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

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Sat Aug 22, 2026 at 1:24 AM CEST, bot+bpf-ci wrote:
>> 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?

This one also looks like a real bug, we should probably save the type before the
call as suggested and done elsewhere.

>
>
> ---
> 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