Re: [PATCH bpf-next v2 2/2] selftests/bpf: Cover mixed arena and stack atomics

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2026-08-16 at 16:17 -0700, Eduard Zingerman wrote:
> On Sun, 2026-08-16 at 10:56 +0000, Yiyang Chen wrote:
> > Add a verifier test with one atomic RMW instruction reached through
> > PTR_TO_ARENA and PTR_TO_STACK paths. The verifier must reject the
> > shared instruction with the existing incompatible-pointer diagnostic.
> > 
> > Signed-off-by: Yiyang Chen <chenyy23@mails.tsinghua.edu.cn>
> > ---
> >  tools/testing/selftests/bpf/progs/verifier_arena.c | 39 ++++++++++++++++++++++
> >  1 file changed, 39 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/bpf/progs/verifier_arena.c b/tools/testing/selftests/bpf/progs/verifier_arena.c
> > index b241bbcf54a8a..b22bab33301ab 100644
> > --- a/tools/testing/selftests/bpf/progs/verifier_arena.c
> > +++ b/tools/testing/selftests/bpf/progs/verifier_arena.c
> > @@ -637,6 +637,45 @@ int non_arena_ptr_add_to_arena_ptr(void *ctx)
> >  
> >  #endif
> >  
> > +static const struct bpf_insn addr_space_cast_insn = {
> > +	.code = 0xbf,
> > +	.dst_reg = BPF_REG_7,
> > +	.src_reg = BPF_REG_7,
> > +	.off = 1,
> > +	.imm = 1,
> > +};
> > +
> > +SEC("socket")
> > +__description("arena and stack atomic at the same instruction")
> > +__failure __msg("same insn cannot be used with different pointers")
> > +__arch_x86_64
> > +__load_if_JITed()
> > +__naked void mixed_arena_stack_atomic(void)
> > +{
> > +	asm volatile ("					\
> > +	r1 = %[arena] ll;				\
> > +	r6 = r10;					\
> > +	r6 += -8;					\
> > +	r9 = 0;					\
> > +	*(u64 *)(r6 + 0) = r9;			\
> > +	r7 = 8192;					\
> > +	.8byte %[addr_space_cast];			\
> 
> I'm going to fix this to:
> 
> 	r7 = addr_space_cast(r7, 0, 1);			\
> 
> as in the test case above.
> Waiting for CI [1].
> 
> [1] https://github.com/kernel-patches/bpf/pull/13325


A...and it requires another exception for GCC-BPF:
https://github.com/kernel-patches/bpf/blob/bpf-next_base/ci/vmtest/configs/DENYLIST.test_progs-bpf_gcc

Ihor, it's a third time GCC-BPF strikes this weekend.
What do people think about downgrading it to may-fail until the
DENYLIST decreases to something reasonable?

Anyway, I'll wrap-up with this patch-set in the evening.

> > +	call %[bpf_get_prandom_u32];			\
> > +	if w0 != 0 goto 1f;				\
> > +	r8 = r6;					\
> > +	goto 2f;					\
> > +1:	r8 = r7;					\
> > +2:	r9 = 1;					\
> > +	lock *(u64 *)(r8 + 0) += r9;			\
> > +	r0 = 0;					\
> > +	exit;						\
> > +"	:
> > +	: __imm_addr(arena),
> > +	  __imm_insn(addr_space_cast, addr_space_cast_insn),
> > +	  __imm(bpf_get_prandom_u32)
> > +	: __clobber_all);
> > +}
> > +
> >  static __noinline
> >  u32 __arena *check_arena_arg_nonglobal(u32 __arena *arg)
> >  {