Re: [PATCH v2 2/3] tpm: Address !chip->auth in tpm_buf_append_name()

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Thu Jul 4, 2024 at 8:21 PM EEST, James Bottomley wrote:
> On Thu, 2024-07-04 at 10:07 -0700, Linus Torvalds wrote:
> > On Wed, 3 Jul 2024 at 13:11, James Bottomley
> > <James.Bottomley@hansenpartnership.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.
>
> I actually like the latter version, but instinct tells me that if this
> is the first time the kernel has ever needed something like this then
> perhaps we should go with the former because that's how everyone must
> have handled it in the past.

I'll go with the former given it is somewhat idiomatic and familiar
pattern.

> James

BR, Jarkko