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BTF maximum vlen is encoded using 16 bits with a maximum vlen of 65535. This has sufficed for structs, function parameters and enumerated type values. However, with upcoming BTF location information - in particular information about inline sites - this limit is surpassed. Use bits 16-23 - currently unused in BTF info - to extend to 24 bits, giving a max vlen of (2^24 - 1), or 16 million. Also extend BTF kind encoding from 5 to 7 bits, giving a maximum available number of kinds of 128. Since with the BTF location work we use another 3 kinds, we are fast approaching the current limit of 32. Convert BTF_MAX_* values to enums to allow them to be encoded in kernel BTF; this will allow us to detect if the running kernel supports a 24-bit vlen or not. Add one for max _possible_ (not used) kind. Fix up a few places in the kernel where a 16-bit vlen is assumed; remove BTF_INFO_MASK as now all bits are used. The vlen expansion was suggested by Andrii in [1]; the kind expansion is tackled here too as it may be needed also to support new kinds in BTF. [1] https://lore.kernel.org/bpf/CAEf4BzZx=X6vGqcA8SPU6D+v6k+TR=ZewebXMuXtpmML058piw@mail.gmail.com/ Suggested-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Alan Maguire <alan.maguire@oracle.com> Acked-by: Mykyta Yatsenko <yatsenko@meta.com> Link: https://lore.kernel.org/r/20260417143023.1551481-2-alan.maguire@oracle.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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.