linux-kernel - [PATCH] linux/bits.h: Fix compilation error with GENMASK

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Date:   Tue, 11 May 2021 22:37:15 +0200
From:   Rikard Falkeborn <rikard.falkeborn@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Yury Norov <yury.norov@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH] linux/bits.h: Fix compilation error with GENMASK

GENMASK() has an input check which uses __builtin_choose_expr() to enable
a compile time sanity check of its inputs if they are known at compile
time. However, it turns out that __builtin_constant_p() does not always
return a compile time constant [0]. It was thought this problem was fixed
with gcc 4.9 [1], but apparently this is not the case [2].

Switch to use __is_constexpr() instead which always returns a compile
time constant, regardless of its inputs.

[0]: https://lore.kernel.org/lkml/42b4342b-aefc-a16a-0d43-9f9c0d63ba7a@rasmusvillemoes.dk
[1]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449
[2]: https://lore.kernel.org/lkml/1ac7bbc2-45d9-26ed-0b33-bf382b8d858b@I-love.SAKURA.ne.jp

Reported-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@...il.com>
---
Feedback on placing __is_constexpr() in const.h is welcome, at least the
name is appropriate...

 include/linux/bits.h        |  2 +-
 include/linux/const.h       |  8 ++++++++
 include/linux/minmax.h      | 10 ++--------
 tools/include/linux/bits.h  |  2 +-
 tools/include/linux/const.h |  8 ++++++++
 5 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/include/linux/bits.h b/include/linux/bits.h
index 7f475d59a097..87d112650dfb 100644
--- a/include/linux/bits.h
+++ b/include/linux/bits.h
@@ -22,7 +22,7 @@
 #include <linux/build_bug.h>
 #define GENMASK_INPUT_CHECK(h, l) \
 	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
-		__builtin_constant_p((l) > (h)), (l) > (h), 0)))
+		__is_constexpr((l) > (h)), (l) > (h), 0)))
 #else
 /*
  * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
diff --git a/include/linux/const.h b/include/linux/const.h
index 81b8aae5a855..435ddd72d2c4 100644
--- a/include/linux/const.h
+++ b/include/linux/const.h
@@ -3,4 +3,12 @@
 
 #include <vdso/const.h>
 
+/*
+ * This returns a constant expression while determining if an argument is
+ * a constant expression, most importantly without evaluating the argument.
+ * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
+ */
+#define __is_constexpr(x) \
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+
 #endif /* _LINUX_CONST_H */
diff --git a/include/linux/minmax.h b/include/linux/minmax.h
index c0f57b0c64d9..5433c08fcc68 100644
--- a/include/linux/minmax.h
+++ b/include/linux/minmax.h
@@ -2,6 +2,8 @@
 #ifndef _LINUX_MINMAX_H
 #define _LINUX_MINMAX_H
 
+#include <linux/const.h>
+
 /*
  * min()/max()/clamp() macros must accomplish three things:
  *
@@ -17,14 +19,6 @@
 #define __typecheck(x, y) \
 	(!!(sizeof((typeof(x) *)1 == (typeof(y) *)1)))
 
-/*
- * This returns a constant expression while determining if an argument is
- * a constant expression, most importantly without evaluating the argument.
- * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
- */
-#define __is_constexpr(x) \
-	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
-
 #define __no_side_effects(x, y) \
 		(__is_constexpr(x) && __is_constexpr(y))
 
diff --git a/tools/include/linux/bits.h b/tools/include/linux/bits.h
index 7f475d59a097..87d112650dfb 100644
--- a/tools/include/linux/bits.h
+++ b/tools/include/linux/bits.h
@@ -22,7 +22,7 @@
 #include <linux/build_bug.h>
 #define GENMASK_INPUT_CHECK(h, l) \
 	(BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
-		__builtin_constant_p((l) > (h)), (l) > (h), 0)))
+		__is_constexpr((l) > (h)), (l) > (h), 0)))
 #else
 /*
  * BUILD_BUG_ON_ZERO is not available in h files included from asm files,
diff --git a/tools/include/linux/const.h b/tools/include/linux/const.h
index 81b8aae5a855..435ddd72d2c4 100644
--- a/tools/include/linux/const.h
+++ b/tools/include/linux/const.h
@@ -3,4 +3,12 @@
 
 #include <vdso/const.h>
 
+/*
+ * This returns a constant expression while determining if an argument is
+ * a constant expression, most importantly without evaluating the argument.
+ * Glory to Martin Uecker <Martin.Uecker@....uni-goettingen.de>
+ */
+#define __is_constexpr(x) \
+	(sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8)))
+
 #endif /* _LINUX_CONST_H */
-- 
2.31.1