mirror: Fix flags usage for static build

The flag we've been using and documenting for static builds does the
work, but is not correct:

- It should be "-static" (compiler option, as expected by gcc for
  example) and not "--static" (linker option, gcc translates the former
  into the latter when invoking the linker).

- It should not be passed with the CFLAGS (or in our case,
  EXTRA_CFLAGS), but instead with the LDFLAGS (or EXTRA_LDFLAGS),
  because it is a flag that will be passed to the linker.

Fix it in the README.md and in CI/release workflows. We also need to
adjust feature detection to take the EXTRA_LDFLAGS (so far ignored) into
account.

Signed-off-by: Quentin Monnet <[email protected]>

Author: qmonnet

.github/workflows/build.yaml

@@ -75,7 +75,7 @@ jobs:
       - name: Build bpftool, with libbfd, static build
         run: |
           make -C src clean
-          EXTRA_CFLAGS=--static make -j -C src V=1
+          EXTRA_LDFLAGS=-static make -j -C src V=1
           ./src/bpftool 2>&1 | grep -q Usage
           ./src/bpftool -p version | \
               tee /dev/stderr | \

.github/workflows/release.yaml

@@ -87,7 +87,7 @@ jobs:
         env:
           LLVM_PATH: ${{ env[format('LLVM_{0}', matrix.arch)] }}
         run: |
-          EXTRA_CFLAGS=--static \
+          EXTRA_LDFLAGS=-static \
               LLVM_CONFIG="${GITHUB_WORKSPACE}/${{ env.LLVM_PATH }}/bin/llvm-config" \
               LLVM_STRIP="${GITHUB_WORKSPACE}/${{ env.LLVM_PATH }}/bin/llvm-strip" \
               HOSTAR="${GITHUB_WORKSPACE}/${{ env.LLVM_PATH }}/bin/llvm-ar" \
@@ -112,7 +112,7 @@ jobs:
                    apt-get install -y make pkg-config gcc \
                        libelf-dev libcap-dev libstdc++-11-dev zlib1g-dev && \
                    cd /build/bpftool && \
-                   EXTRA_CFLAGS=--static \
+                   EXTRA_LDFLAGS=-static \
                        LLVM_CONFIG='/build/${{ env.LLVM_PATH }}/bin/llvm-config' \
                        LLVM_STRIP='/build/${{ env.LLVM_PATH }}/bin/llvm-strip' \
                        CLANG='/build/${{ env.LLVM_PATH }}/bin/clang' \

.github/workflows/static-build.yaml

@@ -43,7 +43,7 @@ jobs:
       - name: Build bpftool (static build, default LLVM disassembler)
         working-directory: 'bpftool'
         run: |
-          EXTRA_CFLAGS=--static \
+          EXTRA_LDFLAGS=-static \
               LLVM_CONFIG="${GITHUB_WORKSPACE}/${LLVM_PATH}/bin/llvm-config" \
               LLVM_STRIP="${GITHUB_WORKSPACE}/${LLVM_PATH}/bin/llvm-strip" \
               make -j -C src V=1

README.md

@@ -123,7 +123,7 @@ example, we can create a static build with the following commands:
 
 ```console
 $ cd src
-$ EXTRA_CFLAGS=--static make
+$ EXTRA_LDFLAGS=-static make
 ```
 
 Note that to use the LLVM disassembler with static builds, we need a static
@@ -150,12 +150,12 @@ version of the LLVM library installed on the system:
       $ make -j -C llvm_build llvm-config llvm-libraries
       ```
 
-2.  Build bpftool with `EXTRA_CFLAGS` set to `--static`, and by passing the
+2.  Build bpftool with `EXTRA_LDFLAGS` set to `-static`, and by passing the
     path to the relevant `llvm-config`.
 
     ```console
     $ cd bpftool
-    $ LLVM_CONFIG=../../llvm_build/bin/llvm-config EXTRA_CFLAGS=--static make -j -C src
+    $ LLVM_CONFIG=../../llvm_build/bin/llvm-config EXTRA_LDFLAGS=-static make -j -C src
     ```
 
 ### Build bpftool's man pages

src/Makefile.feature

@@ -6,6 +6,9 @@ CFLAGS_BACKUP := $(CFLAGS)
 CFLAGS := $(EXTRA_CFLAGS)
 CFLAGS += -Wno-unused-command-line-argument
 
