linux-kernel - Re: [PATCH v4 44/45] mm: fs: initialize fsdata passed to write_begin/write

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Date:   Mon, 4 Jul 2022 21:07:43 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 44/45] mm: fs: initialize fsdata passed to
 write_begin/write_end interface

On Fri, Jul 01, 2022 at 04:23:09PM +0200, Alexander Potapenko wrote:
> Functions implementing the a_ops->write_end() interface accept the
> `void *fsdata` parameter that is supposed to be initialized by the
> corresponding a_ops->write_begin() (which accepts `void **fsdata`).
> 
> However not all a_ops->write_begin() implementations initialize `fsdata`
> unconditionally, so it may get passed uninitialized to a_ops->write_end(),
> resulting in undefined behavior.

... wait, passing an uninitialised variable to a function *which doesn't
actually use it* is now UB?  What genius came up with that rule?  What
purpose does it serve?