bpf: btf: Change how section is supported in btf_header

There are currently unused section descriptions in the btf_header.  Those
sections are here to support future BTF use cases.  For example, the
func section (func_off) is to support function signature (e.g. the BPF
prog function signature).

Instead of spelling out all potential sections up-front in the btf_header.
This patch makes changes to btf_header such that extending it (e.g. adding
a section) is possible later.  The unused ones can be removed for now and
they can be added back later.

This patch:
1. adds a hdr_len to the btf_header.  It will allow adding
sections (and other info like parent_label and parent_name)
later.  The check is similar to the existing bpf_attr.
If a user passes in a longer hdr_len, the kernel
ensures the extra tailing bytes are 0.

2. allows the section order in the BTF object to be
different from its sec_off order in btf_header.

3. each sec_off is followed by a sec_len.  It must not have gap or
overlapping among sections.

The string section is ensured to be at the end due to the 4 bytes
alignment requirement of the type section.

The above changes will allow enough flexibility to
add new sections (and other info) to the btf_header later.

This patch also removes an unnecessary !err check
at the end of btf_parse().

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

1 files changed, 2 insertions, 6 deletions

diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
index bcb56ee47014..4fa479741a02 100644
--- a/include/uapi/linux/btf.h
+++ b/include/uapi/linux/btf.h
@@ -12,15 +12,11 @@ struct btf_header {
 	__u16	magic;
 	__u8	version;
 	__u8	flags;
-
-	__u32	parent_label;
-	__u32	parent_name;
+	__u32	hdr_len;
 
 	/* All offsets are in bytes relative to the end of this header */
-	__u32	label_off;	/* offset of label section	*/
-	__u32	object_off;	/* offset of data object section*/
-	__u32	func_off;	/* offset of function section	*/
 	__u32	type_off;	/* offset of type section	*/
+	__u32	type_len;	/* length of type section	*/
 	__u32	str_off;	/* offset of string section	*/
 	__u32	str_len;	/* length of string section	*/
 };