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[98.128.228.126]) by smtp.gmail.com with ESMTPSA id p28sm12304016ljn.24.2020.04.15.12.40.34 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Apr 2020 12:40:35 -0700 (PDT) From: Rikard Falkeborn X-Google-Original-From: Rikard Falkeborn Date: Wed, 15 Apr 2020 21:40:32 +0200 To: Sasha Levin Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Rikard Falkeborn , Andrew Morton , Masahiro Yamada , Kees Cook , Borislav Petkov , Geert Uytterhoeven , Haren Myneni , Joe Perches , Johannes Berg , Ingo Molnar , Thomas Gleixner , Linus Torvalds Subject: Re: [PATCH AUTOSEL 5.4 52/84] linux/bits.h: add compile time sanity check of GENMASK inputs Message-ID: <20200415194032.GA935@rikard> References: <20200415114442.14166-1-sashal@kernel.org> <20200415114442.14166-52-sashal@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200415114442.14166-52-sashal@kernel.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 15, 2020 at 07:44:09AM -0400, Sasha Levin wrote: > From: Rikard Falkeborn > > [ Upstream commit 295bcca84916cb5079140a89fccb472bb8d1f6e2 ] > > 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 > Signed-off-by: Andrew Morton > Reviewed-by: Masahiro Yamada > Reviewed-by: Kees Cook > Cc: Borislav Petkov > Cc: Geert Uytterhoeven > Cc: Haren Myneni > Cc: Joe Perches > Cc: Johannes Berg > Cc: lkml > Cc: Ingo Molnar > Cc: Thomas Gleixner > Link: http://lkml.kernel.org/r/20200308193954.2372399-1-rikard.falkeborn@gmail.com > Signed-off-by: Linus Torvalds > Signed-off-by: Sasha Levin > --- > 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 669d69441a625..f108302a3121c 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 > +#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 */ > -- > 2.20.1 > This does not really fix anything, it's compile time prevention, so I don't know how appropriate this is for stable (it was also picked for 5.5 and 5.6, but I'm just replying here now, I can ping the other selections if necessary if the patch should be dropped)? Also, for 5.4, it does somewhat depend on commit 8788994376d8 ("linux/build_bug.h: change type to int"). Without it, there may be a subtle integer promotion issue if sizeof(size_t) > sizeof(unsigned long) (I don't *think* such platform exists, but I don't have a warm a fuzzy feeling about it). Rikard