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

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Date:   Wed, 31 Aug 2022 15:32:54 +0200
From:   Alexander Potapenko <glider@...gle.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Segher Boessenkool <segher@...nel.crashing.org>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 Fri, Aug 26, 2022 at 9:41 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, Aug 25, 2022 at 3:10 PM Segher Boessenkool
> <segher@...nel.crashing.org> wrote:
> >
> > But UB is defined in terms of the abstract machine (like *all* of C),
> > not in terms of the generated machine code.  Typically things will work
> > fine if they "become invisible" by inlining, but this does not make the
> > program a correct program ever.  Sorry :-(
>
> Yeah, and the abstract machine model based on "abstract syntax" is
> just wrong, wrong, wrong.
>
> I really wish the C standard people had the guts to just fix it.  At
> some point, relying on tradition when the tradition is bad is not a
> great thing.
>
> It's the same problem that made all the memory ordering discussions
> completely untenable. The language to allow the whole data dependency
> was completely ridiculous, because it became about the C language
> syntax and theory, not about the actual code generation and actual
> *meaning* that the whole thing was *about*.
>
> Java may be a horrible language that a lot of people hate, but it
> avoided a lot of problems by just making things about an actual
> virtual machine and describing things within a more concrete model of
> a virtual machine.
>
> Then you can just say "this code sequence generates this set of
> operations, and the compiler can optimize it any which way it likes as
> long as the end result is equivalent".
>
> Oh well.
>
> I will repeat: a paper standard that doesn't take reality into account
> is less useful than toilet paper. It's scratchy and not very
> absorbent.
>
> And the kernel will continue to care more about reality than about a C
> standard that does bad things.
>
> Inlining makes the use of the argument go away at the call site and
> moves the code of the function into the body. That's how things
> *work*. That's literally the meaning of inlining.
>
> And inlining in C is so important because macros are weak, and other
> facilities like templates don't exist.
>
> But in the kernel, we also often use it because the actual semantics
> of "not a function call" in terms of code generation is also important
> (ie we have literal cases where "not generating the 'call'
> instruction" is a correctness issue).
>
> If the C standard thinks "undefined argument even for inlining use is
> UB", then it's a case of that paperwork that doesn't reflect reality,
> and we'll treat it with the deference it deserves - is less than
> toilet paper.

Just for posterity, in the case of KMSAN we are only dealing with
cases where the function call survived inlining and dead code
elimination.


> We have decades of history of doing that in the kernel. Sometimes the
> standards are just wrong, sometimes they are just too far removed from
> reality to be relevant, and then it's just not worth worrying about
> them.
>
>           Linus



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Alexander Potapenko
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