> On Nov 5, 2021, at 2:16 PM, Alexei Starovoitov <ast@xxxxxx> wrote:
>
> On 11/5/21 2:11 PM, Song Liu wrote:
>>> On Nov 5, 2021, at 8:23 AM, Hengqi Chen <hengqi.chen@xxxxxxxxx> wrote:
>>>
>>> Hi, Song
>>>
>>> On 2021/11/5 5:31 AM, 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.
>>>>
>>>> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>>>> Signed-off-by: Song Liu <songliubraving@xxxxxx>
>>>> ---
>>>
>>> [...]
>>>
>>>>
>>>> -BTF_ID_LIST(btf_task_file_ids)
>>>> -BTF_ID(struct, file)
>>>> -BTF_ID(struct, vm_area_struct)
>>>> -
>>>> static const struct bpf_iter_seq_info task_seq_info = {
>>>> .seq_ops = &task_seq_ops,
>>>> .init_seq_private = init_seq_pidns,
>>>> @@ -586,9 +583,74 @@ static struct bpf_iter_reg task_vma_reg_info = {
>>>> .seq_info = &task_vma_seq_info,
>>>> };
>>>>
>>>> +BPF_CALL_5(bpf_find_vma, struct task_struct *, task, u64, start,
>>>> + bpf_callback_t, callback_fn, void *, callback_ctx, u64, flags)
>>>> +{
>>>> + struct mmap_unlock_irq_work *work = NULL;
>>>> + struct vm_area_struct *vma;
>>>> + bool irq_work_busy = false;
>>>> + struct mm_struct *mm;
>>>> + int ret = -ENOENT;
>>>> +
>>>> + if (flags)
>>>> + return -EINVAL;
>>>> +
>>>> + if (!task)
>>>> + return -ENOENT;
>>>> +
>>>> + mm = task->mm;
>>>> + if (!mm)
>>>> + return -ENOENT;
>>>> +
>>>> + irq_work_busy = bpf_mmap_unlock_get_irq_work(&work);
>>>> +
>>>> + if (irq_work_busy || !mmap_read_trylock(mm))
>>>> + return -EBUSY;
>>>> +
>>>> + vma = find_vma(mm, start);
>>>> +
>>>
>>> I found that when a BPF program attach to security_file_open which is in
>>> the bpf_d_path helper's allowlist, the bpf_d_path helper is also allowed
>>> to be called inside the callback function. So we can have this in callback
>>> function:
>>>
>>> bpf_d_path(&vma->vm_file->f_path, path, sizeof(path));
>>>
>>>
>>> I wonder whether there is a guarantee that vma->vm_file will never be null,
>>> as you said in the commit message, a backing file.
>> I don't think we can guarantee vma->vm_file never be NULL here, so this is
>> a real problem. Let me see how to fix it.
>
> It's unrelated. There was a separate thread about this.
I see. I will not fix it here then.
Thanks,
Song
