linux-kernel - Re: [PATCH] selftests/lkdtm: Use /bin/sh not $SHELL

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Date:   Fri, 18 Jun 2021 20:02:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Guillaume Tucker <guillaume.tucker@...labora.com>
Cc:     Shuah Khan <shuah@...nel.org>, stable@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/lkdtm: Use /bin/sh not $SHELL

On Fri, Jun 18, 2021 at 08:29:57PM +0100, Guillaume Tucker wrote:
> There's a bit more to it...  The lkdtm tests make use of the
> process substitution feature with the <() syntax which is
> specific to Bash.  The tests run by KernelCI use Debian, where
> /bin/sh points to /bin/dash by default which doesn't support this
> feature.  So one way to fix it would be:
> 
>   (/bin/bash -c 'cat <(echo '"$test"') >'"$TRIGGER")

Argh. I always forget that <() is a bash-ism. Thank you for tracking
this down!

> However, this might break others' workflows.
> 
> In fact the LAVA jobs run by KernelCI do define the $SHELL
> environment variable except it's defined to be /bin/sh - and that
> means /bin/dash gets called and we're back to the issue explained
> above.
> 
> I've manually run a modified test job which defines
> SHELL=/bin/bash and that works:
> 
>   https://lava.collabora.co.uk/scheduler/job/4055547#L2835

Yay!!

> So to avoid hitting the same issue in other places, as it seems
> like there is an implicit dependency on Bash, we can just change
> KernelCI kselftest jobs to always export SHELL=/bin/bash.
> 
> I suppose an even better fix would be to use standard shell
> features that would work with any /bin/sh implementation, but
> this is there to kill the sub-shell rather than the main script
> process so I'm not entirely sure if we can easily do that
> differently.  Maybe we can pipe the output to cat rather than the
> substitution syntax, e.g.:
> 
>   (/bin/sh -c '(echo '"$test"') | cat >'"$TRIGGER") || true

Yeah, this is the right fix. There's no reason anything should depend
on bash; I was just not thinking when I wrote this originally. :)

> So I think the "safest" solution is to not change the kselftest
> script and export SHELL=/bin/bash in the KernelCI jobs.  If the
> pipe approach is good enough at catching signals then it could be
> done on top of this patch as it's standard and should work with
> any /bin/sh implementation.  What do you think?

If you set SHELL=/bin/bash for now, the lkdtm tests should work as they
are, and once the v2 patch lands, they'll continue to work, and
SHELL=/bin/bash can be removed.

Thank you so much!

-Kees

-- 
Kees Cook