netdev - Re: [PATCH v5 bpf-next 1/2] bpf: introduce helper bpf_find

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Date:   Mon, 8 Nov 2021 14:43:02 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "kpsingh@...nel.org" <kpsingh@...nel.org>,
        "hengqi.chen@...il.com" <hengqi.chen@...il.com>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH v5 bpf-next 1/2] bpf: introduce helper bpf_find_vma



On 11/8/21 2:27 PM, Eric Dumazet wrote:
> 
> 
> On 11/8/21 1:59 PM, Song Liu wrote:
>>
>>
>>> On Nov 8, 2021, at 10:36 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>
>>>
>>>
>>> On 11/5/21 4:23 PM, Song Liu wrote:
>>>> In some profiler use cases, it is necessary to map an address to the
>>>> backing file, e.g., a shared library. bpf_find_vma helper provides a
>>>> flexible way to achieve this. bpf_find_vma maps an address of a task to
>>>> the vma (vm_area_struct) for this address, and feed the vma to an callback
>>>> BPF function. The callback function is necessary here, as we need to
>>>> ensure mmap_sem is unlocked.
>>>>
>>>> It is necessary to lock mmap_sem for find_vma. To lock and unlock mmap_sem
>>>> safely when irqs are disable, we use the same mechanism as stackmap with
>>>> build_id. Specifically, when irqs are disabled, the unlocked is postponed
>>>> in an irq_work. Refactor stackmap.c so that the irq_work is shared among
>>>> bpf_find_vma and stackmap helpers.
>>>>
>>>> Acked-by: Yonghong Song <yhs@...com>
>>>> Tested-by: Hengqi Chen <hengqi.chen@...il.com>
>>>> Signed-off-by: Song Liu <songliubraving@...com>
>>>> ---
>>>
>>> ...
>>>
>>>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>>>> index dbc3ad07e21b6..cdb0fba656006 100644
>>>> --- a/kernel/bpf/btf.c
>>>> +++ b/kernel/bpf/btf.c
>>>> @@ -6342,7 +6342,10 @@ const struct bpf_func_proto bpf_btf_find_by_name_kind_proto = {
>>>> 	.arg4_type	= ARG_ANYTHING,
>>>> };
>>>>
>>>> -BTF_ID_LIST_GLOBAL_SINGLE(btf_task_struct_ids, struct, task_struct)
>>>> +BTF_ID_LIST_GLOBAL(btf_task_struct_ids)
>>>> +BTF_ID(struct, task_struct)
>>>> +BTF_ID(struct, file)
>>>> +BTF_ID(struct, vm_area_struct)
>>>
>>> $ nm -v vmlinux |grep -A3 btf_task_struct_ids
>>> ffffffff82adfd9c R btf_task_struct_ids
>>> ffffffff82adfda0 r __BTF_ID__struct__file__715
>>> ffffffff82adfda4 r __BTF_ID__struct__vm_area_struct__716
>>> ffffffff82adfda8 r bpf_skb_output_btf_ids
>>>
>>> KASAN thinks btf_task_struct_ids has 4 bytes only.
>>
>> I have KASAN enabled, but couldn't repro this issue. I think
>> btf_task_struct_ids looks correct:
>>
>> nm -v vmlinux | grep -A3 -B1 btf_task_struct_ids
>> ffffffff83cf8260 r __BTF_ID__struct__task_struct__1026
>> ffffffff83cf8260 R btf_task_struct_ids
>> ffffffff83cf8264 r __BTF_ID__struct__file__1027
>> ffffffff83cf8268 r __BTF_ID__struct__vm_area_struct__1028
>> ffffffff83cf826c r bpf_skb_output_btf_ids
>>
>> Did I miss something?
>>
>> Thanks,
>> Song
>>
> 
> I will release the syzbot bug, so that you can use its .config
> 
> Basically, we have
> 
> u32 btf_task_struct_ids[1];

That is, if  

# CONFIG_DEBUG_INFO_BTF is not set

> 
> xxxx = btf_task_struct_ids[2];  /* trap */
> 
> 
>