linux-kernel - Re: [PATCH] iomap: avoid avoid truncating 64-bit offset to 32 bits

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Message-ID: <20250109043846.GJ1387004@frogsfrogsfrogs>
Date: Wed, 8 Jan 2025 20:38:46 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Marco Nelissen <marco.nelissen@...il.com>
Cc: brauner@...nel.org, linux-xfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iomap: avoid avoid truncating 64-bit offset to 32 bits

On Wed, Jan 08, 2025 at 08:11:50PM -0800, Marco Nelissen wrote:
> on 32-bit kernels, iomap_write_delalloc_scan() was inadvertently using a
> 32-bit position due to folio_next_index() returning an unsigned long.
> This could lead to an infinite loop when writing to an xfs filesystem.
> 
> Signed-off-by: Marco Nelissen <marco.nelissen@...il.com>
> ---
>  fs/iomap/buffered-io.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 54dc27d92781..d303e6c8900c 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -1138,7 +1138,7 @@ static void iomap_write_delalloc_scan(struct inode *inode,
>  				start_byte, end_byte, iomap, punch);
>  
>  		/* move offset to start of next folio in range */
> -		start_byte = folio_next_index(folio) << PAGE_SHIFT;
> +		start_byte = folio_pos(folio) + folio_size(folio);

eeek.  Yeah, I guess that would happen towards the upper end of the 16T
range on 32-bit.

I wonder if perhaps pagemap.h should have:

static inline loff_t folio_next_pos(struct folio *folio)
{
	return folio_pos(folio) + folio_size(folio);
}

But I think this is the only place in the kernel that uses this
construction?  So maybe not worth the fuss.

Reviewed-by: "Darrick J. Wong" <djwong@...nel.org>

--D

>  		folio_unlock(folio);
>  		folio_put(folio);
>  	}
> -- 
> 2.39.5
>