libbpf: Skip re-installing headers file if source is older than target

The "install_headers" target in libbpf's Makefile would unconditionally
export all API headers to the target directory. When those headers are
installed to compile another application, this means that make always
finds newer dependencies for the source files relying on those headers,
and deduces that the targets should be rebuilt.

Avoid that by making "install_headers" depend on the source header
files, and (re-)install them only when necessary.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211007194438.34443-2-quentin@isovalent.com

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index 41e4f78dbad5b1d82ee0322105bfeb57b6087077..9c6804ca5b451e3537d78d70ec41240b981017d3 100644 (file)
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -241,15 +241,24 @@ install_lib: all_cmd
                $(call do_install_mkdir,$(libdir_SQ)); \
                cp -fpR $(LIB_FILE) $(DESTDIR)$(libdir_SQ)
 
-INSTALL_HEADERS = bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h xsk.h \
-                 bpf_helpers.h $(BPF_GENERATED) bpf_tracing.h               \
-                 bpf_endian.h bpf_core_read.h skel_internal.h               \
-                 libbpf_version.h
-
-install_headers: $(BPF_GENERATED)
-       $(call QUIET_INSTALL, headers)                                       \
-               $(foreach hdr,$(INSTALL_HEADERS),                            \
-                       $(call do_install,$(hdr),$(prefix)/include/bpf,644);)
+SRC_HDRS := bpf.h libbpf.h btf.h libbpf_common.h libbpf_legacy.h xsk.h      \
+           bpf_helpers.h bpf_tracing.h bpf_endian.h bpf_core_read.h         \
+           skel_internal.h libbpf_version.h
+GEN_HDRS := $(BPF_GENERATED)
+
+INSTALL_PFX := $(DESTDIR)$(prefix)/include/bpf
+INSTALL_SRC_HDRS := $(addprefix $(INSTALL_PFX)/, $(SRC_HDRS))
+INSTALL_GEN_HDRS := $(addprefix $(INSTALL_PFX)/, $(notdir $(GEN_HDRS)))
+
+$(INSTALL_SRC_HDRS): $(INSTALL_PFX)/%.h: %.h
+       $(call QUIET_INSTALL, $@) \
+               $(call do_install,$<,$(prefix)/include/bpf,644)
+
+$(INSTALL_GEN_HDRS): $(INSTALL_PFX)/%.h: $(OUTPUT)%.h
+       $(call QUIET_INSTALL, $@) \
+               $(call do_install,$<,$(prefix)/include/bpf,644)
+
+install_headers: $(BPF_GENERATED) $(INSTALL_SRC_HDRS) $(INSTALL_GEN_HDRS)
 
 install_pkgconfig: $(PC_FILE)
        $(call QUIET_INSTALL, $(PC_FILE)) \