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

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Date:   Wed, 23 Jun 2021 15:46:46 -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 04:27:47PM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 23 June 2021 17:19
> > 
> > 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
> 
> You can probably even do:
> 
> echo "$test" | /bin/sh -c cat >$TRIGGER || true
> 
> (moving the redirect to the outer shell).

Actually, it looks like the "write" is already happening in the exec'd
process, so this can just be:

echo "$test" | cat >$TRIGGER || true

But it still can't be:

echo "$test" >$TRIGGER

which is what I had over-engineered a solution to. :)

-- 
Kees Cook