[Bug 1814905] Re: dash exits when receiving wall message

[Seth Arnold]

Hello Imre, thanks for the report. I can't reproduce this issue myself,
nor could I find anything in the source code that would suggest dash
would try to quit. In fact I'm not even sure that dash knows that it's
happened.

Here's the tail end of strace output (run on the same terminal as dash,
to force intermingling of syscall output and terminal output), showing
dash handling a write(1) without trouble, and checking for completed
jobs after I hit enter several times:

geteuid()                               = 1000
getegid()                               = 1000
rt_sigaction(SIGINT, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGINT, {sa_handler=0x55a82089e200, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
rt_sigaction(SIGQUIT, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGQUIT, {sa_handler=SIG_IGN, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
rt_sigaction(SIGTERM, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGTERM, {sa_handler=SIG_IGN, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
openat(AT_FDCWD, "/dev/tty", O_RDWR)    = 3
fcntl(3, F_DUPFD, 10)                   = 10
close(3)                                = 0
fcntl(10, F_SETFD, FD_CLOEXEC)          = 0
ioctl(10, TIOCGPGRP, [23306])           = 0
getpgrp()                               = 23306
rt_sigaction(SIGTSTP, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGTSTP, {sa_handler=SIG_IGN, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
rt_sigaction(SIGTTOU, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGTTOU, {sa_handler=SIG_IGN, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
rt_sigaction(SIGTTIN, NULL, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
rt_sigaction(SIGTTIN, {sa_handler=SIG_DFL, sa_mask=~[RTMIN RT_1], sa_flags=SA_RESTORER, sa_restorer=0x7fd3be19df20}, NULL, 8) = 0
setpgid(0, 23308)                       = 0
ioctl(10, TIOCSPGRP, [23308])           = 0
wait4(-1, 0x7ffcdf89717c, WNOHANG|WSTOPPED, NULL) = -1 ECHILD (No child processes)
write(2, "$ ", 2$ )                       = 2
read(0, 
Message from sarnold at hunt on pts/13 at 17:41 ...
hello
EOF

"\n", 8192)                     = 1
wait4(-1, 0x7ffcdf89717c, WNOHANG|WSTOPPED, NULL) = -1 ECHILD (No child processes)
write(2, "$ ", 2$ )                       = 2
read(0, 
"\n", 8192)                     = 1
wait4(-1, 0x7ffcdf89717c, WNOHANG|WSTOPPED, NULL) = -1 ECHILD (No child processes)
write(2, "$ ", 2$ )                       = 2
read(0, ^C0x55a820aa9a60, 8192)           = ? ERESTARTSYS (To be restarted if SA_RESTART is set)
--- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} ---
rt_sigprocmask(SIG_SETMASK, [], ~[KILL STOP RTMIN RT_1], 8) = 0
write(2, "\n", 1
)                       = 1
wait4(-1, 0x7ffcdf89717c, WNOHANG|WSTOPPED, NULL) = -1 ECHILD (No child processes)
write(2, "$ ", 2$ )                       = 2
read(0, "", 8192)                       = 0
write(2, "\n", 1
)                       = 1
ioctl(10, TIOCSPGRP, [23306])           = 0
setpgid(0, 23306)                       = 0
close(10)                               = 0
exit_group(0)                           = ?
+++ exited with 0 +++


I tried both dash in a terminal (urxvt) and ssh -t sarnold at localhost /bin/dash -- both behaved identically.

Could you try to reproduce this while you're attached to the shell with
gdb or perhaps perf trace?

Thanks

** Changed in: dash (Ubuntu)
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1814905

Title:
  dash exits when receiving wall message

Status in dash package in Ubuntu:
  Incomplete

Bug description:
  When using "/bin/sh" (dash) as a shell for ssh, or just starting
  /bin/sh in a terminal, when the shell receives a wall message (from
  another user or root), it just quits.

  How to reproduce:
  - start a terminal (xterm, gnome-terminal, etc), usually starts with bash
  - from bash run 'sh' command to open a dash shell
  - start another terminal, and run: echo "foo" | wall
  - the broadcast message appears on both terminals
  - in the first terminal you are thrown back to bash, because dash exits

  I've tried this on Ubuntu 14.04, 16.04, 18.04 (64bits) latest updates
  and it's happening on all of them. I've also tried with latest dash
  from git, compiled on Ubuntu 16.04, same problem, that's why I don't
  think it's dash's problem.

  Also tried on CentOS 7, there is no problem there.

  The problem appears this morning (6 February) after updating to latest
  version of 16.04, 18.04. I also have a box with 16.04 but not the
  latest updates, and on that box the problem does not happen.

  If one uses dash as a login shell, and enters on a remote box with SSH
  for example, the connection can be terminated by another user simply
  wall'ing a broadcast message (or a system daemon like NUT). This could
  be taken as a potential security issue.

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