linux-kernel - Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

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Message-ID: <CANiq72kNgO7indoEsVqZX8GuM9TyyiuVaS1OgFzvTVMX=5P-XA@mail.gmail.com>
Date:   Sat, 17 Mar 2018 02:49:27 +0100
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Florian Weimer <fweimer@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...lab.com>,
        Ian Abbott <abbotti@....co.uk>,
        linux-input <linux-input@...r.kernel.org>,
        linux-btrfs <linux-btrfs@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

On Fri, Mar 16, 2018 at 9:14 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Mar 16, 2018 at 1:03 PM, Miguel Ojeda
> <miguel.ojeda.sandonis@...il.com> wrote:
>>>
>>> Kees - is there some online "gcc-4.4 checker" somewhere? This does
>>> seem to work with my gcc. I actually tested some of those files you
>>> pointed at now.
>>
>> I use this one:
>>
>>   https://godbolt.org/
>
> Well, my *test* code works on that one and -Wvla -Werror.
>
> It does not work with gcc-4.1.x, but works with gcc-4.4.x.
>
> I can't seem to see the errors any way, I wonder if
> __builtin_choose_expr() simply didn't exist back then.
>
> Odd that you can't view warnings/errors with it.
>
> But it's possible that it fails on more complex stuff in the kernel.
>
> I've done a "allmodconfig" build with that patch, and the only issue
> it found was that (real) type issue in tpm_tis_core.h.

Just tested your max() with a Python script I wrote yesterday to try a
lot of combinations for this thing. Your version gives some warnings
in some cases, like:

  warning: signed and unsigned type in conditional expression [-Wsign-compare]
   #define __cmp(a,b,op) ((a)op(b)?(a):(b))

  warning: comparison between signed and unsigned integer expressions
[-Wsign-compare]
   #define const_max(a,b)              __careful_cmp(a,b,>)

  warning: comparison of distinct pointer types lacks a cast
           (!!(sizeof((typeof(a)*)1==(typeof(b)*)1)))

But it fails on something like (with warnings):

  int a[const_max(-30, 60u)];

Sorry... :-(

Has anyone taken a look at the last one I sent? Patch attached with
the draft changes on the kernel. It compiles fine the cases Kees
cleaned up in the other patch, but also works without a explicit type,
for mixed types, and for both positive and negative values.

Cheers,
Miguel

View attachment "patch.diff" of type "text/x-patch" (2260 bytes)