[RFC bpf-next 09/12] kbuild: Header guards for types from include/uapi/*.h in kernel BTF

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

Use pahole --header_guards_db flag to enable encoding of header guard
information in kernel BTF. The actual correspondence between header
file and guard string is computed by the scripts/infer_header_guards.pl.

The encoded header guard information could be used to restore the
original guards in the vmlinux.h, e.g.:

    include/uapi/linux/tcp.h:

      #ifndef _UAPI_LINUX_TCP_H
      #define _UAPI_LINUX_TCP_H
      ...
      union tcp_word_hdr {
    	struct tcphdr hdr;
    	__be32        words[5];
      };
      ...
      #endif /* _UAPI_LINUX_TCP_H */

    vmlinux.h:

      ...
      #ifndef _UAPI_LINUX_TCP_H

      union tcp_word_hdr {
    	struct tcphdr hdr;
    	__be32 words[5];
      };

      #endif /* _UAPI_LINUX_TCP_H */
      ...

Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
 scripts/link-vmlinux.sh | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index 918470d768e9..f57f621eda1f 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -110,6 +110,7 @@ vmlinux_link()
 gen_btf()
 {
 	local pahole_ver
+	local extra_flags
 
 	if ! [ -x "$(command -v ${PAHOLE})" ]; then
 		echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available"
@@ -122,10 +123,20 @@ gen_btf()
 		return 1
 	fi
 
+	if [ "${pahole_ver}" -ge "124" ]; then
+		scripts/infer_header_guards.pl \
+			include/uapi \
+			include/generated/uapi \
+			arch/${SRCARCH}/include/uapi \
+			arch/${SRCARCH}/include/generated/uapi \
+			> .btf.uapi_header_guards || return 1;
+		extra_flags="--header_guards_db .btf.uapi_header_guards"
+	fi
+
 	vmlinux_link ${1}
 
 	info "BTF" ${2}
-	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${1}
+	LLVM_OBJCOPY="${OBJCOPY}" ${PAHOLE} -J ${PAHOLE_FLAGS} ${extra_flags} ${1}
 
 	# Create ${2} which contains just .BTF section but no symbols. Add
 	# SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all
-- 
2.34.1