Re: WARNING in __mark_chain_precision

[Yonghong Song <yhs@meta.com>]

On 12/30/22 1:44 AM, Hao Sun wrote:
> 
> 
> Andrii Nakryiko <andrii.nakryiko@gmail.com> 于2022年12月30日周五 06:16写道：
>>
>> On Tue, Dec 27, 2022 at 9:24 PM Yonghong Song <yhs@meta.com> wrote:
>>>
>>>
>>>
>>> On 12/20/22 4:30 PM, Andrii Nakryiko wrote:
>>>> On Mon, Dec 19, 2022 at 11:13 AM <sdf@google.com> wrote:
>>>>>
>>>>> On 12/19, Hao Sun wrote:
>>>>>> Hi,
>>>>>
>>>>>> The following backtracking bug can be triggered on the latest bpf-next and
>>>>>> Linux 6.1 with the C prog provided. I don't have enough knowledge about
>>>>>> this part in the verifier, don't know how to fix this.
>>>>>
>>>>> Maybe something related to commit be2ef8161572 ("bpf: allow precision
>>>>> tracking
>>>>> for programs with subprogs") and/or the related ones?
>>>>>
>>>>>
>>>>>> This can be reproduced on:
>>>>>
>>>>>> HEAD commit: 0e43662e61f2 tools/resolve_btfids: Use pkg-config to locate
>>>>>> libelf
>>>>>> git tree: bpf-next
>>>>>> console log: https://pastebin.com/raw/45hZ7iqm
>>>>>> kernel config: https://pastebin.com/raw/0pu1CHRm
>>>>>> C reproducer: https://pastebin.com/raw/tqsiezvT
>>>>>
>>>>>> func#0 @0
>>>>>> 0: R1=ctx(off=0,imm=0) R10=fp0
>>>>>> 0: (18) r2 = 0x8000000000000          ; R2_w=2251799813685248
>>>>>> 2: (18) r6 = 0xffff888027358000       ;
>>>>>> R6_w=map_ptr(off=0,ks=3032,vs=3664,imm=0)
>>>>>> 4: (18) r7 = 0xffff88802735a000       ;
>>>>>> R7_w=map_ptr(off=0,ks=156,vs=2624,imm=0)
>>>>>> 6: (18) r8 = 0xffff88802735e000       ;
>>>>>> R8_w=map_ptr(off=0,ks=2396,vs=76,imm=0)
>>>>>> 8: (18) r9 = 0x8e9700000000           ; R9_w=156779191205888
>>>>>> 10: (36) if w9 >= 0xffffffe3 goto pc+1
>>>>>> last_idx 10 first_idx 0
>>>>>> regs=200 stack=0 before 8: (18) r9 = 0x8e9700000000
>>>>>> 11: R9_w=156779191205888
>>>>>> 11: (85) call #0
>>>>>> 12: (cc) w2 s>>= w7
>>>>
>>>> w2 should have been set to NOT_INIT (because r1-r5 are clobbered by
>>>> calls) and rejected here as !read_ok (see check_reg_arg()) before
>>>> attempting to mark precision for r2. Can you please try to debug and
>>>> understand why that didn't happen here?
>>>
>>> The verifier is doing the right thing here and the 'call #0' does
>>> implicitly cleared r1-r5.
>>>
>>> So for 'w2 s>>= w7', since w2 is used, the verifier tries to find
>>> its definition by backtracing. It encountered 'call #0', which clears
>>
>> and that's what I'm saying is incorrect. Normally we'd get !read_ok
>> error because s>>= is both READ and WRITE on w2, which is
>> uninitialized after call instruction according to BPF ABI. And that's
>> what actually seems to happen correctly in my (simpler) tests locally.
>> But something is special about this specific repro that somehow either
>> bypasses this logic, or attempts to mark precision before we get to
>> that test. That's what we should investigate. I haven't tried to run
>> this specific repro locally yet, so can't tell for sure.
>>
> 
> So, the reason why w2 is not marked as uninit is that the kfunc call in
> the BPF program is invalid, "call #0", imm is zero, right?

Yes, "call #0" is invalid. As the code below

 > /* skip for now, but return error when we find this in 
fixup_kfunc_call */
 >   if (!insn->imm)
 >   return 0;

The error report will be delayed later in fixup_kfunc_call().

static int fixup_kfunc_call(struct bpf_verifier_env *env, struct 
bpf_insn *insn,
                             struct bpf_insn *insn_buf, int insn_idx, 
int *cnt)
{
         const struct bpf_kfunc_desc *desc;

         if (!insn->imm) {
                 verbose(env, "invalid kernel function call not 
eliminated in verifier pass\n");
                 return -EINVAL;
         }


> In check_kfunc_call(), it skips this error temporarily:
> 
> /* skip for now, but return error when we find this in fixup_kfunc_call */
>   if (!insn->imm)
>   return 0;
> 
> So the kfunc call is the previous instruction before "w2 s>>= w7", this
> leads to the warning in backtrack_insn():
> 
> /* regular helper call sets R0 */
> *reg_mask &= ~1;
> if (*reg_mask & 0x3f) {
> 	/* if backtracing was looking for registers R1-R5
> 	* they should have been found already.
> 	*/
> 	verbose(env, "BUG regs %x\n", *reg_mask);
> 	WARN_ONCE(1, "verifier backtracking bug”);
> 	return -EFAULT;
> }

The main triggering the backtrack_insn() is due to

                         } else {
                                 /* scalar += pointer
                                  * This is legal, but we have to 
reverse our
                                  * src/dest handling in computing the range
                                  */
                                 err = mark_chain_precision(env, 
insn->dst_reg);
                                 if (err)
                                         return err;
                                 return adjust_ptr_min_max_vals(env, insn,
                                                                src_reg, 
dst_reg);
                         }


unc#0 @0
0: R1=ctx(off=0,imm=0) R10=fp0
0: (18) r2 = 0x8000000000000          ; R2_w=2251799813685248
2: (18) r6 = 0xffff888100d29000       ; 
R6_w=map_ptr(off=0,ks=3032,vs=3664,imm=0)
4: (18) r7 = 0xffff888100d2a000       ; 
R7_w=map_ptr(off=0,ks=156,vs=2624,imm=0)
6: (18) r8 = 0xffff888100d2ac00       ; 
R8_w=map_ptr(off=0,ks=2396,vs=76,imm=0)
8: (18) r9 = 0x8e9700000000           ; R9_w=156779191205888
10: (36) if w9 >= 0xffffffe3 goto pc+1
last_idx 10 first_idx 0
regs=200 stack=0 before 8: (18) r9 = 0x8e9700000000
11: R9_w=156779191205888
11: (85) call #0
12: (cc) w2 s>>= w7
last_idx 12 first_idx 12
parent didn't have regs=4 stack=0 marks: R1=ctx(off=0,imm=0) 
R2_rw=P2251799813685248 R6_w=map_ptr(off=0,ks=3032,vs=3664,imm=0) 
R7_rw=map_ptr(off=0,ks=156,vs=2624,imm=0) R8_w=map_ptr(off=0,ks=2396,v0
last_idx 11 first_idx 0
regs=4 stack=0 before 11: (85) call #0
BUG regs 4

For insn 12, 'w2 s>>= w7', w2 is a scalar and w7 is a map_ptr. Hence, 
based on the above verifier code, mark_chain_precision() is triggered.

Not sure what is the purpose of this test. But to make it succeed,
first "call #0" need to change to a valid kfunc call, and second, you
might want to change 'w2 s>>= w7' to e.g., 'w9 s>>= w7' to avoid
precision tracking.

> 
> Any idea or hint on how to fix this?
>