From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 63BAEC98328 for ; Sat, 26 Sep 2026 10:02:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=wBsv8tb6LH2O7UOw2NPC4tXK8Z8EIGx6ZkJ06PXH8gw=; b=iEy4FLv9E+AUhp TdC+SGNOnQNA9wiBtOEXqd5ddtsd8aoLTm8BpIu+5/6GInjeSTrbOxbGyGjbGH2Keqg+RyNSKLuQ8 aXJYKsHRwO5mZAB7woXlD9YIgQDyVDuFbZ6gi9aoY3LE5aAcZhBxw1TdjCa3IX6rN7B7szRn02hhk bY7Q3cr6PDYs7VB+HZbd6QFoo6ClD48x3hsq2RoC4gEKz3nbuMmoa0um1cxTpXPqN6BqzWxVCoh/+ P0HKluApb/90szIUAHsuiaJg3oeznPxuZt8JlVehEIC39pr+deG8nkVbAASu13upiL1nRrzjdNSWM la1vhJYBnVWA8D2PCrRg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1xAPEx-0000000FFBP-3v4E; Sat, 26 Sep 2026 10:02:43 +0000 Received: from tor.source.kernel.org ([2600:3c04:e001:324:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1xAPEw-0000000FFAt-1E5q; Sat, 26 Sep 2026 10:02:42 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 1DBCD6020C; Sat, 26 Sep 2026 10:02:41 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A5C31F000FF; Sat, 26 Sep 2026 10:02:37 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=zx2c4.com header.i=@zx2c4.com header.a=rsa-sha256 header.s=20210105 header.b=OfuqufY7 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1790416955; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=b4UQCU/ng7lvOfRadkfffYlo+ZYxoOXVPEaR6txbrSk=; b=OfuqufY7Zlzrp2MAniYFVjK13GMq17YgmuZGEkdWoZWzrVashWdc1VY7gG6iw5f1ay+UGR OIx/M6Kn9cb9R3rhwYAPHY8tFbYHBh11+x4RCQ49ugzXpzm2kx17CtXFw7mIf9ioCavt+m sUgBH7n/oWVrLYceHgWDzsrCpDBOGJA= Received: by mail.zx2c4.com (OpenSMTPD) with ESMTPSA id da1c29b1 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Sat, 26 Sep 2026 10:02:35 +0000 (UTC) Date: Sat, 26 Sep 2026 12:02:26 +0200 From: "Jason A. Donenfeld" To: Nick Desaulniers Cc: Nathan Chancellor , Andy Lutomirski , Thomas Gleixner , Theodore Ts'o , Vincenzo Frascino , Bill Wendling , Justin Stitt , Catalin Marinas , Will Deacon , Mark Rutland , Huacai Chen , WANG Xuerui , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data() Message-ID: References: <20260925-vdso-getrandom-avoid-memset-llvm-24-v2-1-ce640f872393@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-BeenThere: linux-riscv@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: base64 Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org T24gRnJpLCBTZXAgMjUsIDIwMjYgYXQgMDM6MDA6NDZQTSAtMDcwMCwgTmljayBEZXNhdWxuaWVy 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da1c29b1 (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Sat, 26 Sep 2026 10:02:35 +0000 (UTC) Date: Sat, 26 Sep 2026 12:02:26 +0200 From: "Jason A. Donenfeld" To: Nick Desaulniers Cc: Nathan Chancellor , Andy Lutomirski , Thomas Gleixner , Theodore Ts'o , Vincenzo Frascino , Bill Wendling , Justin Stitt , Catalin Marinas , Will Deacon , Mark Rutland , Huacai Chen , WANG Xuerui , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Heiko Carstens , Vasily Gorbik , Alexander Gordeev , Christian Borntraeger , Sven Schnelle , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, llvm@lists.linux.dev Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data() Message-ID: References: <20260925-vdso-getrandom-avoid-memset-llvm-24-v2-1-ce640f872393@kernel.org> Precedence: bulk X-Mailing-List: llvm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Sep 25, 2026 at 03:00:46PM -0700, Nick Desaulniers wrote: > On Fri, Sep 25, 2026 at 2:56 PM Nick Desaulniers > wrote: > > > > On Fri, Sep 25, 2026 at 2:53 PM Nick Desaulniers > > wrote: > > > > > > On Fri, Sep 25, 2026 at 2:46 PM Nathan Chancellor wrote: > > > > > > > > After a recent change in LLVM [1], builds with the random vDSO > > > > implementation, such as PowerPC and RISC-V, fail when checking the vDSO: > > > > > > > > arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not supported > > > > arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported > > > > > > > > memset() is now generated when zeroing params->reserved for some builds > > > > because LLVM has an optimization (now run in more instances) that can > > > > recognize at compile time when it is assigning a static value to a > > > > contiguous area of memory and turn that into a call to memset(). Both > > > > clang and GCC assume