Do not allow defining the license from CFLAGS or command line

Signed-off-by: Oriol Arcas <oriol@starflownetworks.com>

docs/reference_guide.md

@@ -1550,7 +1550,7 @@ Exception: Failed to load BPF program kretprobe__inet_csk_accept
 
 This error happens when a GPL-only helper is called from a non-GPL BPF program. To fix this error, do not use GPL-only helpers from a proprietary BPF program, or relicense the BPF program under a GPL-compatible license. Check which [BPF helpers](https://github.com/iovisor/bcc/blob/master/docs/kernel-versions.md#helpers) are GPL-only, and what licenses are considered GPL-compatible.
 
-Example calling `bpf_get_stackid()`, a GPL-only BPF helper, from a proprietary program (`cflags=['-DBPF_LICENSE=Proprietary']`):
+Example calling `bpf_get_stackid()`, a GPL-only BPF helper, from a proprietary program (`#define BPF_LICENSE Proprietary`):
 
 ```
 bpf: Failed to load program: Invalid argument

src/cc/frontends/clang/b_frontend_action.cc

@@ -1279,6 +1279,9 @@ void BFrontendAction::DoMiscWorkAround() {
   // CONFIG_CC_STACKPROTECTOR properly based on other configs, so it relieved any bpf
   // program (using task_struct, etc.) of patching the below code.
   rewriter_->getEditBuffer(rewriter_->getSourceMgr().getMainFileID()).InsertText(0,
+    "#if defined(BPF_LICENSE)\n"
+    "#error BPF_LICENSE cannot be specified through cflags\n"
+    "#endif\n"
     "#if !defined(CONFIG_CC_STACKPROTECTOR)\n"
     "#if defined(CONFIG_CC_STACKPROTECTOR_AUTO) \\\n"
     "    || defined(CONFIG_CC_STACKPROTECTOR_REGULAR) \\\n"

tests/python/test_license.py

@@ -65,51 +65,35 @@ int license_program(struct pt_regs *ctx) {
         b = BPF(text=self.gpl_only_text)
         self.load_bpf_code(b)
 
-    def test_gpl_helper(self):
-        b = BPF(text=self.gpl_only_text, cflags=["-DBPF_LICENSE=GPL"])
-        self.load_bpf_code(b)
-
     def test_gpl_helper_macro(self):
         b = BPF(text=self.gpl_only_text + self.license('GPL'))
         self.load_bpf_code(b)
 
-    def test_proprietary(self):
-        b = BPF(text=self.proprietary_text, cflags=["-DBPF_LICENSE=Proprietary"])
-        self.load_bpf_code(b)
-
     def test_proprietary_macro(self):
         b = BPF(text=self.proprietary_text + self.license('Proprietary'))
         self.load_bpf_code(b)
 
-    def test_gpl_compatible(self):
-        b = BPF(text=self.gpl_only_text, cflags=["-DBPF_LICENSE=Dual BSD/GPL"])
-        self.load_bpf_code(b)
-
     def test_gpl_compatible_macro(self):
         b = BPF(text=self.gpl_only_text + self.license('Dual BSD/GPL'))
         self.load_bpf_code(b)
 
-    def test_proprietary_words(self):
-        b = BPF(text=self.proprietary_text, cflags=["-DBPF_LICENSE=Proprietary license"])
-        self.load_bpf_code(b)
-
     def test_proprietary_words_macro(self):
         b = BPF(text=self.proprietary_text + self.license('Proprietary license'))
         self.load_bpf_code(b)
 
     @unittest.expectedFailure
-    def test_empty_fail(self):
-        b = BPF(text=self.gpl_only_text, cflags=["-DBPF_LICENSE="])
+    def test_cflags_fail(self):
+        b = BPF(text=self.gpl_only_text, cflags=["-DBPF_LICENSE=GPL"])
         self.load_bpf_code(b)
 
     @unittest.expectedFailure
-    def test_empty_fail_macro(self):
-        b = BPF(text=self.gpl_only_text + self.license(''))
+    def test_cflags_macro_fail(self):
+        b = BPF(text=self.gpl_only_text + self.license('GPL'), cflags=["-DBPF_LICENSE=GPL"])
         self.load_bpf_code(b)
 
     @unittest.expectedFailure
-    def test_proprietary_fail(self):
-        b = BPF(text=self.gpl_only_text, cflags=["-DBPF_LICENSE=Proprietary license"])
+    def test_empty_fail_macro(self):
+        b = BPF(text=self.gpl_only_text + self.license(''))
         self.load_bpf_code(b)
 
     @unittest.expectedFailure
@@ -117,5 +101,10 @@ int license_program(struct pt_regs *ctx) {
         b = BPF(text=self.gpl_only_text + self.license('Proprietary license'))
         self.load_bpf_code(b)
 
+    @unittest.expectedFailure
+    def test_proprietary_cflags_fail(self):
+        b = BPF(text=self.proprietary_text, cflags=["-DBPF_LICENSE=Proprietary"])
+        self.load_bpf_code(b)
+
 if __name__ == "__main__":
     unittest.main()