build: pick the BTF kernel by capability, not by directory name

Makefile

@@ -159,12 +159,58 @@ $(BPFTOOL): | $(BPFTOOL_OUTPUT)
 
 
 # Generate Ceph BTF header
+#
+# Which kernel do we take ceph.ko's BTF from?
+#
+# "The newest kernel under /lib/modules" is not a safe answer, because it
+# assumes every directory directly under /lib/modules is named after a kernel
+# release. Distributions are not obliged to comply. Kylin Linux Advanced Server
+# V11 ships /lib/modules/ksaf: a kernel-release-independent directory holding
+# Kylin's own LSM modules (ksaf_main.ko from ksaf-main-module, kysec_exectl.ko
+# from kysec2-exectl-module). It is deliberately kept out of
+# /lib/modules/<release>/ so that the dracut module 98ksaf_main can copy one
+# fixed path into the initramfs for every kernel, and the security modules can
+# be loaded as early as possible. There is no ceph.ko in it.
+#
+# GNU `sort -V` ranks a run of letters ABOVE a run of digits, so a name that
+# starts with a letter sorts above every real version number:
+#
+#     5.4.241-24.0017.26
+#     6.6.0-32.12.v2505.ky11.x86_64
+#     6.6.0-32.21.v2505.ky11.x86_64
+#     6.6.0-32.22.v2505.ky11.x86_64
+#     6.6.0-32.24.v2505.ky11.x86_64   <- the running kernel
+#     ksaf                            <- ... so `tail -1` returns this one
+#
+# This recipe then searched /lib/modules/ksaf for ceph.ko and aborted with
+# "No ceph.ko* found under /lib/modules/ksaf". Because ceph_btf_local.h is a
+# prerequisite of the .bpf.o targets, that killed the build before a single
+# object was compiled -- which makes the failure look unrelated to kernel
+# discovery. It is only observable when the header must be regenerated, so it
+# stays hidden until a `make clean`, `git clean`, or fresh checkout removes it.
+#
+# Select by CAPABILITY instead: a candidate must actually contain a ceph.ko*,
+# which excludes any non-kernel directory whatever it happens to be called.
+# Prefer the running kernel when it qualifies, because tracing attaches to the
+# live kernel, and the base BTF fallback /sys/kernel/btf/vmlinux below is only
+# valid for the running kernel.
 $(OSDTRACE_SRC)/ceph_btf_local.h: | $(OUTPUT) $(BPFTOOL)
 	@$(call msg,GEN-BTF,$@)
 	@set -eu; \
 	src=""; tmp=""; kver=""; kdir=""; base_btf=""; \
-	kver=$$(find /lib/modules -mindepth 1 -maxdepth 1 -type d -printf '%f\n' 2>/dev/null | sort -V | tail -1); \
-	[ -n "$$kver" ] || { echo "No installed kernels found under /lib/modules" >&2; exit 1; }; \
+	running_kver=$$(uname -r); \
+	if [ -d "/lib/modules/$$running_kver" ] && \
+	   [ -n "$$(find "/lib/modules/$$running_kver" -type f -name 'ceph.ko*' -print -quit 2>/dev/null)" ]; then \
+		kver="$$running_kver"; \
+	fi; \
+	if [ -z "$$kver" ]; then \
+		kver=$$(for d in /lib/modules/*/; do \
+			[ -d "$$d" ] || continue; \
+			[ -n "$$(find "$$d" -type f -name 'ceph.ko*' -print -quit 2>/dev/null)" ] || continue; \
+			basename "$$d"; \
+		done | sort -V | tail -1); \
+	fi; \
+	[ -n "$$kver" ] || { echo "No installed kernel under /lib/modules provides a ceph.ko* module" >&2; exit 1; }; \
 	kdir="/lib/modules/$$kver"; \
 	CEPH_KO=$$(find "$$kdir" -type f -name 'ceph.ko*' 2>/dev/null | head -1); \
 	[ -n "$$CEPH_KO" ] || { echo "No ceph.ko* found under $$kdir" >&2; exit 1; }; \