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Date:   Sun, 30 Aug 2020 17:30:33 -0700
From:   "Darrick J. Wong" <darrick.wong@...cle.com>
To:     Qian Cai <cai@....pw>
Cc:     hch@...radead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iomap: fix WARN_ON_ONCE() from unprivileged users

On Sat, Aug 29, 2020 at 09:30:57PM -0400, Qian Cai wrote:
> It is trivial to trigger a WARN_ON_ONCE(1) in iomap_dio_actor() by
> unprivileged users which would taint the kernel, or worse - panic if
> panic_on_warn or panic_on_taint is set. Hence, just convert it to
> pr_warn_ratelimited() to let users know their workloads are racing.
> Thanks Dave Chinner for the initial analysis of the racing reproducers.
> 
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  fs/iomap/direct-io.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index c1aafb2ab990..6a6b4bc13269 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -389,7 +389,14 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	case IOMAP_INLINE:
>  		return iomap_dio_inline_actor(inode, pos, length, dio, iomap);
>  	default:
> -		WARN_ON_ONCE(1);
> +		/*
> +		 * DIO is not serialised against mmap() access at all, and so
> +		 * if the page_mkwrite occurs between the writeback and the
> +		 * iomap_apply() call in the DIO path, then it will see the
> +		 * DELALLOC block that the page-mkwrite allocated.
> +		 */
> +		pr_warn_ratelimited("page_mkwrite() is racing with DIO read (iomap->type = %u).\n",
> +				    iomap->type);

Shouldn't we log the name of triggering process and the file path?  Sort
of like what dio_warn_stale_pagecache does?

--D

>  		return -EIO;
>  	}
>  }
> -- 
> 2.18.4
> 

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