Added new footer.h header where BPF_LICENSE is set if not defined

[oriolarcas]

The helpers.h header specifies the BPF program license as 'GPL'.
However, other GPL-compatible licenses are possible (e.g., Dual BSD/GPL)
or even proprietary licenses (e.g., cachetop can run with a proprietary
license).

With this commit, the user can specify a BPF_LICENSE macro in the source
code:

#define BPF_LICENSE Custom license

Note it supports multiple words and the absence of quotes. If the BPF
doesn't have a GPL-compatible license and it uses any GPL-only helpers,
the kernel will reject it with:

cannot call GPL only function from proprietary program

If no license is specified, it will fall back to GPL (the current
behavior before this commit) so that the BCC tools and examples remain
usable.

Updated the documentation with BPF_LICENSE description, licensing error
description, and licenses for each helper.

Signed-off-by: Oriol Arcas [email protected]

[yonghong-song]

[buildbot, ok to test]

[yonghong-song]

Look good. But ubuntu 16.04 test failed. ubuntu 16.04 is linux 4.4 which does not have bpf_get_stackid support. I think you can just remove it since bpf_ktime_get_ns also requires gpl and is available on 4.4.

[4ast]

please remove the ability to specify the license via flags. I don't think it was ever done by any open source project. The license should only be explicitly set in the source code (either .h or .c)

[yonghong-song]

FYI, the following change

iff --git a/src/cc/frontends/clang/b_frontend_action.cc b/src/cc/frontends/clang/b_frontend_action.cc
index 593859a..d07f09e 100644
--- a/src/cc/frontends/clang/b_frontend_action.cc
+++ b/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"
-bash-4.2$ 

can prevent user defined macro BPF_LICENSE

[4ast]

this part of comment is obsolete?
Other than this bit the patch looks good to me.

[oriolarcas]

Thanks for the feedback, I changed it in 639217f. Any other fixes?