Re: [RFC PATCH bpf-next 05/16] bpf: create file or anonymous dumpers

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Wed, Apr 8, 2020 at 4:26 PM Yonghong Song <yhs@fb.com> wrote:
>
> Given a loaded dumper bpf program, which already
> knows which target it should bind to, there
> two ways to create a dumper:
>   - a file based dumper under hierarchy of
>     /sys/kernel/bpfdump/ which uses can
>     "cat" to print out the output.
>   - an anonymous dumper which user application
>     can "read" the dumping output.
>
> For file based dumper, BPF_OBJ_PIN syscall interface
> is used. For anonymous dumper, BPF_PROG_ATTACH
> syscall interface is used.
>
> To facilitate target seq_ops->show() to get the
> bpf program easily, dumper creation increased
> the target-provided seq_file private data size
> so bpf program pointer is also stored in seq_file
> private data.
>
> Further, a seq_num which represents how many
> bpf_dump_get_prog() has been called is also
> available to the target seq_ops->show().
> Such information can be used to e.g., print
> banner before printing out actual data.
>
> Note the seq_num does not represent the num
> of unique kernel objects the bpf program has
> seen. But it should be a good approximate.
>
> A target feature BPF_DUMP_SEQ_NET_PRIVATE
> is implemented specifically useful for
> net based dumpers. It sets net namespace
> as the current process net namespace.
> This avoids changing existing net seq_ops
> in order to retrieve net namespace from
> the seq_file pointer.
>
> For open dumper files, anonymous or not, the
> fdinfo will show the target and prog_id associated
> with that file descriptor. For dumper file itself,
> a kernel interface will be provided to retrieve the
> prog_id in one of the later patches.
>
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---
>  include/linux/bpf.h            |   5 +
>  include/uapi/linux/bpf.h       |   6 +-
>  kernel/bpf/dump.c              | 338 ++++++++++++++++++++++++++++++++-
>  kernel/bpf/syscall.c           |  11 +-
>  tools/include/uapi/linux/bpf.h |   6 +-
>  5 files changed, 362 insertions(+), 4 deletions(-)
>

[...]

>
> +struct dumper_inode_info {
> +       struct bpfdump_target_info *tinfo;
> +       struct bpf_prog *prog;
> +};
> +
> +struct dumper_info {
> +       struct list_head list;
> +       /* file to identify an anon dumper,
> +        * dentry to identify a file dumper.
> +        */
> +       union {
> +               struct file *file;
> +               struct dentry *dentry;
> +       };
> +       struct bpfdump_target_info *tinfo;
> +       struct bpf_prog *prog;
> +};

This is essentially a bpf_link. Why not do it as a bpf_link from the
get go? Instead of having all this duplication for anonymous and
pinned dumpers, it would always be a bpf_link-based dumper, but for
those pinned bpf_link itself is going to be pinned. You also get a
benefit of being able to list all dumpers through existing bpf_link
API (also see my RFC patches with bpf_link_prime/bpf_link_settle,
which makes using bpf_link safe and simple).

[...]

> +
> +static void anon_dumper_show_fdinfo(struct seq_file *m, struct file *filp)
> +{
> +       struct dumper_info *dinfo;
> +
> +       mutex_lock(&anon_dumpers.dumper_mutex);
> +       list_for_each_entry(dinfo, &anon_dumpers.dumpers, list) {

this (and few other places where you search in a loop) would also be
simplified, because struct file* would point to bpf_dumper_link, which
then would have a pointer to bpf_prog, dentry (if pinned), etc. No
searching at all.

> +               if (dinfo->file == filp) {
> +                       seq_printf(m, "target:\t%s\n"
> +                                     "prog_id:\t%u\n",
> +                                  dinfo->tinfo->target,
> +                                  dinfo->prog->aux->id);
> +                       break;
> +               }
> +       }
> +       mutex_unlock(&anon_dumpers.dumper_mutex);
> +}
> +
> +#endif
> +

[...]