Re: [BUG] Redirection bug in subshell's EXIT trap

[Harald van Dijk]

On 19/10/17 19:44, Vitaly Sinilin wrote:

Hi,

I've recently written a quite unusual script and faced a strange dash
behavior that seems not POSIX-ly correct to me.

Here is the script with a lot of debugging extra output.

(I am sorry about posting a pretty lengthy script, but I feel like a
shorten version will look just insane without the context.)


A shortened version is easier to debug:

  dash -c 'f()(trap cat EXIT);f<&-'

This is supposed to print error messages. With dash, it doesn't.


The problem here is that although getch is run in a subshell and it
got FD#4 as stdin it somehow has parent's stdin in its EXIT trap.

POSIX says: "The environment in which the shell executes a trap on
EXIT shall be identical to the environment immediately after the
last command executed before the trap on EXIT was taken."


Yes, it is a bug. The problem is that dash, just before checking whether 
to execute the EXIT handler, has already reset everything to continue 
parsing and evaluating the next command. Which is not supposed to 
happen, never going to happen, and messes up the execution of the handler.


This resetting is done by the reset() function, and although the simple 
fix would be to move its call after exitshell(), the old ChangeLog shows 
that it was originally (about 15 years ago) the other way around and 
intentionally switched to how it is now, because some of what reset() 
does is necessary even here:



 -- Herbert Xu   Sat, 26 Oct 2002 21:28:33 +1000

dash (0.4.2) unstable; urgency=low

  [...]
  * Call reset() before exitshell() is called.  This fixes the bug where
returning an error from a function running under set -e caused the exit
trap to be taken with evalskip set.


Aside from the issue with redirections, the current order has other 
problems too:


  dash -c 'f()(local cmd="echo good";trap \$cmd EXIT);cmd="echo bad";f'

This is supposed to print "good". With dash, it prints "bad". With the 
call to reset() moved after exitshell(), it prints "good".


I suspect reset() needs to be split into two separate functions, but it 
may be a bit tricky to determine exactly what is supposed to go where.


Cheers,
Harald van Dijk
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[BUG] Redirection bug in subshell's EXIT trap

[Vitaly Sinilin]

Hi,

I've recently written a quite unusual script and faced a strange dash
behavior that seems not POSIX-ly correct to me.

Here is the script with a lot of debugging extra output.

(I am sorry about posting a pretty lengthy script, but I feel like a
shorten version will look just insane without the context.)

#!/bin/dash
#
# This helper script takes a terminal from stdin to interact with user
# and prints uuid of the selected entity to stdout that is not
# necessarily connected to the same terminal.
#
# It's meant to be run as part of command substitution, e.g.
#
# uuid=$(pickthing "$@")
#

die() {
echo "$@" >&2
exit 2
}

getch() ( # need a subshell here, so round braces instead of curly ones
dump_self_fds
trap 'dump_self_fds; stty -cbreak echo' EXIT
stty cbreak -echo
head -c 1
dump_self_fds
)

dump_self_fds() {
{ read void; read void; read header pid; } < /proc/self/status
printf "%d:" $pid
for fd in 0 1 2; do
printf " %s" "$(readlink /proc/$pid/fd/$fd)"
done
echo
} >&2

test -t 0 || die "Standard input is not a terminal"

exec 3>&1 # 3 will be initial stdout
exec 1>$(tty) # switch stdout to terminal
exec 4<&0 # 4 will be initial stdin (for use in pipelines)
exec 0<&-

