openssh bugs - Jun 2009 - [Bug 396] sshd orphans processes when no pty allocated

https://bugzilla.mindrot.org/show_bug.cgi?id=396


Marc Herbert <marc.herbert+mindrot at gmail.com> changed:

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--- Comment #14 from Marc Herbert <marc.herbert+mindrot at gmail.com> 
2009-06-11 04:29:55 ---
(In reply to comment #9)
> Bug #52 is sshd waiting for descriptors to close on a clean shutdown.  This
one
> is subprocesses not knowing that sshd (and thus their stdin/stdout) has
gone
> away on abnormal termination, usually because they don't check if their
read()
> calls on stdin return zero.
People like me not comfortable with patching their ssh server can
probably take advantage of this workaround I wrote. This is just a "EOF
to SIGHUP" converter implemented in 4 lines of shell script (it simply
inserts a "cat" between sshd and the no longer orphan process).

TUBE=/tmp/myfifo.$$; mkfifo "$TUBE"
<"$TUBE"   yourApplicationIgnoringEOFMoreOftenThanNot   & 
appPID=$!
# unlike the above, cat WILL notice the EOF and politely die
cat >"$TUBE"
kill -HUP -$appPID; rm "$TUBE"

Feedback is welcome; do not hesitate to email me.

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https://bugzilla.mindrot.org/show_bug.cgi?id=396





--- Comment #15 from Marc Herbert <marc.herbert+mindrot at gmail.com> 
2009-07-02 00:33:14 ---
(In reply to comment #14)
>(it simply inserts a "cat" between sshd and the no longer orphan
process).
Ahem, except sshd seems to create standard input in non-blocking mode,
which makes cat intermittently fail:

  cat: -: Resource temporarily unavailable

See this discussion:
http://lists.mindrot.org/pipermail/openssh-unix-dev/2005-July/023090.html

I initially used a recent ssh version 5.1 and did NOT hit this
non-blocking problem. Then I tried to use my cat trick with ssh version
3.4 and version 4.3 and DID intermittently hit this non-blocking
problem. Could this be due to version 5.1 now passing 3 pipes for
stdin/out/err as opposed to a socketpair in earlier versions?


Anyway, I switch to using socat instead of cat, and socat seems to deal
with non-blocking stdin in a more robust fashion. Here is the updated
workaround:

TUBE=/tmp/myfifo.$$; mkfifo "$TUBE"
<"$TUBE"   yourApplicationIgnoringEOFMoreOftenThanNot   & 
appPID=$!
# unlike the above, socat WILL notice the EOF and politely die
socat -u STDIN "PIPE:$TUBE"
kill -HUP -$appPID; rm "$TUBE"

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