linux-kernel - Re: [PATCH v2] selftests/lkdtm: Use /bin/sh not $SHELL

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Message-ID: <202106230917.FE2F587@keescook>
Date:   Wed, 23 Jun 2021 09:18:44 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     'Guillaume Tucker' <guillaume.tucker@...labora.com>,
        Shuah Khan <shuah@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] selftests/lkdtm: Use /bin/sh not $SHELL

On Wed, Jun 23, 2021 at 01:43:04PM +0000, David Laight wrote:
> From: Guillaume Tucker
> > Sent: 23 June 2021 13:40
> ...
> > > diff --git a/tools/testing/selftests/lkdtm/run.sh b/tools/testing/selftests/lkdtm/run.sh
> > > index bb7a1775307b..0f9f22ac004b 100755
> > > --- a/tools/testing/selftests/lkdtm/run.sh
> > > +++ b/tools/testing/selftests/lkdtm/run.sh
> > > @@ -78,8 +78,9 @@ dmesg > "$DMESG"
> > >
> > >  # Most shells yell about signals and we're expecting the "cat" process
> > >  # to usually be killed by the kernel. So we have to run it in a sub-shell
> > > -# and silence errors.
> > > -($SHELL -c 'cat <(echo '"$test"') >'"$TRIGGER" 2>/dev/null) || true
> > > +# to avoid terminating this script. Leave stderr alone, just in case
> > > +# something _else_ happens.
> > > +(/bin/sh -c '(echo '"$test"') | cat >'"$TRIGGER") || true
> 
> I was having trouble parsing that command - and I'm good
> at shell scripts.
> I think the extra subshell the 'echo' is in doesn't help.
> In fact, is either subshell needed?
> Surely:
> /bin/sh -c "echo '$test' | cat >$trigger" || true
> will work just as well?

Ah yeah, and I just tested it to double check, it can be even simpler:

echo "$test" | /bin/sh -c "cat >$TRIGGER" || true

I'll adjust and resend...

-Kees

-- 
Kees Cook