# Even when run as sh, bash doesn't interrupt script execution if SIGINT
# is received during execution of a foreground pipeline (dash does). So
# we need to explicitly instruct it to do so.
# https://www.cons.org/cracauer/sigint.html
trap 'trap - INT; kill -INT $$' INT

ask() {
echo "Take $2? (y/n)"
while :; do
#key=$(exec 0<&4; dump_self_fds; getch) # works in dash
key=$(dump_self_fds; getch <&4) # looks better but
# doesn't work in dash
case $key in
[yY]) echo $1 >&3; return 0;;
[nN]) break;;
esac
done
return 1
}

lsthings() { # just a stub instead of a real command
cat <<-"EOF"
df66e01c-1dcf-487c-aedc-a8b1c1859b49 item a
fff43fed-7560-48f7-9b38-c5509d74693f item b
EOF
}

dump_self_fds
lsthings "$@" | { while read uuid info; do
dump_self_fds
ask "$uuid" "$info" && exit 0
done; exit 1; } || ask "$*" "$* (initial literal value)"

# End of script

N.B. dump_self_fds() dumps PID and file descriptors 0, 1 and 2. It is very
Linux-specific. So there is no point in trying this script in other
environments.

N.B. This script will mess up your terminal settings. Use 'reset' or
'stty echo' after run to fix the terminal.

This script involves 3 levels of subshells.
[1] pipeline with a while loop.
 `--[2] command substitution in ask() key=$(...)
 `--[3] getch() body

Let's see what happens when the script is run (my comments inlined):

$ ./pickthing test
18406: pipe:[4898025] /dev/pts/1 /dev/pts/1 # toplevel shell
18412: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [1]
Take item a? (y/n)
18417: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [2] got stdin from [1]
18421: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] got FD#4 as stdin
# I answered n here
18421: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3] last line
18421: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [3] EXIT trap. WTF???
stty: standard input: Inappropriate ioctl for device

# Since I answered no, second iteration happened.

18412: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [1]
Take item b? (y/n)
18439: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [2]
18443: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3]
# I answered n here
18443: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3]
18443: pipe:[4898032] /dev/pts/1 /dev/pts/1 # [3] WTF???
stty: standard input: Inappropriate ioctl for device
Take test (initial literal value)? (y/n)

# It's the last line ask, so no pipeline here anymore.

18461: pipe:[4898099] /dev/pts/1 /dev/pts/1 # [2]
18465: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3]
18465: /dev/pts/1 /dev/pts/1 /dev/pts/1 # [3]
18465: pipe:[4898118] /dev/pts/1 /dev/pts/1 # [3] WTF???
stty: standard input: Bad file descriptor   # WTF?!?!?

The problem here is that although getch is run in a subshell and it
got FD#4 as stdin it somehow has parent's stdin in its EXIT trap.

POSIX says: "The environment in which the shell executes a trap on
EXIT shall be identical to the environment immediately after the
last command executed before the trap on EXIT was taken."

As you can see the last command of getch is dump_self_fds and stdin
is OK at that time.

In bash this script works as expected. For dash I had to invent
a workaround (commented out line key=$(...)).

-- 
Thanks,
Vitaly Sinilin
(I am not subscribed to the list, so please CC me.)
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Re: [NO-BUG] quoted substring parameter expansion ignores single-quotes in the arg

[G.raud]

Wrong bug report.

In fact the beahviour of dash on "a=\\'a\\'; echo \"\${a%'}\"" is that
of POSIX and of zsh 5.  However 'bash --posix', 'mksh -o posix' and
pdksh fail to parse the command and ksh does not remove the quote from
$a.

On Tue, Sep 26, 2017 at 05:41:48AM +0200, G.raud wrote:
> Subject: [BUG] quoted substring parameter expansion ignores single-quotes
>  in the arg
> 
> To test that the single quotes are considered ordinary chars when
> double-quoting the parameter expansion:
> 
> $ dash -c "a=\\'a\\'; echo \"\${a%'}\""
> 
> The result is surprising and non consistent with a non double-quoted
> expansion where the single-quotes act as quotes:
> 
> $ dash -c "a=a%; echo \${a%'%'}"
> 
> Additionally the manual states that the arg to substring processing is
> processed as an ordinary word (as probably POSIX too).

-- 
G.raud Meyer
gr...@gmx.com
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