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

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Date:   Mon, 8 Nov 2021 14:27:45 -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 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];

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