Re: Re: [PATCH bpf 2/2] selftests/bpf: Cover stack nospec slot indexing

[Nuoqi Gui]

> -----Original Messages-----
> From: "Emil Tsalapatis" <emil@xxxxxxxxxxxxxxx>
> Send time:Wednesday, 17/06/2026 05:14:12
> To: "Nuoqi Gui" <gnq25@xxxxxxxxxxxxxxxxxxxxx>, "Alexei Starovoitov" <ast@xxxxxxxxxx>, "Daniel Borkmann" <daniel@xxxxxxxxxxxxx>, "Andrii
>  Nakryiko" <andrii@xxxxxxxxxx>, "Eduard Zingerman" <eddyz87@xxxxxxxxx>, "Kumar Kartikeya Dwivedi" <memxor@xxxxxxxxx>
> Cc: "John Fastabend" <john.fastabend@xxxxxxxxx>, "Martin KaFai Lau" <martin.lau@xxxxxxxxx>, "Luis Gerhorst" <luis.gerhorst@xxxxxx>, "Shuah
>  Khan" <shuah@xxxxxxxxxx>, bpf@xxxxxxxxxxxxxxx, linux-kselftest@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH bpf 2/2] selftests/bpf: Cover stack nospec slot indexing
> 
> On Tue Jun 16, 2026 at 12:57 PM EDT, Nuoqi Gui wrote:
> > Add a verifier test for the fixed-offset stack write case where two 4-byte
> > stores initialize opposite halves of the same stack slot.
> >
> > The test uses the CAP_BPF-without-CAP_PERFMON loader lane so Spectre v4
> > mitigation remains active. It expects both half-slot writes to emit nospec
> > in the translated program.
> >
> > Signed-off-by: Nuoqi Gui <gnq25@xxxxxxxxxxxxxxxxxxxxx>
> 
> Reviewed-by: Emil Tsalapatis <emil@xxxxxxxxxxxxxxx>
> 
> Trivial style nits.
> 
> > ---
> >  .../testing/selftests/bpf/progs/verifier_unpriv.c  | 23 ++++++++++++++++++++++
> >  1 file changed, 23 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_unpriv.c b/tools/testing/selftests/bpf/progs/verifier_unpriv.c
> > index c16f8382cf17d..9ebbd4b531df1 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_unpriv.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_unpriv.c
> > @@ -976,4 +976,27 @@ l0_%=:	exit;						\
> >  	: __clobber_all);
> >  }
> >  
> > +SEC("socket")
> > +__description("noperfmon: Spectre v4 stack write slot index")
> > +__success __success_unpriv
> > +__caps_unpriv(CAP_BPF)
> > +__retval(0)
> > +#ifdef SPEC_V4
> > +__xlated_unpriv("r0 = 0")
> > +__xlated_unpriv("*(u32 *)(r10 -4) = r0")
> > +__xlated_unpriv("nospec")
> > +__xlated_unpriv("*(u32 *)(r10 -8) = r0")
> 
> No whitespace to the right of -
> 
> > +__xlated_unpriv("nospec")
> > +__xlated_unpriv("exit")
> > +#endif
> > +__naked void stack_write_nospec_slot_index(void)
> > +{
> > +	asm volatile ("					\
> > +	r0 = 0;						\
> > +	*(u32 *)(r10 - 4) = r0;			\
> > +	*(u32 *)(r10 - 8) = r0;			\
> 
> Unaligned \ 
> 
> > +	exit;						\
> > +"	::: __clobber_all);
> > +}
> > +
> >  char _license[] SEC("license") = "GPL";

Thanks.

I'll fix the selftest formatting by adding the missing whitespace in the
__xlated_unpriv() strings and aligning the inline asm '\' markers.