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Current cgroup prog ordering is appending at attachment time. This is not ideal. In some cases, users want specific ordering at a particular cgroup level. To address this, the existing mprog API seems an ideal solution with supporting BPF_F_BEFORE and BPF_F_AFTER flags. But there are a few obstacles to directly use kernel mprog interface. Currently cgroup bpf progs already support prog attach/detach/replace and link-based attach/detach/replace. For example, in struct bpf_prog_array_item, the cgroup_storage field needs to be together with bpf prog. But the mprog API struct bpf_mprog_fp only has bpf_prog as the member, which makes it difficult to use kernel mprog interface. In another case, the current cgroup prog detach tries to use the same flag as in attach. This is different from mprog kernel interface which uses flags passed from user space. So to avoid modifying existing behavior, I made the following changes to support mprog API for cgroup progs: - The support is for prog list at cgroup level. Cross-level prog list (a.k.a. effective prog list) is not supported. - Previously, BPF_F_PREORDER is supported only for prog attach, now BPF_F_PREORDER is also supported by link-based attach. - For attach, BPF_F_BEFORE/BPF_F_AFTER/BPF_F_ID/BPF_F_LINK is supported similar to kernel mprog but with different implementation. - For detach and replace, use the existing implementation. - For attach, detach and replace, the revision for a particular prog list, associated with a particular attach type, will be updated by increasing count by 1. Signed-off-by: Yonghong Song <yonghong.song@linux.dev> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Link: https://lore.kernel.org/bpf/20250606163141.2428937-1-yonghong.song@linux.dev
Why we want a copy of kernel headers in tools?
==============================================
There used to be no copies, with tools/ code using kernel headers
directly. From time to time tools/perf/ broke due to legitimate kernel
hacking. At some point Linus complained about such direct usage. Then we
adopted the current model.
The way these headers are used in perf are not restricted to just
including them to compile something.
There are sometimes used in scripts that convert defines into string
tables, etc, so some change may break one of these scripts, or new MSRs
may use some different #define pattern, etc.
E.g.:
$ ls -1 tools/perf/trace/beauty/*.sh | head -5
tools/perf/trace/beauty/arch_errno_names.sh
tools/perf/trace/beauty/drm_ioctl.sh
tools/perf/trace/beauty/fadvise.sh
tools/perf/trace/beauty/fsconfig.sh
tools/perf/trace/beauty/fsmount.sh
$
$ tools/perf/trace/beauty/fadvise.sh
static const char *fadvise_advices[] = {
[0] = "NORMAL",
[1] = "RANDOM",
[2] = "SEQUENTIAL",
[3] = "WILLNEED",
[4] = "DONTNEED",
[5] = "NOREUSE",
};
$
The tools/perf/check-headers.sh script, part of the tools/ build
process, points out changes in the original files.
So its important not to touch the copies in tools/ when doing changes in
the original kernel headers, that will be done later, when
check-headers.sh inform about the change to the perf tools hackers.
Another explanation from Ingo Molnar:
It's better than all the alternatives we tried so far:
- Symbolic links and direct #includes: this was the original approach but
was pushed back on from the kernel side, when tooling modified the
headers and broke them accidentally for kernel builds.
- Duplicate self-defined ABI headers like glibc: double the maintenance
burden, double the chance for mistakes, plus there's no tech-driven
notification mechanism to look at new kernel side changes.
What we are doing now is a third option:
- A software-enforced copy-on-write mechanism of kernel headers to
tooling, driven by non-fatal warnings on the tooling side build when
kernel headers get modified:
Warning: Kernel ABI header differences:
diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
...
The tooling policy is to always pick up the kernel side headers as-is,
and integate them into the tooling build. The warnings above serve as a
notification to tooling maintainers that there's changes on the kernel
side.
We've been using this for many years now, and it might seem hacky, but
works surprisingly well.