Messages in this thread | | | Date | Mon, 9 Mar 2020 09:40:47 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v5] linux/bits.h: Add compile time sanity check of GENMASK inputs |
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On Sun, Mar 08, 2020 at 08:39:54PM +0100, Rikard Falkeborn wrote: > 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 aa4c0c9091b0 ("net: stmmac: Fix misuses of GENMASK macro") and > commit 9bdd7bb3a844 ("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 95b980d62d52 ("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>
Reviewed-by: Kees Cook <keescook@chromium.org>
-Kees
> Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com> > --- > Another attempt to get this merged. I've test built allmodconfig for > i386, x86_64 and arm64 for linux-next 20200306 without issues. Also, the > last known GENMASK issue was just merged into Linus tree [1]. > > [1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=96b4ea324ae92386db2b0c73ace597c80cde1ecb > > Changes in v5: > - Added Masahiros Reviewed-by > - Waited for bugfixes to get merged > > Changes in v4: > - Disable the argument check for GCC < 4.9 due to a compiler bug. > > Changes in v3: > - Changed back to shorter macro argument names > - Remove casts and use 0 instead of UL(0) in GENMASK_INPUT_CHECK(), > since all results in GENMASK_INPUT_CHECK() are now ints. Update > commit message to reflect that. > > Changes in v2: > - Add comment about why inputs are not checked when used in asm file > - Use UL(0) instead of 0 > - Extract mask creation in a separate macro to improve readability > - Use high and low instead of h and l (part of this was extracted to a > separate patch) > - Updated commit message > > Joe Perches sent a series to fix the existing misuses of GENMASK(). > Those patches have been merged into Linus tree except two places where > the GENMASK misuse is in unused macros, which will not fail to build. > There was also a patch by Nathan Chancellor that have now been merged. > > include/linux/bits.h | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bits.h b/include/linux/bits.h > index 669d69441a62..f108302a3121 100644 > --- a/include/linux/bits.h > +++ b/include/linux/bits.h > @@ -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 */ > -- > 2.25.1 >
-- Kees Cook
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