Re: [PATCH bpf-next v7 1/9] bpf: mark instructions accessing program stack

> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index 004b06785521..412a13a664fa 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -725,6 +725,8 @@ struct bpf_insn_aux_data {
>  	u16 const_reg_map_mask;
>  	u16 const_reg_subprog_mask;
>  	u32 const_reg_vals[10];
> +	/* instruction can access non-stack memory */
> +	bool non_stack_access;
>  };

[ ... ]

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index e036ae20bf6b..040af75d9550 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3272,6 +3272,11 @@ static void mark_indirect_target(struct bpf_verifier_env *env, int idx)
>  	env->insn_aux_data[idx].indirect_target = true;
>  }
>
> +static void mark_non_stack_access(struct bpf_verifier_env *env, int idx)
> +{
> +	env->insn_aux_data[idx].non_stack_access = true;
> +}
> +

[ ... ]

> @@ -6654,6 +6659,10 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, struct b
>  				clear_scalar_id(&regs[value_regno]);
>  		}
>  	}
> +
> +	if (!err && reg->type != PTR_TO_STACK)
> +		mark_non_stack_access(env, insn_idx);
> +
>  	return err;
>  }

Does this test read the correct register type? For a load instruction where
dst_reg == src_reg (e.g., rX = *(u64 *)(rX + off)), check_mem_access()
receives reg = &regs[src_reg] and value_regno = dst_reg, so reg points to
the same slot that gets overwritten by the load.

When the pointer is PTR_TO_STACK, check_stack_read() runs:

check_stack_read_fixed_off() {
    state->regs[dst_regno] = *reg;  /* or mark_reg_stack_read() */
}

By the time execution reaches the new test, reg->type is the loaded value's
type (typically SCALAR_VALUE), not PTR_TO_STACK. The condition evaluates to
true and marks a stack-only access as non-stack.

Both check_load_mem() and check_store_reg() snapshot the pointer type before
calling check_mem_access():

    /* verifier.c check_load_mem */
    src_reg_type = regs[insn->src_reg].type;
    err = check_mem_access(env, env->insn_idx, ...);

    /* verifier.c check_store_reg */
    dst_reg_type = regs[insn->dst_reg].type;
    err = check_mem_access(env, env->insn_idx, ...);

Could check_mem_access() snapshot reg->type at entry and test that snapshot
instead?


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/32534828209