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

[Nick Desaulniers]

On Thu, Sep 17, 2026 at 10:39 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
>
> On Thu, Sep 17, 2026 at 11:26:46AM +0200, Jason A. Donenfeld wrote:
> > 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

/me nods

> >
> > 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.

Ideally, yeah.
Pragmatically:
the compiler doesn't know what you will link against or not; so it
just emits relocations that the linker will (hopefully) resolve.  I've
definitely looked at how llvm decides when to emit libcalls to
compiler-rt/libgcc ("the compiler runtime") and thought "I wonder how
this works with compiler runtime version N-1?"

There's also the requirement gcc and clang have about memcpy, memmove,
memset, memcmp always being available (-ffreestanding or not) noted
below.

> > 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.
>
> As I mentioned in the commit message, compilers require all environments
> (hosted or not) to provide memset(), so the "doing something else" is
> nothing :)
>
>   GCC requires the freestanding environment provide memcpy, memmove,
>   memset and memcmp.

+1

>
> I guess another option is to just include a basic memset() like the one
> in lib/string.c so that it is only used if the compiler makes this sort
> of transformation, while leaving all other architectures alone.

It's also possible to get GCC to emit the libcall, even in the
presence of -ffreestanding: https://godbolt.org/z/6hTrYq9z8

And it's not the first time this code in particular has had this issue;

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b7bad082e113640fc81200ff869e5c2d7a9c29a2

(is this patch a Fixes: for that?)

Clang has __builtin_memset_inline to avoid explicit libcalls; AFAICT
GCC does not.  So we could use that here to provide such a guarantee
with clang; the code would remain brittle and likely break again with
GCC.

>
> --
> Cheers,
> Nathan



-- 
Thanks,
~Nick Desaulniers