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

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Date:   Thu, 25 Aug 2022 17:39:24 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>
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>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 44/45] mm: fs: initialize fsdata passed to
 write_begin/write_end interface

On Mon, Jul 4, 2022 at 10:07 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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?
>

Hi Matthew,

There is a discussion at [1], with Segher pointing out a reason for
this rule [2] and Linus requesting that we should be warning about the
cases where uninitialized variables are passed by value.

Right now there are only a handful cases in the kernel where such
passing is performed (we just need one more patch in addition to this
one for KMSAN to boot cleanly). So we are in a good position to start
enforcing this rule, unless there's a reason not to.

I am not sure standard compliance alone is a convincing argument, but
from KMSAN standpoint, performing parameter check at callsites
noticeably eases handling of values passed between instrumented and
non-instrumented code. This lets us avoid some low-level hacks around
instrumentation_begin()/instrumentation_end() (some context available
at [4]).

Let me know what you think,
Alex

[1] - https://lore.kernel.org/lkml/CAFKCwrjBjHMquj-adTf0_1QLYq3Et=gJ0rq6HS-qrAEmVA7Ujw@mail.gmail.com/T/
[2] - https://lore.kernel.org/lkml/20220615164655.GC25951@gate.crashing.org/
[3] - https://lore.kernel.org/lkml/CAHk-=whjz3wO8zD+itoerphWem+JZz4uS3myf6u1Wd6epGRgmQ@mail.gmail.com/
[4] https://lore.kernel.org/lkml/20220426164315.625149-29-glider@google.com/