Commit 295bcca8 authored Apr 06, 2020 by Rikard Falkeborn Committed by Linus Torvalds Apr 07, 2020

linux/bits.h: add compile time sanity check of GENMASK inputs

GENMASK() and GENMASK_ULL() are supposed to be called with the high bit as
the first argument and the low bit as the second argument.  Mixing them
will return a mask with zero bits set.

Recent commits show getting this wrong is not uncommon, see e.g.  commit
aa4c0c90 ("net: stmmac: Fix misuses of GENMASK macro") and commit
9bdd7bb3 ("clocksource/drivers/npcm: Fix misuse of GENMASK macro").

To prevent such mistakes from appearing again, add compile time sanity
checking to the arguments of GENMASK() and GENMASK_ULL().  If both
arguments are known at compile time, and the low bit is higher than the
high bit, break the build to detect the mistake immediately.

Since GENMASK() is used in declarations, BUILD_BUG_ON_ZERO() must be used
instead of BUILD_BUG_ON().

__builtin_constant_p does not evaluate is argument, it only checks if it
is a constant or not at compile time, and __builtin_choose_expr does not
evaluate the expression that is not chosen.  Therefore, GENMASK(x++, 0)
does only evaluate x++ once.

Commit 95b980d6 ("linux/bits.h: make BIT(), GENMASK(), and friends
available in assembly") made the macros in linux/bits.h available in
assembly.  Since BUILD_BUG_OR_ZERO() is not asm compatible, disable the
checks if the file is included in an asm file.

Due to bugs in GCC versions before 4.9 [0], disable the check if building
with a too old GCC compiler.

[0]: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19449



Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Haren Myneni <haren@us.ibm.com>
Cc: Joe Perches <joe@perches.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: lkml <linux-kernel@vger.kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20200308193954.2372399-1-rikard.falkeborn@gmail.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8f0259c2

include/linux/bits.h

+20 −2

Original line number	Diff line number	Diff line
		@@ -18,12 +18,30 @@
		* position @h. For example
		* GENMASK_ULL(39, 21) gives us the 64bit vector 0x000000ffffe00000.
		*/
		#define GENMASK(h, l) \
		#if !defined(__ASSEMBLY__) && \
		(!defined(CONFIG_CC_IS_GCC) \|\| CONFIG_GCC_VERSION >= 49000)
		#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)))
		#else
		/*
		* BUILD_BUG_ON_ZERO is not available in h files included from asm files,
		* disable the input check if that is the case.
		*/
		#define GENMASK_INPUT_CHECK(h, l) 0
		#endif

		#define __GENMASK(h, l) \
		(((~UL(0)) - (UL(1) << (l)) + 1) & \
		(~UL(0) >> (BITS_PER_LONG - 1 - (h))))
		#define GENMASK(h, l) \
		(GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))

		#define GENMASK_ULL(h, l) \
		#define __GENMASK_ULL(h, l) \
		(((~ULL(0)) - (ULL(1) << (l)) + 1) & \
		(~ULL(0) >> (BITS_PER_LONG_LONG - 1 - (h))))
		#define GENMASK_ULL(h, l) \
		(GENMASK_INPUT_CHECK(h, l) + __GENMASK_ULL(h, l))

		#endif /* __LINUX_BITS_H */

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