linux-kernel - Re: [PATCH v3 2/2] bug: introduce ASSERT_VAR_CAN

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Date: Fri, 22 Mar 2024 15:37:38 +0000
From: Will Deacon <will@...nel.org>
To: Petr Tesarik <petrtesarik@...weicloud.com>
Cc: Christoph Hellwig <hch@....de>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>,
	Michael Kelley <mhklinux@...look.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:DMA MAPPING HELPERS" <iommu@...ts.linux.dev>,
	Roberto Sassu <roberto.sassu@...weicloud.com>,
	Petr Tesarik <petr@...arici.cz>
Subject: Re: [PATCH v3 2/2] bug: introduce ASSERT_VAR_CAN_HOLD()

On Thu, Mar 21, 2024 at 06:19:02PM +0100, Petr Tesarik wrote:
> From: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> 
> Introduce an ASSERT_VAR_CAN_HOLD() macro to check at build time that a
> variable can hold the given value.
> 
> Use this macro in swiotlb to make sure that the list and pad_slots fields
> of struct io_tlb_slot are big enough to hold the maximum possible value of
> IO_TLB_SEGSIZE.
> 
> Signed-off-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> ---
>  include/linux/build_bug.h | 10 ++++++++++
>  kernel/dma/swiotlb.c      |  2 ++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
> index 3aa3640f8c18..6e2486508af0 100644
> --- a/include/linux/build_bug.h
> +++ b/include/linux/build_bug.h
> @@ -86,4 +86,14 @@
>  		"Offset of " #field " in " #type " has changed.")
>  
>  
> +/*
> + * Compile time check that a variable can hold the given value
> + */
> +#define ASSERT_VAR_CAN_HOLD(var, value) ({		\
> +	typeof(value) __val = (value);			\
> +	typeof(var) __tmp = __val;			\
> +	BUILD_BUG_ON_MSG(__tmp != __val,		\
> +		#var " cannot hold " #value ".");	\
> +})

nit, but I think this prevents putting negative values into unsigned
types. Not sure whether we care? Arguably it's even correct to complain.

e.g.

	u16 s;
	ASSERT_VAR_CAN_HOLD(s, -1);

explodes for me.

Will