+LDFLAGS_BACKUP := $(LDFLAGS)
+LDFLAGS := $(EXTRA_LDFLAGS)
+
 ifeq ($(V),1)
   LOG=$(warning $(1))
   LOG_RES = (echo $(1) && >&2 echo result: $(1))
@@ -43,7 +46,7 @@ LIBBFD_PROBE += '	bfd_demangle(0, 0, 0);'
 LIBBFD_PROBE += '	return 0;'
 LIBBFD_PROBE += '}'
 LIBBFD_PROBE_CMD = printf '%b\n' $(LIBBFD_PROBE) | \
-  $(CC) $(CFLAGS) -Wall -Werror -x c -DPACKAGE='"bpftool"' - $(1) -o /dev/null >/dev/null
+  $(CC) $(CFLAGS) -Wall -Werror -x c -DPACKAGE='"bpftool"' - $(LDFLAGS) $(1) -o /dev/null >/dev/null
 
 define libbfd_build
   $(call detect,$(LIBBFD_PROBE_CMD))
@@ -76,7 +79,7 @@ DISASSEMBLER_PROBE += '	return 0;'
 DISASSEMBLER_PROBE += '}'
 
 DISASSEMBLER_PROBE_CMD = printf '%b\n' $(1) | \
-  $(CC) $(CFLAGS) -Wall -Werror -x c -DPACKAGE='"bpftool"' - -lbfd -lopcodes -S -o - >/dev/null
+  $(CC) $(CFLAGS) -Wall -Werror -x c -DPACKAGE='"bpftool"' - $(LDFLAGS) -lbfd -lopcodes -S -o - >/dev/null
 define disassembler_build
   $(call detect,$(DISASSEMBLER_PROBE_CMD))
 endef
@@ -109,7 +112,7 @@ LIBCAP_PROBE += '	cap_free(0);'
 LIBCAP_PROBE += '	return 0;'
 LIBCAP_PROBE += '}'
 LIBCAP_PROBE_CMD = printf '%b\n' $(LIBCAP_PROBE) | \
-  $(CC) $(CFLAGS) -Wall -Werror -x c - -lcap -S -o - >/dev/null
+  $(CC) $(CFLAGS) -Wall -Werror -x c - $(LDFLAGS) -lcap -S -o - >/dev/null
 
 define libcap_build
   $(call detect,$(LIBCAP_PROBE_CMD))
@@ -130,19 +133,15 @@ LLVM_PROBE += '	LLVMDisposeMessage(triple);'
 LLVM_PROBE += '	return 0;'
 LLVM_PROBE += '}'
 
-# We need some adjustments for the flags.
-# - $(CFLAGS) was set to parent $(EXTRA_CFLAGS) at the beginning of this file.
-# - $(EXTRA_LDFLAGS) from parent Makefile should be kept as well.
-# - Libraries to use depend on whether we have a static or shared version of
-#   LLVM, pass the llvm-config flag and adjust the list of libraries
-#   accordingly.
+# Libraries to use depend on whether we have a static or shared version of
+# LLVM, pass the llvm-config flag and adjust the list of libraries accordingly.
 FEATURE_LLVM_CFLAGS := $(CFLAGS) $(shell $(LLVM_CONFIG) --cflags 2>/dev/null)
 FEATURE_LLVM_LIBS := $(shell $(LLVM_CONFIG) --libs target 2>/dev/null)
 ifeq ($(shell $(LLVM_CONFIG) --shared-mode 2>/dev/null),static)
   FEATURE_LLVM_LIBS += $(shell $(LLVM_CONFIG) --system-libs target 2>/dev/null)
   FEATURE_LLVM_LIBS += -lstdc++
 endif
-FEATURE_LDFLAGS := $(EXTRA_LDFLAGS) $(shell $(LLVM_CONFIG) --ldflags 2>/dev/null)
+FEATURE_LDFLAGS := $(LDFLAGS) $(shell $(LLVM_CONFIG) --ldflags 2>/dev/null)
 
 LLVM_PROBE_CMD = printf '%b\n' $(LLVM_PROBE) | \
   $(CC) $(FEATURE_LLVM_CFLAGS) $(FEATURE_LDFLAGS) \
@@ -174,4 +173,5 @@ $(foreach feature,$(filter-out libbfd%,$(FEATURE_DISPLAY)), \
   $(call feature_print_status,$(feature-$(feature)),$(feature)))
 
 CFLAGS := $(CFLAGS_BACKUP)
+LDFLAGS := $(LDFLAGS_BACKUP)
 undefine LOG LOG_RES