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Date: Sat, 17 Mar 2018 02:49:27 +0100
From: Miguel Ojeda <>
To: Linus Torvalds <>
Cc: Al Viro <>, Florian Weimer <>, 
	Kees Cook <>, Andrew Morton <>, 
	Josh Poimboeuf <>, Rasmus Villemoes <>, 
	Randy Dunlap <>, Ingo Molnar <>, 
	David Laight <>, Ian Abbott <>, 
	linux-input <>, linux-btrfs <>, 
	Network Development <>, 
	Linux Kernel Mailing List <>, 
	Kernel Hardening <>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()

On Fri, Mar 16, 2018 at 9:14 PM, Linus Torvalds
<> wrote:
> On Fri, Mar 16, 2018 at 1:03 PM, Miguel Ojeda
> <> 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:
> 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
   #define const_max(a,b)              __careful_cmp(a,b,>)

  warning: comparison of distinct pointer types lacks a cast

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.


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

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