linux-kernel - Re: [PATCH] vboxsf: fix old signature detection

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Date:   Tue, 28 Sep 2021 11:39:17 +0200
From:   Hans de Goede <hdegoede@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...nel.org>
Cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Sparse Mailing-list <linux-sparse@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] vboxsf: fix old signature detection

Hi Linus,

On 9/27/21 8:33 PM, Linus Torvalds wrote:
> On Mon, Sep 27, 2021 at 6:22 AM Arnd Bergmann <arnd@...nel.org> wrote:
>>
>> More specifically, ' think '\377' may be either -1 or 255 depending on
>> the architecture.
>> On most architectures, 'char' is implicitly signed, but on some others
>> it is not.
> 
> Yeah. That code is just broken.
> 
> And Arnd, your patch may be "conceptually minimal", in that it keeps
> thed broken code and makes it work. But it just dials up the oddity to
> 11.
> 
> The proper patch is just this appended thing that stops playing silly
> games, and just uses "memcmp()".
> 
> I've verified that with sane build configurations, it just generates
> 
>         testq   %rsi, %rsi
>         je      .L25
>         cmpl    $-33620224, (%rsi)
>         je      .L31
> 
> for that
> 
>         if (data && !memcmp(data, VBSF_MOUNT_SIGNATURE, 4)) {
> 
> test. With a lot of crazy debug options you'll actually see the
> "memcmp()", but the bad code generation is the least of your options
> in that case.

I agree that your suggestion is to be the best solution,
so how do we move forward with this, do I turn this into a
proper patch with you as the author and Arnd as Reported-by and
if yes may I add your Signed-off-by to the patch ?

Or do I make myself author and set you as Suggested-by ?

Regards,

Hans