lkml.org 
[lkml]   [2018]   [Mar]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Date
SubjectRe: [PATCH v5 0/2] Remove false-positive VLAs when using max()
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
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
\
 
 \ /
  Last update: 2018-03-17 02:51    [W:0.066 / U:1.616 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site