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

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Date:   Tue, 22 Nov 2022 03:40:31 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexander Potapenko <glider@...gle.com>,
        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, Nov 21, 2022 at 11:48:40AM -0800, Andrew Morton wrote:
> 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?

It's a weird one.  It used to be not-a-bug.  Then we changed from
std=gnu99 to std=gnu11 or something.  And in the intervening years,
the C standards ctte decided that passing an uninitialised pointer to a
function was UB.  So we start by passing a pointer to the pointer to
->write_begin().  Some ->write_begin functions initialise that pointer;
others don't.  Then we pass the pointer directly to ->write_end.  If
->write_begin initialised the pointer, that's fine, and if not, it's UB.
Of course the ->write_end doesn't use it if the ->write_begin didn't
initialise it, but it's too late because merely calling the function
was UB.  Thanks, Itanium!