linux-kernel - Re: [PATCH] tpm/ppi: replace assertion code with recovery in tpm_eval

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Date:   Thu, 19 Dec 2019 02:02:18 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Jason Gunthorpe <jgg@...pe.ca>, Aditya Pakki <pakki001@....edu>
Cc:     kjlu@....edu, Peter Huewe <peterhuewe@....de>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tpm/ppi: replace assertion code with recovery in
 tpm_eval_dsm

On Wed, 2019-12-18 at 09:45 -0400, Jason Gunthorpe wrote:
> On Sun, Dec 15, 2019 at 12:23:14PM -0600, Aditya Pakki wrote:
> > In tpm_eval_dsm, BUG_ON on ppi_handle is used as an assertion.
> > By returning NULL to the callers, instead of crashing, the error
> > can be better handled.
> > 
> > Signed-off-by: Aditya Pakki <pakki001@....edu>
> >  drivers/char/tpm/tpm_ppi.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/char/tpm/tpm_ppi.c b/drivers/char/tpm/tpm_ppi.c
> > index b2dab941cb7f..4b6f6a9c0b48 100644
> > +++ b/drivers/char/tpm/tpm_ppi.c
> > @@ -42,7 +42,9 @@ static inline union acpi_object *
> >  tpm_eval_dsm(acpi_handle ppi_handle, int func, acpi_object_type type,
> >  	     union acpi_object *argv4, u64 rev)
> >  {
> > -	BUG_ON(!ppi_handle);
> > +	if (!ppi_handle)
> > +		return NULL;
> 
> If it can't happen the confusing if should either be omitted entirely
> or written as 
> 
> if (WARN_ON(!ppi_handle))
>        return NULL;
> 
> Leaving it as apparently operational code just creates confusion for
> the reader that now has the task to figure out why ppi_handle can be
> null.
> 
> I favour not including tests for impossible conditions. The kernel
> will crash immediately if ppi_handle is null anyhow.
> 
> Jason

Absolutely should be changed WARN_ON() as it never should happen. I'll
update the patch before sending PR to Linus since I have it already
applied.

Thanks Jason for the remark!

/Jarkko