linux-kernel - Re: [PATCH v2 2/3] tpm: Address !chip->auth in tpm_buf_append

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Message-ID: <CAHk-=wiM=Cyw-07EkbAH66pE50VzJiT3bVHv9CS=kYR6zz5mTQ@mail.gmail.com>
Date: Thu, 4 Jul 2024 10:07:54 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Jarkko Sakkinen <jarkko@...nel.org>, linux-integrity@...r.kernel.org, 
	Thorsten Leemhuis <regressions@...mhuis.info>, stable@...r.kernel.org, 
	Stefan Berger <stefanb@...ux.ibm.com>, Peter Huewe <peterhuewe@....de>, 
	Jason Gunthorpe <jgg@...pe.ca>, Mimi Zohar <zohar@...ux.ibm.com>, David Howells <dhowells@...hat.com>, 
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, Ard Biesheuvel <ardb@...nel.org>, 
	Mario Limonciello <mario.limonciello@....com>, linux-kernel@...r.kernel.org, 
	keyrings@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v2 2/3] tpm: Address !chip->auth in tpm_buf_append_name()

On Wed, 3 Jul 2024 at 13:11, James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
>
> if (__and(IS_ENABLED(CONFIG_TCG_TPM2_HMAC), chip->auth))

Augh. Please don't do this.

That "__and()" thing may work, but it's entirely accidental that it does.

It's designed for config options _only_, and the fact that it then
happens to work for "first argument is config option, second argument
is C conditional".

The comment says that it's implementing "&&" using preprocessor
expansion only, but it's a *really* limited form of it. The arguments
are *not* arbitrary.

So no. Don't do this.

Just create a helper inline like

    static inline struct tpm2_auth *chip_auth(struct tpm_chip *chip)
    {
    #ifdef CONFIG_TCG_TPM2_HMAC
        return chip->auth;
    #else
        return NULL;
    #endif
    }

and if we really want to have some kind of automatic way of doing
this, we will *NOT* be using __and(), we'd do something like

        /* Return zero or 'value' depending on whether OPTION is
enabled or not */
        #define IF_ENABLED(option, value) __and(IS_ENABLED(option), value)

that actually would be documented and meaningful.

Not this internal random __and() implementation that is purely a
kconfig.h helper macro and SHOULD NOT be used anywhere else.

             Linus