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 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

> +	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)
>  {