If the calling thread is blocking SIGTTOU  sig‐
>        nals  or  the  process is ignoring SIGTTOU signals, the process
>        shall be allowed to perform the operation,  and  no  signal  is
>        sent.
> 
> Ordinarily, when job control is enabled, SIGTTOU is ignored. However, 
> when a trap action is specified for SIGTTOU, the signal is not ignored, 
> and there is no blocking in place either, so the tcsetpgrp() call is not 
> allowed.
> 
> The lowest impact change to make here, the one that otherwise preserves 
> the existing shell behaviour, is to block signals before calling 
> tcsetpgrp and unblocking them afterwards. This ensures SIGTTOU does not 
> get raised here, but also ensures that if SIGTTOU is sent to the shell 
> for another reason, there is no window where it gets silently ignored.
> 
> Another way to fix this is by not trying to make the shell start a new 
> process group, or at least not make it the foreground process group. 
> Most other shells appear to not try to do this.

This patch implements the blocking of SIGTTOU (and everything else)
while we call tcsetpgrp.

Reported-by: Steffen Nurpmeso <steffen@sdaoden.eu>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

diff --git a/src/jobs.c b/src/jobs.c
index 516786f..809f37c 100644
--- a/src/jobs.c
+++ b/src/jobs.c
@@ -1512,7 +1512,13 @@ showpipe(struct job *jp, struct output *out)
 STATIC void
 xtcsetpgrp(int fd, pid_t pgrp)
 {
-	if (tcsetpgrp(fd, pgrp))
+	int err;
+
+	sigblockall(NULL);
+	err = tcsetpgrp(fd, pgrp);
+	sigclearmask();
+
+	if (err)
 		sh_error("Cannot set tty process group (%s)", strerror(errno));
 }
 #endif
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

  parent reply	other threads:[~2021-01-06  4:46 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-19 17:28 dash 0.5.11.2, busybox sh 1.32.0, FreeBSD 12.2 sh: spring TTOU but should not i think Steffen Nurpmeso
2020-12-19 22:21 ` Steffen Nurpmeso
2020-12-19 23:52   ` Harald van Dijk
2020-12-21 16:24     ` Jilles Tjoelker
2020-12-21 19:43       ` Steffen Nurpmeso
2020-12-23 20:18       ` Harald van Dijk
2020-12-24 15:29         ` Jilles Tjoelker
2021-01-10 23:56         ` Harald van Dijk
2021-01-06  4:46       ` Herbert Xu
2021-01-06  4:45     ` Herbert Xu [this message]
2021-01-06 21:16       ` [PATCH] jobs: Block signals during tcsetpgrp Harald van Dijk
2021-01-06 22:41         ` Jilles Tjoelker
2021-01-07  7:36         ` Denys Vlasenko

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