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[bug-diffutils] bug#24116: bug#24116: [platform-testers] new snapshot av

From: Dave Gordon
Subject: [bug-diffutils] bug#24116: bug#24116: [platform-testers] new snapshot available: diffutils-3.3.50-0353
Date: Fri, 5 Aug 2016 15:09:03 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

On 05/08/16 14:13, Harald van Dijk wrote:
On 5-8-2016 14:46, Dave Gordon wrote:
On 01/08/16 01:36, Jim Meyering wrote:
On Sun, Jul 31, 2016 at 10:17 AM, Assaf Gordon <address@hidden>
Hello Jim

On Jul 31, 2016, at 03:08, Jim Meyering <address@hidden> wrote:

diffutils snapshot:

The "colors" test seems to succeed on Fedora/CentOS/SUSE systems (of
various versions), but fail on others (Ubuntu, Debian, FreeBSD, Mac
OS X).

Attached are logs from 3 systems. From a cursory look it seems the
exact same failure, but I haven't looked deeper.
No other test failures found, but I'll have more results later today.

Hi Assaf,
Thank you for all the speedy testing.
I've looked into the failure on a Debian system for which /bin/sh is
dash 0.5.8-2.2.
dash's printf builtin handles \e differently -- that's easy to work
around: use \033, which *is* portable.
More surprising is that this generates no output:

  dash -c 'f() { local t=$(printf '\''\t\t'\''); printf "$t"; }; f'

I.e., piping it into wc -c prints 0.
With bash, it prints the expected pair of TAB bytes.
I found that I could work around this nonsensical behavior by hoisting
the "tab=..." definition up/out of those two functions, or by adding
standard-says-never-necessary double quotes like this:

  dash -c 'f() { local t="$(printf '\''\t\t'\'')"; printf "$t"; }; f'

However, I prefer not to work around it here (and in every other test
script where this comes up), and will insulate all of our test scripts
by rejecting any shell with that misbehavior, so plan to adjust
init.sh to select another shell when it finds this flaw.

On second thought, I will make the local change now, and sleep on the
idea of making init.sh reject dash.
Done in the attached patch.

No, that's definitely a dash(1) bug, and quite a serious one. Here's a
variant that makes it more obvious:

# Define our test string, without too much complicated quoting
$ X='f() { local t=$(printf "abc"); printf "$t"; }; f'
$ bash -c "$X" | hd
00000000  61 62 63                                          |abc|
$ dash -c "$X" | hd
00000000  61 62 63                                          |abc|
# As expected, we get the same result from bash(1) and dash(1).

# Now try a different test string:
$ X='f() { local t=$(printf "a\tc"); printf "$t"; }; f'
$ bash -c "$X" | hd
00000000  61 09 63                                          |a.c|
$ dash -c "$X" | hd
00000000  61                                                |a|
# Wibble! dash(1) has truncated the string at the TAB :(

# In fact it's worse that that
$ X='f() { local t=$(printf "a\tc=d"); printf "$t+$c"; }; f'
$ bash -c "$X" | hd
00000000  61 09 63 3d 64 2b                                 |a.c=d+|
$ dash -c "$X" | hd
00000000  61 2b 64                                          |a+d|

What dash(1) appears to have done is silently take the TAB as
the terminator of the containing double-quoted string, AND of
the containing $() construct, as well as a whitespace, so that
the "c=d" is taken as the next argument to the 'local' builtin.

I suspect this unexpected termination of the inner quoted-string
could be quite exploitable!

This gets reported relatively frequently. The local command is
non-standard but a common extension in shells. In the shells that
provide it, it gets treated the same, syntax-wise, as the standard
export command, including in dash.

Unfortunately, POSIX currently requires the export command to not have
any magic quoting, and any POSIX-conforming shell will make

    a="b c=d"
    export a=$a

set a to b, and c to d. Not so with bash, but that's because bash simply
isn't POSIX-conforming, even if invoked as sh.

POSIX will require special quoting rules for the export command in the
future, similar to what bash does today. When it does, dash is likely to
change to match that, and the local command will likely be changed to
work the same way.

Right now, though, since the special quoting behaviour is non-standard,
this is a bug in the script unless the script is explicitly stated to
work only with specific shells. If the script is meant to be portable,
even if only across shells that provide the local command, quoting
$(...) is the right thing to do.


    local a

should work too, including in dash. Since a=$(...) is not an argument to
any command here, since it's the shell syntax that says it's an
assignment rather than the semantics of a particular command, field
splitting won't happen here.

Harald van Dijk


thanks for the explanation :) I had devised a few more tests and
realised that dash is applying word-splitting after substitution,
as would be expected for ordinary external commands e.g.

$ X="256 if=foo"
$ dd bs=$X
dd: failed to open ‘foo’: No such file or directory

where one would always expect to write bs="$X" with quotes if one wanted to ensure that it was taken as a single parameter and without quotes if one wanted it to be broken into multiple words.

It was just a surprise to find this (rather than bash's implicit
quoting) applying to dash builtin commands!

Another variant that does work, this time by escaping rather than quoting the TAB, and deferring conversion of '\t' into a TAB until after the word-splitting:

$ X='f() { local t=$(printf a\\\\tc=d); printf "$t+$c"; }; f'
$ bash -c "$X" | hd
00000000  61 09 63 3d 64 2b                                 |a.c=d+|
$ dash -c "$X" | hd
00000000  61 09 63 3d 64 2b                                 |a.c=d+|


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