linux-ext4 - Re: [PATCH 1/5] fs: ext4: initialize fsdata in pagecache

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Date:   Mon, 21 Nov 2022 11:48:40 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     linux-kernel@...r.kernel.org, tytso@....edu,
        adilger.kernel@...ger.ca, jaegeuk@...nel.org, chao@...nel.org,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        Eric Biggers <ebiggers@...nel.org>,
        syzbot+9767be679ef5016b6082@...kaller.appspotmail.com
Subject: Re: [PATCH 1/5] fs: ext4: initialize fsdata in pagecache_write()

On Mon, 21 Nov 2022 12:21:30 +0100 Alexander Potapenko <glider@...gle.com> wrote:

> When aops->write_begin() does not initialize fsdata, KMSAN reports
> an error passing the latter to aops->write_end().
> 
> Fix this by unconditionally initializing fsdata.
> 
> ...
>

I'm assuming that this is not-a-bug, and that these changes are purely
workarounds for a KMSAN shortcoming?

If true, this important info should be included in each changelog,
please.

If false, please provide a full description of the end-user visible
effects of the bug.

Also, it would be helpful to describe why it is not considered
practical to teach KMSAN to handle this?

> --- a/fs/ext4/verity.c
> +++ b/fs/ext4/verity.c
> @@ -79,7 +79,7 @@ static int pagecache_write(struct inode *inode, const void *buf, size_t count,
>  		size_t n = min_t(size_t, count,
>  				 PAGE_SIZE - offset_in_page(pos));
>  		struct page *page;
> -		void *fsdata;
> +		void *fsdata = NULL;
>  		int res;
>  
>  		res = aops->write_begin(NULL, mapping, pos, n, &page, &fsdata);