memset() is always available [2]. > > > > > > > > clang provides a builtin, __builtin_memset_inline [3][4], that can be > > > > used to ensure an external function call is not generated when zeroing > > > > this memory. Use it when it is available. > > > > > > > > While GCC has no issues with the current code, it has generated memset() > > > > before, as seen in commit b7bad082e113 ("random: vDSO: avoid call to out > > > > of line memset()"). GCC 14 provides '-finline-stringops=memset' [5][6] > > > > with a similar guarantee to the clang builtin, so use it and > > > > __builtin_memset() when available. > > > > > > > > If no option is available, provide a simple memset_inline() like the one > > > > from lib/string.c to avoid adding an ugly ifdef. > > > > > > > > Link: https://github.com/llvm/llvm-project/commit/90cebef1411617fc3eedd359bdf00cb44b1c2439 [1] > > > > Link: https://gcc.gnu.org/onlinedocs/gcc-16.2.0/gcc/Standards.html#index-ffreestanding [2] > > > > Link: https://github.com/llvm/llvm-project/commit/38637ee477541370a90b37f149069d8e5c0c2efd [3] > > > > Link: https://clang.llvm.org/docs/LanguageExtensions.html#guaranteed-inlined-memset [4] > > > > Link: https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=1ff6d9f7428b0668cd8ab0b3e3ab94f1d733124d [5] > > > > Link: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-finline-stringops [6] > > > > Suggested-by: "Jason A. Donenfeld" > > > > Suggested-by: Nick Desaulniers > > > > Signed-off-by: Nathan Chancellor > > > > --- > > > > Changes in v2: > > > > - Switch approach entirely (Jason, Nick) > > > > - clang: use __builtin_memset_inline() to avoid external call > > > > - GCC 14+: use __builtin_memset + -finline-stringops=memset (hence the > > > > massive CC list increase) > > > > - GCC < 14: no issues currently but avoid ifdef with simple memset > > > > implementation > > > > - Link to v1: https://patch.msgid.link/20260916-vdso-getrandom-avoid-memset-llvm-24-v1-1-80a92f2e225a@kernel.org > > > > --- > > > > arch/arm64/kernel/vdso/Makefile | 2 +- > > > > arch/loongarch/vdso/Makefile | 1 + > > > > arch/powerpc/kernel/vdso/Makefile | 1 + > > > > arch/riscv/kernel/vdso/Makefile | 1 + > > > > arch/s390/kernel/vdso/Makefile | 1 + > > > > arch/x86/entry/vdso/vdso64/Makefile | 2 +- > > > > init/Kconfig | 7 +++++++ > > > > lib/vdso/getrandom.c | 19 +++++++++++++++++-- > > > > 8 files changed, 30 insertions(+), 4 deletions(-) > > > > > > > > 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/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile > > > > index 368759f81708..0d1a49529885 100644 > > > > --- a/arch/powerpc/kernel/vdso/Makefile > > > > +++ b/arch/powerpc/kernel/vdso/Makefile > > > > @@ -43,6 +43,7 @@ ccflags-y := -fno-common -fno-builtin -DBUILD_VDSO > > > > ccflags-y += $(DISABLE_LATENT_ENTROPY_PLUGIN) > > > > ccflags-y += $(call cc-option, -fno-stack-protector) > > > > ccflags-y += -DDISABLE_BRANCH_PROFILING > > > > +ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET) > > > > ccflags-y += -ffreestanding -fasynchronous-unwind-tables > > > > ccflags-remove-y := $(CC_FLAGS_FTRACE) > > > > ldflags-y := -Wl,--hash-style=both -nostdlib -shared -z noexecstack $(CLANG_FLAGS) > > > > 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 8583d9f06c52..ff3f8475dd2f 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..f5cad5641985 100644 > > > > --- a/lib/vdso/getrandom.c > > > > +++ b/lib/vdso/getrandom.c > > > > @@ -29,6 +29,22 @@ > > > > } \ > > > > } 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) > > > > +#else > > > > +static inline void *memset_inline(void *dst, int value, size_t size) > > > > +{ > > > > + char *d = dst; > > > > + > > > > + while (size--) > > > > + *d++ = value; > > > > + > > > > + return d; > > > > +} > > > > +#endif > > > > > > Does this work? > > > https://godbolt.org/z/Tx6EaGMfb > > > > Hmmm...possibly. > > https://godbolt.org/z/ec9rzd9Yf > > I don't get it... > > Just keep it under 2 pages, it will be fine: > https://godbolt.org/z/qqoMqfxev https://godbolt.org/z/9W9f5a7Wx Apparently not on RISCV, which is what prompted this patch in the first place. So I suspect Nathan's v2 here is the way to go. Jason