busybox/shell
Denys Vlasenko eb60777769 ash: eval: Prevent recursive PS4 expansion
Date: Wed, 27 May 2020 13:19:10 +1000
eval: Prevent recursive PS4 expansion

    Yaroslav Halchenko <yoh@onerussian.com> wrote:
    > I like to (ab)use PS4 and set -x for tracing execution of scripts.
    > Reporting time and PID is very useful in this context.
    >
    > I am not 100% certain if bash's behavior (of actually running the command
    > embedded within PS4 string, probably eval'ing it) is actually POSIX
    > compliant, posh seems to not do that; but I think it is definitely not
    > desired for dash to just stall:
    >
    > - the script:
    > #!/bin/sh
    > set -x
    > export PS4='+ $(date +%T.%N) [$$] '
    > echo "lets go"
    > sleep 1
    > echo "done $var"
    >
    > - bash:
    > /tmp > bash --posix test.sh
    > +export 'PS4=+ $(date +%T.%N) [$$] '
    > +PS4='+ $(date +%T.%N) [$$] '
    > + 09:15:48.982296333 [2764323] echo 'lets go'
    > lets go
    > + 09:15:48.987829613 [2764323] sleep 1
    > + 09:15:49.994485037 [2764323] echo 'done '
    > done
    >
    ...
    > - dash: (stalls it set -x)
    > /tmp > dash test.sh
    > +export PS4=+ $(date +%T.%N) [$$]
    > ^C^C

    This patch fixes the infinite loop caused by repeated expansions
    of PS4.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
2021-09-09 16:29:37 +02:00
..
ash_test ash: parser: Fix alias expansion after heredoc or newlines 2021-09-08 01:43:12 +02:00
hush_test ash: LINENO starts from 0 in -c SCRIPT mode 2021-09-07 18:16:45 +02:00
ash_doc.txt
ash_ptr_hack.c
ash_remove_unnecessary_code_in_backquote_expansion.patch ash: save Ron's patch from oblivion 2021-06-06 13:01:25 +02:00
ash.c ash: eval: Prevent recursive PS4 expansion 2021-09-09 16:29:37 +02:00
brace.txt
Config.src nologin: make it possible to build it as single applet 2020-06-24 15:05:22 +02:00
cttyhack.c config: update size information 2018-12-28 03:20:17 +01:00
hush_doc.txt
hush_leaktool.sh
hush.c hush: fix var_LINENO3.tests failure 2021-09-07 21:44:44 +02:00
Kbuild.src
match.c hush: optimize ${var/pattern/repl} for trivial patterns 2021-07-27 17:53:55 +02:00
match.h
math.c shell: remove FAST_FUNC from a static function 2020-10-01 00:57:05 +02:00
math.h shell: move all definitions of strto_arith_t() together 2019-05-26 14:02:10 +02:00
random.c whitespace fixes 2018-07-17 15:04:17 +02:00
random.h
README
README.job
shell_common.c shells: do not allow bare "read" in non-bash compat configs 2021-06-06 12:08:43 +02:00
shell_common.h hush: fix "export PS1=xyz" and "local PS1=xyz" messing up prompt 2019-05-14 18:56:04 +02:00

http://www.opengroup.org/onlinepubs/9699919799/
Open Group Base Specifications Issue 7


http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html
Shell & Utilities

It says that any of the standard utilities may be implemented
as a regular shell built-in. It gives a list of utilities which
are usually implemented that way (and some of them can only
be implemented as built-ins, like "alias"):

alias
bg
cd
command
false
fc
fg
getopts
jobs
kill
newgrp
pwd
read
true
umask
unalias
wait


http://www.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
Shell Command Language

It says that shell must implement special built-ins. Special built-ins
differ from regular ones by the fact that variable assignments
done on special builtin are *PRESERVED*. That is,

VAR=VAL special_builtin; echo $VAR

should print VAL.

(Another distinction is that an error in special built-in should
abort the shell, but this is not such a critical difference,
and moreover, at least bash's "set" does not follow this rule,
which is even codified in autoconf configure logic now...)

List of special builtins:

. file
: [argument...]
break [n]
continue [n]
eval [argument...]
exec [command [argument...]]
exit [n]
export name[=word]...
export -p
readonly name[=word]...
readonly -p
return [n]
set [-abCefhmnuvx] [-o option] [argument...]
set [+abCefhmnuvx] [+o option] [argument...]
set -- [argument...]
set -o
set +o
shift [n]
times
trap n [condition...]
trap [action condition...]
unset [-fv] name...

In practice, no one uses this obscure feature - none of these builtins
gives any special reasons to play such dirty tricks.

However. This section also says that *function invocation* should act
similar to special built-in. That is, variable assignments
done on function invocation should be preserved after function invocation.

This is significant: it is not unthinkable to want to run a function
with some variables set to special values. But because of the above,
it does not work: variable will "leak" out of the function.