Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

From: Nathan Chancellor <nathan@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	"Christophe Leroy (CS GROUP)" <chleroy@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@kernel.org>, Theodore Ts'o <tytso@mit.edu>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Bill Wendling <morbo@google.com>,
	Justin Stitt <justinstitt@google.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Nicholas Piggin <npiggin@gmail.com>,
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	Albert Ou <aou@eecs.berkeley.edu>,
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	"H. Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()
Date: Wed, 30 Sep 2026 15:38:13 +0200	[thread overview]
Message-ID: <20260930133813.GA3142230@ax162> (raw)
In-Reply-To: <CAKwvOd=BrMfFT88jsvBPhh383qqk5G+J07CsofNu0MVkziyhCg@mail.gmail.com>

On Tue, Sep 29, 2026 at 11:24:11AM -0700, Nick Desaulniers wrote:
> arch/arm64/kvm/hyp/nvhe/Makefile has this pattern for exactly the same
> problem I suspect.  Maybe that's the right tool in the toolbox?
> 
> ```
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index 8dbf2532a573..27fa72d8fb86 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -27,7 +27,7 @@ asflags-y += -DVDSO_CFI=1
>  endif
> 
>  # Files to link into the vdso
> -obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o
> +obj-vdso = $(patsubst %, %.o, $(vdso-syms)) note.o ../../lib/memset.o
> 
>  ifdef CONFIG_VDSO_GETRANDOM
>  obj-vdso += vgetrandom-chacha.o
> ```
> Fixes `make -skj"$(nproc)" ARCH=riscv LLVM=1 mrproper allmodconfig
> vdso_prepare` for me, as per

For the record, this also happens with the 32-bit PowerPC vDSO, as I
noted in the commit message of v2. I should update the issue too, I only
realized this after wider testing. So if this is the route we want to
go, we would need a memset() for that vDSO as well.

> https://github.com/ClangBuiltLinux/linux/issues/2183
> (Nathan, don't forget to link to that in the commit message)

Yes, thanks, I have added it for v3.

> I'm surprised I didn't need -fno-semantic-interposition (or one of the
> related flags... -fvisibility=hidden)
> 
> If we want to get better, (if performance matters here and we want to
> trade source+build system complexity for absolute code perf) I would
> start with that, then worry about clawing back performance via things
> like:
> - __builtin_memset_inline
> - -finline-stringops=memset
> - -ffunction-sections+-Wl,--gc-sections to dead code eliminate the out
> of line copy of memset, though IIRC there's potential for wasted space
> due to alignment requirements (maybe the out of line copy of memset is
> smaller...idk)

Yeah, I guess it is ultimately up to the maintainers what route they
prefer.

-- 
Cheers,
Nathan