linux-kernel - Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not 32/64 bit

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Message-ID: <20191214194626.GA140998@rani.riverdale.lan>
Date:   Sat, 14 Dec 2019 14:46:27 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org,
        Hans de Goede <hdegoede@...hat.com>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arvind Sankar <nivedita@...m.mit.edu>
Subject: Re: [PATCH 05/10] efi/libstub: distinguish between native/mixed not
 32/64 bit

On Sat, Dec 14, 2019 at 06:57:30PM +0100, Ard Biesheuvel wrote:
> +
> +#define efi_table_attr(table, attr, instance) ({			\
> +	__typeof__(((table##_t *)0)->attr) __ret;			\
> +	if (efi_is_native()) {						\
> +		__ret = ((table##_t *)instance)->attr;			\
> +	} else {							\
> +		__typeof__(((table##_32_t *)0)->attr) at;		\
> +		at = (((table##_32_t *)(unsigned long)instance)->attr);	\
> +		__ret = (__typeof__(__ret))(unsigned long)at;		\
> +	}								\
> +	__ret;								\
> +})

The casting of `at' is appropriate if the attr is a pointer type which
needs to be zero-extended to 64-bit, but for other fields it is
unnecessary at best and possibly dangerous.  There are probably no
instances currently where it is called for a non-pointer field, but is
it possible to detect if the type is pointer and avoid the cast if not?