From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DE8195275A3; Fri, 18 Sep 2026 21:07:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789765671; cv=none; b=t43/Go25Gpbi+j+E15iB1NRR1t+XOcwFrW7KY7ZJc4+4artDtEQMNRNxMO5XZE7gd7yWaQTWX6C4gIKv0A70416oCsSniS57LJzpeoLVS5JwSs2FK82TIDL3KnPfchBOwCBlMeMnSe6srsg99+QBERJwbKFKb6Snu7SSB19WChI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789765671; c=relaxed/simple; bh=wuKgIkiS2FHxRxJ4IEELHzmMQy5AtbOWto6qESGTv5E=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LCllADcBZB+ecVvZaVrRbUCMYcbFXNvzRo92a7soCyXPy8NDGdCvtxSGcSgLOIK2H4e7Mo9YQaWd+p7KRp5/4xkfgPA5/EumqZXHU9g29gJ25vC++YGc9LaH6tQzM3CDvQtE8LCJon9ojAWCbq/9AALEIhQLyFyanQpVvsB2yPc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=gNNUYQ80; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="gNNUYQ80" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3B0ED1F000FF; Fri, 18 Sep 2026 21:07:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789765669; bh=zMyHIW5JmXrfN6pCRrMHPR90GnGJt4p1gcDtib6I5ek=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=gNNUYQ80TMa9xylhzSJAqvCbr81AxV4IcdIDUX6qTECYlv3LT9Bvcn9bmVI/8B/29 lFAaY3L09NlcjNEX+E9TTBdKQDfT5lXxbF4bxh0foXJ5XY4k+Mt5LanZAyMNOsZY5p FlYj5TcoOyJuE0ww0tbwW/jzUY8Sit4pdzGIsVptJgWhpOsZpYxHK7ReXz7yTGA5NF L8x05AC/wjRjEWHBGwDcHa8CZS5jRp0lnNEH7k5mN1sqX7iaGsyWxqjlt0wCyJyS8E 9MuMLU2cAerd+4RkRYFcH0f7uvrHD/USH5StwWaEh/X22vGdQKdB23rTiTzy66jntV T2v4SwaXT+/xw== Date: Fri, 18 Sep 2026 14:07:44 -0700 From: Nathan Chancellor To: "Jason A. Donenfeld" Cc: Nick Desaulniers , Andy Lutomirski , Thomas Gleixner , Theodore Ts'o , Vincenzo Frascino , Bill Wendling , Justin Stitt , linux-kernel@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data() Message-ID: <20260918210744.GA235304@ax162> References: <20260916-vdso-getrandom-avoid-memset-llvm-24-v1-1-80a92f2e225a@kernel.org> <20260917173920.GB356152@ax162> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Fri, Sep 18, 2026 at 09:19:00PM +0200, Jason A. Donenfeld wrote: > Untested, but putting this all together I suppose looks like this, if > you want to play around and see: Thanks! > diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c > index 2851afa9154f..713e95dfdedc 100644 > --- a/lib/vdso/getrandom.c > +++ b/lib/vdso/getrandom.c > @@ -29,6 +29,12 @@ > } \ > } while (0) > > +#if __has_builtin(__builtin_memset_inline) > +#define memset(dst, value, size) __builtin_memset_inline(dst, value, size) > +#else > +#define memset(dst, value, size) __builtin_memset(dst, value, size) > +#endif > + > static void memcpy_and_zero_src(void *dst, void *src, size_t len) > { > if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) { > @@ -83,8 +89,7 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_ > params->size_of_opaque_state = sizeof(*state); > params->mmap_prot = PROT_READ | PROT_WRITE; > params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS; > - for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i) > - params->reserved[i] = 0; > + memset(params->reserved, 0, sizeof(params->reserved)); > return 0; > } This will regress GCC < 14 because '-finline-stringops=memset' does not exist yet, meaning __builtin_memset() may still generated a call to memset(). Testing GCC 13 with ARCH=riscv defconfig, I get the same error as originally reported up thread. I am not sure we can get rid of the loop for that reason. I tested the following diff that keeps clang (__builtin_memset_inline) GCC 14+ (-finline-stringops=memset + __builtin_memset) GCC <14 (simple loop) happy (and moves the checks into Kconfig, which is being discussed at a wider level [1]) but it is admittedly slightly uglier... That said, it certainly seems more robust. [1]: https://lore.kernel.org/linux-kbuild/20260917-build-speedup-v3-11-9ecf4163ff36@kernel.org diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile index 7dec05dd33b7..f6619e1cb2ce 100644 --- a/arch/arm64/kernel/vdso/Makefile +++ b/arch/arm64/kernel/vdso/Makefile @@ -41,7 +41,7 @@ CC_FLAGS_REMOVE_VDSO := $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) \ $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) \ -Wmissing-prototypes -Wmissing-declarations -CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables +CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables $(CONFIG_CC_OPT_INLINE_MEMSET) CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_REMOVE_VDSO) CFLAGS_REMOVE_vgetrandom.o = $(CC_FLAGS_REMOVE_VDSO) diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile index 9c9181bb4071..0b84892d084b 100644 --- a/arch/loongarch/vdso/Makefile +++ b/arch/loongarch/vdso/Makefile @@ -16,6 +16,7 @@ ccflags-vdso := \ $(filter -m64,$(KBUILD_CFLAGS)) \ $(filter -march=%,$(KBUILD_CFLAGS)) \ $(filter -m%-float,$(KBUILD_CFLAGS)) \ + $(CONFIG_CC_OPT_INLINE_MEMSET) \ $(CLANG_FLAGS) \ -D__VDSO__ diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile index 8dbf2532a573..c023046a3fd7 100644 --- a/arch/riscv/kernel/vdso/Makefile +++ b/arch/riscv/kernel/vdso/Makefile @@ -36,6 +36,7 @@ endif ccflags-y := -fno-stack-protector ccflags-y += -DDISABLE_BRANCH_PROFILING ccflags-y += -fno-builtin +ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET) ccflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH) ccflags-y += $(CFI_FULL) asflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH) diff --git a/arch/s390/kernel/vdso/Makefile b/arch/s390/kernel/vdso/Makefile index 35c834b895ec..54bcf2984ca1 100644 --- a/arch/s390/kernel/vdso/Makefile +++ b/arch/s390/kernel/vdso/Makefile @@ -29,6 +29,7 @@ KBUILD_CFLAGS_VDSO := $(filter-out -munaligned-symbols,$(KBUILD_CFLAGS_VDSO)) KBUILD_CFLAGS_VDSO := $(filter-out -fno-asynchronous-unwind-tables,$(KBUILD_CFLAGS_VDSO)) KBUILD_CFLAGS_VDSO += -fPIC -fno-common -fno-builtin -fasynchronous-unwind-tables KBUILD_CFLAGS_VDSO += -fno-stack-protector $(DISABLE_KSTACK_ERASE) +KBUILD_CFLAGS_VDSO += $(CONFIG_CC_OPT_INLINE_MEMSET) ldflags-y := -shared -soname=linux-vdso.so.1 \ --hash-style=both --build-id=sha1 \ $(call ld-option, --eh-frame-hdr) -T diff --git a/arch/x86/entry/vdso/vdso64/Makefile b/arch/x86/entry/vdso/vdso64/Makefile index 7c0790065b5e..c2353a065279 100644 --- a/arch/x86/entry/vdso/vdso64/Makefile +++ b/arch/x86/entry/vdso/vdso64/Makefile @@ -14,7 +14,7 @@ vobjs-$(CONFIG_X86_SGX) += vsgx.o vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK) += vfutex.o # Compilation flags -flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small +flags-y := -DBUILD_VDSO64 -m64 -mcmodel=small $(CONFIG_CC_OPT_INLINE_MEMSET) # The location of this include matters! include $(src)/../common/Makefile.include diff --git a/init/Kconfig b/init/Kconfig index 6e82b4957a5d..09251b5f79d4 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -173,6 +173,13 @@ config CC_HAS_ALLOC_TOKEN config CC_HAS_MULTIDIMENSIONAL_NONSTRING def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror) +config CC_HAS_OPT_INLINE_MEMSET + def_bool $(cc-option,-finline-stringops=memset) + +config CC_OPT_INLINE_MEMSET + string + default "-finline-stringops=memset" if CC_HAS_OPT_INLINE_MEMSET + config LD_CAN_USE_KEEP_IN_OVERLAY # ld.lld prior to 21.0.0 did not support KEEP within an overlay description # https://github.com/llvm/llvm-project/pull/130661 diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c index 2851afa9154f..83831dab08b2 100644 --- a/lib/vdso/getrandom.c +++ b/lib/vdso/getrandom.c @@ -29,6 +29,12 @@ } \ } while (0) +#if __has_builtin(__builtin_memset_inline) +#define memset_inline(dst, value, size) __builtin_memset_inline(dst, value, size) +#elif IS_ENABLED(CONFIG_CC_HAS_OPT_INLINE_MEMSET) +#define memset_inline(dst, value, size) __builtin_memset(dst, value, size) +#endif + static void memcpy_and_zero_src(void *dst, void *src, size_t len) { if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) { @@ -83,8 +89,12 @@ __cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_ params->size_of_opaque_state = sizeof(*state); params->mmap_prot = PROT_READ | PROT_WRITE; params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS; +#ifdef memset_inline + memset_inline(params->reserved, 0, sizeof(params->reserved)); +#else for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i) params->reserved[i] = 0; +#endif return 0; } -- Cheers, Nathan