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

From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Florian Weimer <fweimer@redhat.com>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Randy Dunlap <rdunlap@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	David Laight <David.Laight@aculab.com>,
	Ian Abbott <abbotti@mev.co.uk>,
	linux-input <linux-input@vger.kernel.org>,
	linux-btrfs <linux-btrfs@vger.kernel.org>,
	Network Development <netdev@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>
Subject: Re: [PATCH v5 0/2] Remove false-positive VLAs when using max()
Date: Sat, 17 Mar 2018 02:49:27 +0100	[thread overview]
Message-ID: <CANiq72kNgO7indoEsVqZX8GuM9TyyiuVaS1OgFzvTVMX=5P-XA@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFxLpdfyfTqhkHQoedQuqTcLRw3bYOgyz1s0eRW6cBmFTA@mail.gmail.com>

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On Fri, Mar 16, 2018 at 9:14 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Mar 16, 2018 at 1:03 PM, Miguel Ojeda
> <miguel.ojeda.sandonis@gmail.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

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diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 3fd291503576..f83658a4f15d 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -819,6 +819,49 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
 	      __UNIQUE_ID(max1_), __UNIQUE_ID(max2_),	\
 	      x, y)
 
+size_t __error_not_const_arg(void) \
+__compiletime_error("const_max() used with non-compile-time constant arg");
+size_t __error_too_big(void) \
+__compiletime_error("const_max() used with an arg too big");
+
+#define INTMAXT_MAX LLONG_MAX
+typedef int64_t intmax_t;
+
+#define const_cmp(x, y, op)                                           \
+        __builtin_choose_expr(                                        \
+                !__builtin_constant_p(x) || !__builtin_constant_p(y), \
+                __error_not_const_arg(),                              \
+                __builtin_choose_expr(                                \
+                        (x) > INTMAXT_MAX || (y) > INTMAXT_MAX,       \
+                        __error_too_big(),                            \
+                        __builtin_choose_expr(                        \
+                                (intmax_t)(x) op (intmax_t)(y),       \
+                                (x),                                  \
+                                (y)                                   \
+                        )                                             \
+                )                                                     \
+        )
+
+/**
+ * const_min - returns minimum of two compile-time constant values
+ * @x: first compile-time constant value
+ * @y: second compile-time constant value
+ *
+ * The result has the type of the winner of the comparison. Works for any
+ * value up to LLONG_MAX.
+ */
+#define const_min(x, y) const_cmp((x), (y), <=)
+
+/**
+ * const_max - returns maximum of two compile-time constant values
+ * @x: first compile-time constant value
+ * @y: second compile-time constant value
+ *
+ * The result has the type of the winner of the comparison. Works for any
+ * value up to LLONG_MAX.
+ */
+#define const_max(x, y) const_cmp((x), (y), >=)
+
 /**
  * min3 - return minimum of three values
  * @x: first value

