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.
If that is not something to be concerned, feel free to add:
Tested-by: Hengqi Chen <hengqi.chen@xxxxxxxxx>
> + if (vma && vma->vm_start <= start && vma->vm_end > start) {
> + callback_fn((u64)(long)task, (u64)(long)vma,
> + (u64)(long)callback_ctx, 0, 0);
> + ret = 0;
> + }
> + bpf_mmap_unlock_mm(work, mm);
> + return ret;
> +}
> +
> +const struct bpf_func_proto bpf_find_vma_proto = {
> + .func = bpf_find_vma,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_PTR_TO_BTF_ID,
> + .arg1_btf_id = &btf_task_struct_ids[0],
> + .arg2_type = ARG_ANYTHING,
> + .arg3_type = ARG_PTR_TO_FUNC,
> + .arg4_type = ARG_PTR_TO_STACK_OR_NULL,
> + .arg5_type = ARG_ANYTHING,
> +};
> +
[...]
Cheers,
--
Hengqi
