linux-kernel - Re: [PATCH] iommu: Convert unreachable() to BUG()

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Message-ID: <7ffc46e5-60ff-4bef-8fdc-9ab0c67ebd6b@paulmck-laptop>
Date: Thu, 27 Mar 2025 07:58:55 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
	iommu@...ts.linux.dev, Peter Zijlstra <peterz@...radead.org>,
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] iommu: Convert unreachable() to BUG()

On Wed, Mar 26, 2025 at 10:28:46PM -0700, Josh Poimboeuf wrote:
> Bare unreachable() should be avoided as it generates undefined behavior,
> e.g. falling through to the next function.  Use BUG() instead so the
> error is defined.
> 
> Fixes the following warnings:
> 
>   drivers/iommu/dma-iommu.o: warning: objtool: iommu_dma_sw_msi+0x92: can't find jump dest instruction at .text+0x54d5
>   vmlinux.o: warning: objtool: iommu_dma_get_msi_page() falls through to next function __iommu_dma_unmap()
> 
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Closes: https://lore.kernel.org/314f8809-cd59-479b-97d7-49356bf1c8d1@infradead.org
> Reported-by: Paul E. McKenney <paulmck@...nel.org>
> Closes: https://lore.kernel.org/5dd1f35e-8ece-43b7-ad6d-86d02d2718f6@paulmck-laptop
> Fixes: 6aa63a4ec947 ("iommu: Sort out domain user data")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>

Thank you for the quick response!

Tested-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  drivers/iommu/dma-iommu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 7b2734de2ba9..6054d0ab8023 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1753,7 +1753,7 @@ static size_t cookie_msi_granule(const struct iommu_domain *domain)
>  	case IOMMU_COOKIE_DMA_MSI:
>  		return PAGE_SIZE;
>  	default:
> -		unreachable();
> +		BUG();
>  	};
>  }
>  
> @@ -1765,7 +1765,7 @@ static struct list_head *cookie_msi_pages(const struct iommu_domain *domain)
>  	case IOMMU_COOKIE_DMA_MSI:
>  		return &domain->msi_cookie->msi_page_list;
>  	default:
> -		unreachable();
> +		BUG();
>  	};
>  }
>  
> -- 
> 2.48.1
>