netdev - Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf

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Date:   Wed, 16 Dec 2020 16:34:46 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 1/4] bpf: introduce task_vma bpf_iter

On Tue, Dec 15, 2020 at 3:37 PM Song Liu <songliubraving@...com> wrote:
>
> Introduce task_vma bpf_iter to print memory information of a process. It
> can be used to print customized information similar to /proc/<pid>/maps.
>
> task_vma iterator releases mmap_lock before calling the BPF program.
> Therefore, we cannot pass vm_area_struct directly to the BPF program. A
> new __vm_area_struct is introduced to keep key information of a vma. On
> each iteration, task_vma gathers information in __vm_area_struct and
> passes it to the BPF program.
>
> If the vma maps to a file, task_vma also holds a reference to the file
> while calling the BPF program.
>
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  include/linux/bpf.h    |   2 +-
>  kernel/bpf/task_iter.c | 205 ++++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 205 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 07cb5d15e7439..49dd1e29c8118 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1325,7 +1325,7 @@ enum bpf_iter_feature {
>         BPF_ITER_RESCHED        = BIT(0),
>  };
>
> -#define BPF_ITER_CTX_ARG_MAX 2
> +#define BPF_ITER_CTX_ARG_MAX 3
>  struct bpf_iter_reg {
>         const char *target;
>         bpf_iter_attach_target_t attach_target;
> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
> index 0458a40edf10a..15a066b442f75 100644
> --- a/kernel/bpf/task_iter.c
> +++ b/kernel/bpf/task_iter.c
> @@ -304,9 +304,183 @@ static const struct seq_operations task_file_seq_ops = {
>         .show   = task_file_seq_show,
>  };
>
> +/*
> + * Key information from vm_area_struct. We need this because we cannot
> + * assume the vm_area_struct is still valid after each iteration.
> + */
> +struct __vm_area_struct {
> +       __u64 start;
> +       __u64 end;
> +       __u64 flags;
> +       __u64 pgoff;

I'd keep the original names of the fields (vm_start, vm_end, etc). But
there are some more fields which seem useful, like vm_page_prot,
vm_mm, etc.

It's quite unfortunate, actually, that bpf_iter program doesn't get
access to the real vm_area_struct, because it won't be able to do much
beyond using fields that we pre-defined here. E.g., there could be
interesting things to do with vm_mm, but unfortunately it won't be
possible.

Is there any way to still provide access to the original
vm_area_struct and let BPF programs use BTF magic to follow all those
pointers (like vm_mm) safely?

> +};
> +

[...]