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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Wed, Sep 16, 2026 at 02:58:44PM -0700, Nathan Chancellor wrote:
> After a recent change in LLVM [1], RISC-V builds fail when checking the
> vDSO:
> 
>   arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
>   make[4]: *** [arch/riscv/kernel/vdso/Makefile:78: arch/riscv/kernel/vdso/vdso.so.dbg] Error 1
> 
> memset() is now generated when zeroing params->reserved 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].
> 
> Hide the value of the iterator variable from the optimizer using
> OPTIMIZER_HIDE_VAR to inhibit this optimization since it can no longer
> assume that the zeroing is contiguous.
> 
> 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]
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  lib/vdso/getrandom.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
> index 2851afa9154f..d48d1fdb3351 100644
> --- a/lib/vdso/getrandom.c
> +++ b/lib/vdso/getrandom.c
> @@ -4,6 +4,7 @@
>   */
>  
>  #include <linux/array_size.h>
> +#include <linux/compiler.h>
>  #include <linux/minmax.h>
>  #include <vdso/datapage.h>
>  #include <vdso/getrandom.h>
> @@ -83,8 +84,11 @@ __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)
> +		for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i) {
> +			/* prevent compiler from turning loop into memset() */
> +			OPTIMIZER_HIDE_VAR(i);
>  			params->reserved[i] = 0;
> +		}
>  		return 0;
>  	}
 
I don't suspect this is the right change. On x86_64, this changes to
code from:

    rep stosq

into:

    loc_2D9:
    mov     dword ptr [rbx+rax*4+0Ch], 0
    add     rax, 1
    cmp     rax, 0Ch
    jbe     short loc_2D9

Which is a lot less compact. It seems like the actual solution is for
gcc&clang to emit this inline memset mnemonic when the platform has a
good one, and otherwise not. But disabling optimizations for all
platforms, because it's broken on one, seems bad.

In this case, the compiler is being smart: it identifies a loop and
rightly turns it into memset. But if this isn't a compilation
environment that has an outline function, it should do something else.

Jason