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[158.174.22.22]) by smtp.gmail.com with ESMTPSA id v5sm26129lji.73.2020.06.04.16.30.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Jun 2020 16:30:09 -0700 (PDT) From: Rikard Falkeborn X-Google-Original-From: Rikard Falkeborn Date: Fri, 5 Jun 2020 01:30:03 +0200 To: Andy Shevchenko Cc: Rikard Falkeborn , Andrew Morton , Arnd Bergmann , emil.l.velikov@gmail.com, Kees Cook , Linus Walleij , Linux-Arch , Linux Kernel Mailing List , Syed Nayyar Waris , William Breathitt Gray , Masahiro Yamada , kbuild test robot Subject: Re: [PATCH] linux/bits.h: fix unsigned less than zero warnings Message-ID: <20200604233003.GA102768@rikard> References: <20200603215314.GA916134@rikard> <20200603220226.916269-1-rikard.falkeborn@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 04, 2020 at 09:41:29AM +0300, Andy Shevchenko wrote: > On Thu, Jun 4, 2020 at 1:03 AM Rikard Falkeborn > wrote: > > > > When calling the GENMASK and GENMASK_ULL macros with zero lower bit and > > an unsigned unknown high bit, some gcc versions warn due to the > > comparisons of the high and low bit in GENMASK_INPUT_CHECK. > > > > To silence the warnings, cast the inputs to int before doing the > > comparisons. The only valid inputs to GENMASK() and GENMASK_ULL() are > > are 0 to 31 or 63. Anything outside this is undefined due to the shifts > > in GENMASK()/GENMASK_ULL(). Therefore, casting the inputs to int do not > > change the values for valid known inputs. For unknown values, the check > > does not change anything since it's a compile-time check only. > > > > As an example of the warning, kindly reported by the kbuild test robot: > > > > from drivers/mfd/atmel-smc.c:11: > > drivers/mfd/atmel-smc.c: In function 'atmel_smc_cs_encode_ncycles': > > include/linux/bits.h:26:28: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] > > 26 | __builtin_constant_p((l) > (h)), (l) > (h), 0))) > > | ^ > > include/linux/build_bug.h:16:62: note: in definition of macro 'BUILD_BUG_ON_ZERO' > > 16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) > > | ^ > > include/linux/bits.h:39:3: note: in expansion of macro 'GENMASK_INPUT_CHECK' > > 39 | (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) > > | ^~~~~~~~~~~~~~~~~~~ > > >> drivers/mfd/atmel-smc.c:49:25: note: in expansion of macro 'GENMASK' > > 49 | unsigned int lsbmask = GENMASK(msbpos - 1, 0); > > | ^~~~~~~ > > > > Thank you for the patch! > > I think there is still a possibility to improve (as I mentioned there > are test cases that are absent right now). > What if we will have unsigned long value 0x100000001? Would it be 1 > after casting? > > Maybe cast to (long) or (long long) more appropriate? It you're entering 0x10000001 you're going to get a compiler warning since it's going to be shifted out of range, when I wrote the check I figured that should be enough, but perhaps I was wrong? How about requiring both l and h bit to be constant? (that's arguably the way I should have written in the first place). That's going to remove the warnings for GENMASK(x, 0). It will not work as expected when mixing negative and unsigned input, like GENMASK(-1, 0u) is not going to fail the build while GENMASK(1u, -1) will. For GENMASK(-1, 0u), you will get a compiler warning due to the shifts in GENMASK. If we need to handle the negative inputs as well. I think I'd prefer to add explicit checks for negative values over the casting. A macro for that can probably be reused in other places as well. > > Please, add test cases. Will do. Rikard > > > Fixes: 295bcca84916 ("linux/bits.h: add compile time sanity check of GENMASK inputs") > > Reported-by: kbuild test robot > > Reported-by: Emil Velikov > > Reported-by: Syed Nayyar Waris > > Signed-off-by: Rikard Falkeborn > > --- > > include/linux/bits.h | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/bits.h b/include/linux/bits.h > > index 4671fbf28842..293d1ee71a48 100644 > > --- a/include/linux/bits.h > > +++ b/include/linux/bits.h > > @@ -21,9 +21,10 @@ > > #if !defined(__ASSEMBLY__) && \ > > (!defined(CONFIG_CC_IS_GCC) || CONFIG_GCC_VERSION >= 49000) > > #include > > +/* Avoid Wtype-limits warnings by casting the inputs to int */ > > #define GENMASK_INPUT_CHECK(h, l) \ > > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \ > > - __builtin_constant_p((l) > (h)), (l) > (h), 0))) > > + __builtin_constant_p((int)(l) > (int)(h)), (int)(l) > (int)(h), 0))) > > #else > > /* > > * BUILD_BUG_ON_ZERO is not available in h files included from asm files, > > -- > > 2.27.0 > > > > > -- > With Best Regards, > Andy Shevchenko