linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Make the pointer returned by iter next method valid

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Message-ID:
 <AM6PR03MB584807BFB29105F1D7FDC89E99812@AM6PR03MB5848.eurprd03.prod.outlook.com>
Date: Fri, 16 Aug 2024 14:42:47 +0100
From: Juntong Deng <juntong.deng@...look.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
 andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org, Kumar Kartikeya Dwivedi <memxor@...il.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Make the pointer returned by iter next
 method valid

On 8/15/24 18:15, Andrii Nakryiko wrote:
> On Thu, Aug 15, 2024 at 9:11 AM Juntong Deng <juntong.deng@...look.com> wrote:
>>
>> Currently we cannot pass the pointer returned by iter next method as
>> argument to KF_TRUSTED_ARGS kfuncs, because the pointer returned by
>> iter next method is not "valid".
>>
>> This patch sets the pointer returned by iter next method to be valid.
>>
>> This is based on the fact that if the iterator is implemented correctly,
>> then the pointer returned from the iter next method should be valid.
>>
>> This does not make NULL pointer valid. If the iter next method has
>> KF_RET_NULL flag, then the verifier will ask the ebpf program to
>> check NULL pointer.
>>
>> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
>> ---
>>   kernel/bpf/verifier.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index ebec74c28ae3..35a7b7c6679c 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -12832,6 +12832,10 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>>                          /* For mark_ptr_or_null_reg, see 93c230e3f5bd6 */
>>                          regs[BPF_REG_0].id = ++env->id_gen;
>>                  }
>> +
>> +               if (is_iter_next_kfunc(&meta))
>> +                       regs[BPF_REG_0].type |= PTR_TRUSTED;
>> +
> 
> It seems a bit too generic to always assign PTR_TRUSTED to anything
> returned from any iterator. Let's maybe add KF_RET_TRUSTED or
> KF_ITER_TRUSTED or something along those lines to mark such iter_next
> kfuncs explicitly?
> 
> For the numbers iterator, for instance, this PTR_TRUSTED makes no sense.
> 

I had the same idea (KF_RET_TRUSTED) before, but Kumar thought it should
be avoided and pointers returned by iter next method should be trusted
by default [0].

The following are previous related discussions:

 >> For iter_next(), I currently have an idea to add new flags to allow
 >> iter_next() to decide whether the return value is trusted or not,
 >> such as KF_RET_TRUSTED.
 >>
 >> What do you think?
 >
 > Why shouldn't the return value always be trusted?
 > We eventually want to switch over to trusted by default everywhere.
 > It would be nice not to go further in the opposite direction (i.e.
 > having to manually annotate trusted) if we can avoid it.

[0]: 
https://lore.kernel.org/bpf/CAP01T75na=fz7EhrP4Aw0WZ33R7jTbZ4BcmY56S1xTWczxHXWw@mail.gmail.com/

Maybe we can have more discussion?

(This email has been CC Kumar)

>>                  mark_btf_func_reg_size(env, BPF_REG_0, sizeof(void *));
>>                  if (is_kfunc_acquire(&meta)) {
>>                          int id = acquire_reference_state(env, insn_idx);
>> --
>> 2.39.2
>>