From: Andrii Nakryiko [mailto:andrii.nakryiko@gmail.com]
Sent: Wednesday, March 30, 2022 1:52 AM
On Mon, Mar 28, 2022 at 10:52 AM Roberto Sassu
[off-list ref] wrote:

quoted

The first part of the preload code generation consists in generating the
static variables to be used by the code itself: the links and maps to be
pinned, and the skeleton. Generation of the preload variables and

methods

quoted

is enabled with the option -P added to 'bpftool gen skeleton'.

The existing variables maps_link and progs_links in bpf_preload_kern.c

have

quoted

been renamed respectively to dump_bpf_map_link and

dump_bpf_prog_link, to

quoted

match the name of the variables in the main structure of the light
skeleton.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
---
 kernel/bpf/preload/bpf_preload_kern.c         |  35 +-
 kernel/bpf/preload/iterators/Makefile         |   2 +-
 .../bpf/preload/iterators/iterators.lskel.h   | 378 +++++++++---------
 .../bpf/bpftool/Documentation/bpftool-gen.rst |   5 +
 tools/bpf/bpftool/bash-completion/bpftool     |   2 +-
 tools/bpf/bpftool/gen.c                       |  27 ++
 tools/bpf/bpftool/main.c                      |   7 +-
 tools/bpf/bpftool/main.h                      |   1 +
 8 files changed, 254 insertions(+), 203 deletions(-)

[...]

quoted

+__attribute__((unused)) static void
+iterators_bpf__assert(struct iterators_bpf *s)
+{
+#ifdef __cplusplus
+#define _Static_assert static_assert
+#endif
+#ifdef __cplusplus
+#undef _Static_assert
+#endif
+}
+
+static struct bpf_link *dump_bpf_map_link;
+static struct bpf_link *dump_bpf_prog_link;
+static struct iterators_bpf *skel;

I don't understand what is this and what for? You are making an
assumption that light skeleton can be instantiated just once, why? And
adding extra bpftool option to light skeleton codegen just to save a
bit of typing at the place where light skeleton is actually
instantiated and used doesn't seems like a right approach.

True, iterator_bpf is simple. Writing the preloading code
for it is simple. But, what if you wanted to preload an LSM
with 10 hooks or more?

Ok, regarding where the preloading code should be, I will
try to move the generated code to the kernel module instead
of the light skeleton.

Thanks

Roberto

HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Zhong Ronghua

Further, even if this is the way to go, please split out bpftool
changes from kernel changes. There is nothing requiring them to be
coupled together.

[...]