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

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Date:   Wed, 18 Dec 2019 09:45:13 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Aditya Pakki <pakki001@....edu>
Cc:     kjlu@....edu, Peter Huewe <peterhuewe@....de>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        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 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